Learn Java/J2EE core concepts and design/coding issues
With
Java/J2EE Job Interview Companion
By
K.Arulkumaran
Technical Reviewers
Craig Malone
Lara D’Albreo
Stuart Watson
Acknowledgementswww.allwalkin.blogspot.com
A. Sivayini
R.Kumaraswamipillai
Cover Design
K. Arulkumaran
A.Sivayini
2
Java/J2EE
Job Interview Companion
Copy Right 2005 K.Arulkumaran
The author has made every effort in the preparation of this book to ensure the accuracy of the information.
However, information in this book is sold without warranty either express or implied. The author will not be held
liable for any damages caused or alleged to be caused either directly or indirectly by this book.
3
Outline
SECTION DESCRIPTION
What this book will do for you?
Motivation for this book
Key Areas index
SECTION 1 Interview questions and answers on:
Javaallwalkin.blogspot.com
Language Fundamentals
Swing
Applet
Performance and memory Leaks.
Personal
SECTION 2 Interview questions and answers on:
Enterprise Java
J2EE
Servlet
JSP
JDBC
JNDI
RMI
EJB
JMS
XML
SQL, Database tuning and O/R mapping
RUP & UML
Struts
Web and Application servers.
Best practices and performance considerations.
Testing and deployment.
Personal
SECTION 3 Putting it all together section.
How would you go about…?
1. How would you go about documenting your Java/J2EE application?
2. How would you go about designing a Java/J2EE application?
3. How would you go about identifying performance problems and/or memory leaks in your Java
application?
4. How would you go about minimising memory leaks in your Java/J2EE application?
5. How would you go about improving performance of your Java/J2EE application?
6. How would you go about identifying any potential thread-safety issues in your Java/J2EE
application?
7. How would you go about identifying any potential transactional issues in your Java/J2EE
application?
8. How would you go about applying the Object Oriented (OO) design concepts in your Java/J2EE
4
application?
9. How would you go about applying the UML diagrams in your Java/J2EE project?
10. How would you go about describing the software development processes you are familiar with?
11. How would you go about applying the design patterns in your Java/J2EE application?
12. How would you go about determining the enterprise security requirements for your Java/J2EE
application?
13. How would you go about describing the open source projects like JUnit (unit testing), Ant (build
tool), CVS (version control system) and log4J (logging tool) which are integral part of most
Java/J2EE projects?
14. How would you go about describing Web services?
SECTION 4 Emerging Technologies/Frameworks
Test Driven Development (TDD).
Aspect Oriented Programming (AOP).
Inversion of Control (IOC) (Also known as Dependency Injection).
Annotations or attributes based programming (xdoclet etc).
Spring framework.
Hibernate framework.
EJB 3.0.
JavaServer Faces (JSF) framework.
SECTION 5 Sample interview questions …
Java
Web Components
Enterprise
Design
General
GLOSSARY OF TERMS
RESOURCES
INDEXallwalkin.blogspot.com
5
Table of contents
Outline ________________________________________________________________________ 3
Table of contents ________________________________________________________________ 5
What this book will do for you? _____________________________________________________ 6
Motivation for this book___________________________________________________________ 7
Key Areas Index _________________________________________________________________ 9
Java – Interview questions & answers_______________________________________________ 10
Java – Language Fundamentals _______________________________________________________ 11
Java – Personal _____________________________________________________________________ 37
Java – Key Points ___________________________________________________________________ 40
GLOSSARY OF TERMS _________________________________________________________ 43
RESOURCES __________________________________________________________________ 45
INDEX _______________________________________________________________________ 47
6
What this book will do for you?
Have you got the time to read 10 or more books and articles to add value prior to the interview? This book has been
written mainly from the perspective of Java/J2EE job seekers and interviewers. There are numerous books and articles
on the market covering specific topics like Java, J2EE, EJB, Design Patterns, ANT, CVS, Multi-Threading, Servlets, JSP,
emerging technologies like AOP (Aspect Oriented Programming), Test Driven Development (TDD), Inversion of Control
(IoC) etc. But from an interview perspective it is not possible to brush up on all these books where each book usually has
from 300 pages to 600 pages. The basic purpose of this book is to cover all the core concepts and design/coding issuesallwalkin.blogspot.com
which, all Java/J2EE developers, designers and architects should be conversant with to perform well in their current jobs
and to launch a successful career by doing well at interviews. The interviewer can also use this book to make sure that
they hire the right candidate depending on their requirements. This book contains a wide range of topics relating to
Java/J2EE development in a concise manner supplemented with diagrams, tables, sample codes and examples. This
book is also appropriately categorised to enable you to choose the area of interest to you.
This book will assist all Java/J2EE practitioners to become better at what they do. Usually it takes years to understand all
the core concepts and design/coding issues when you rely only on your work experience. The best way to fast track this
is to read appropriate technical information and proactively apply these in your work environment. It worked for me and
hopefully it will work for you as well. I was also at one stage undecided whether to name this book “Java/J2EE core
concepts and solving design/coding issues” or “Java/J2EE Job Interview Companion”. The reason I chose
“Java/J2EE Job Interview Companion” is because these core concepts and design/coding issues helped me to be
successful in my interviews and also gave me thumbs up in code reviews.
7
Motivation for this book
I started using Java in 1999 when I was working as a junior developer. During those two years as a permanent employee,
I pro-actively spent many hours studying the core concepts behind Java/J2EE in addition to my hands on practical
experience. Two years later I decided to start contracting. Since I started contracting in 2001, my career had a muchneeded
boost in terms of contract rates, job satisfaction, responsibility etc. I moved from one contract to another with a
view of expanding my skills and increasing my contract rates.
In the last 5 years of contracting, I have worked for 5 different organisations both medium and large on 8 different
projects. For each contract I held, on average I attended 6-8 interviews with different companies. In most cases multiple
job offers were made and consequently I was in a position to negotiate my contract rates and also to choose the job I
liked based on the type of project, type of organisation, technology used, etc. I have also sat for around 10 technical tests
and a few preliminary phone interviews.
The success in the interviews did not come easily. I spent hours prior to each set of interviews wading through various
books and articles as a preparation. The motivation for this book was to collate all this information into a single book,
which will save me time prior to my interviews but also can benefit others in their interviews. What is in this book has
helped me to go from just a Java/J2EE job to a career in Java/J2EE in a short time. It has also given me the job
security that ‘I can find a contract/permanent job opportunity even in the difficult job market’.
I am not suggesting that every one should go contracting but by performing well at the interviews you can be in a position
to pick the permanent role you like and also be able to negotiate your salary package. Those of you who are already inallwalkin.blogspot.com
good jobs can impress your team leaders, solution designers and/or architects for a possible promotion by demonstrating
your understanding of the key areas discussed in this book. You can discuss with your senior team members about
perfomance issues, transactional issues, threading issues (concurrency issues) and memory issues. In most of
my previous contracts I was in a position to impress my team leads and architects by pinpointing some of the critical
performance, memory, transactional and threading issues with the code and subsequently fixing them. Trust me it is not
hard to impress someone if you understand the key areas.
For example:
Struts action classes are not thread-safe (Refer Q113 in Enterprise section).
JSP variable declaration is not thread-safe (Refer Q34 in Enterprise section).
Valuable resources like database connections should be closed properly to avoid any memory and performance
issues (Refer Q45 in Enterprise section).
Throwing an application exception will not rollback the transaction in EJB. (Refer Q77 in Enterprise section).
The other key areas, which are vital to any software development, are a good understanding of some of key design
concepts, design patterns, and a modelling language like UML. These key areas are really worthy of a mention in
your resume and interviews.
For example:
Know how to use inheritance, polymorphism and encapsulation (Refer Q5, Q6, Q7, and Q8 in Java section.).
Why use design patterns? (Refer Q5 in Enterprise section).
Why is UML important? (Refer Q106 in Enterprise section).
If you happen to be in an interview with an organization facing serious issues with regards to their Java application
relating to memory leaks, performance problems or a crashing JVM etc then you are likely to be asked questions on
these topics. Refer Q 63 – Q 65 in Java section and Q123, Q125 in Enterprise section.
Another good reason why these key areas like transactional issues, design concepts, design patterns etc are vital are
because solution designers, architects, team leads, and/or senior developers are usually responsible for conducting the
technical interviews. These areas are their favourite topics because these are essential to any software development.
Some interviewers request you to write a small program during interview or prior to getting to the interview stage. This is
to ascertain that you can code using object oriented concepts and design patterns. So I have included a coding key area
to illustrate what you need to look for while coding.
8allwalkin.blogspot.com
Apply OO concepts like inheritance, polymorphism and encapsulation: Refer Q08 in Java section.
Program to interfaces not to implementations: Refer Q08, Q15 in Java section.
Use of relevant design patterns: Refer Q11 in How would you go about… section.
Use of Java collection API and exceptions correctly: Refer Q15, Q34, and Q35 in Java section.
Stay away from hard coding values: Refer Q04 in Java section.
L a n g u a g e
F u n d am e n ta ls
D e s ig n
C o n c e p ts D e s ig n
P a t t e rn s
S E c u r ity
C o n c u r re n c y
Is s u e s
P e r fo rm a n c e
Is s u e s
M e m o ry
Is s u e s
S c a la b ility
Is s u e s
S p e c if ic a tio n
F u n d am e n ta ls
E x c e p t io n
H a n d lin g
T ra n s a c t io n a l
Is s u e s
B e s t
P r a c t ic e s
S o ftw a re
D e v e lo pm e n t
P ro c e s s
C O d in g
L F D C
D P S F
C I P I
M I S I
S E E H
T I B P
S D
C O
H o w m a n y b o o k s d o I h a v e to re a d to
u n d e rs ta n d a n d p u t to g e th e r a ll th e s e
k e y a re a s ?
H ow m a n y y e a rs o f e x p e rie n c e
s h o u ld I h a v e to u n d e rs ta n d a ll th e s e
k e y a re a s ?
W ill th e s e k e y a re a s h e lp m e
p ro g re s s in m y c a re e r?
W ill th e s e k e y a re a s h e lp m e c u t
q u a lity c o d e ?
This book aims to solve the above dilemma.
My dad keeps telling me to find a permanent job (instead of contracting), which in his view provides better job security but
I keep telling him that in my view in Information Technology the job security is achieved only by keeping your knowledge
and skills sharp and up to date. The 8 contract positions I held over the last 5.5 years have given me broader experience
in Java/J2EE and related technologies. It also kept me motivated since there was always something new to learn in each
assignment, and not all companies will appreciate your skills and expertise until you decide to leave. Do the following
statements sound familiar to you when you hand in your resignation or decide not to extend your contract after getting
another job offer? “Can I tempt you to come back? What can I do to keep you here?” etc. You might even think why you
waited so long. The best way to make an impression in any organisations is to understand and proactively apply and
resolve the issues relating to the Key Areas discussed in the next section. But be a team player, be tactful and don’t
be critical of everything, do not act in a superior way and have a sense of humour.
“Technical skills must be complemented with interpersonal skills.”
Quick Read guide: It is recommended that you go through all the questions in all the sections but if you are pressed for time or would
like to read it just before an interview then follow the steps shown below:
1. Read/Browse Popular Questions in Java and Enterprise Java sections.
2. Read/Browse Key Points in Java and Enterprise Java sections.
3. Read/Browse through “Emerging Technologies/Frameworks” section.
4. Read/Browse “How would you go about…” section excluding Q11 & Q13, which are discussed in detail.
9
Key Areas Index
I have categorised the core concepts and issues into 14 key areas as listed below. These key areas are vital for any
good software development. This index will enable you to refer to the questions based on key areas. Also note that eachwww.allwalkin.blogspot.com
question has an icon next to it to indicate which key area or areas it belongs to. Additional reading is recommended for
beginners in each of the key areas.
Key Areas icon Question Numbers
Java section Enterprise section How
would you
go
about…?
Emerging
Technologies
/Frameworks
Language
Fundamentals
LF Q1-Q4, Q10-Q14, Q16-
Q20, Q22-Q27, Q30-
Q33, Q36-Q43, Q47-Q62
- Q10, Q15,
Q17, Q19
Specification
Fundamentals
SF - Q1-Q19, Q26-Q33, Q35-
Q38, Q41, Q42, Q44, Q46-
Q81, Q89-Q97, Q99, 102,
Q110, Q112-Q115, Q118-
Q119, Q121, Q126, Q127,
Q128
Q14
Design Concepts DC Q5-Q9, Q10, Q13, Q22,
Q49
Q2, Q3, Q19, Q20, Q21,
Q31, Q45, Q98, Q106,
Q107, Q108, Q109, 101,
Q111
Q02, Q08,
Q09
Q3-Q9, Q11,
Q13, Q14,
Q16, Q18,
Q20
Design Patterns DP Q10, Q14, Q20, Q31,
Q45, Q46, Q50, Q54,
Q66
Q5, Q5, Q22, Q24, Q25,
Q83, Q84, Q85, Q86, Q87,
Q88, Q110, Q111, Q116
Q11
Q12
Transactional
Issues
TI - Q43, Q71, Q72, Q73, Q74,
Q75, Q77, Q78, Q79
Q7
Concurrency Issues CI Q13, Q15, Q29, Q36,
Q40, Q53
Q16, Q34, Q113 Q6
Performance Issues PI Q13, Q15 -Q22, Q40,
Q53, Q63.
Q10, Q16, Q43, Q45, Q46,
Q72, Q83-Q88, Q97, Q98,
Q100, Q102, Q123, Q125,
Q128
Q3, Q5
Memory Issues MI Q22, Q29, Q32, Q33,
Q36, Q45, Q64, Q65.
Q45, Q93 Q3, Q4
Scalability Issues SI Q19, Q20 Q20, Q21, Q120, Q122
Exception Handling EH Q34,Q35 Q76, Q77
Security SE Q61 Q12, Q13, Q23, Q35, Q46,
Q51, Q58, Q81
Q12
Best Practices BP Q15, Q21, Q34, Q63,
Q64
Q10, Q16, Q39, Q40, Q46,
Q82, Q124, Q125
Software
Development
Process
SD - Q103-Q109, Q129, Q133,
Q134, Q136
Q1, Q10,
Q13
Q1, Q2
Coding1
CO Q04, Q08, Q10, Q12,
Q13, Q15, Q16, Q17,
Q21, Q34, Q45, Q46
Q10, Q18, Q21, Q23, Q36,
Q38, Q42, Q43, Q45, Q74,
Q75, Q76, Q77, Q112,
Q114, Q127, Q128
Q11
1 Some interviewers request you to write a small program during interview or prior to getting to the interview stage. This is to ascertain
that you can code using object oriented concepts and design patterns. I have included a coding key area to illustrate what you need to
look for while coding. Unlike other key areas, the CO is not always shown against the question but shown above the actual section of
relevance within a question.
Java
10
SECTION ONE
Java – Interview questions & answers
Language Fundamentals LF
Design Concepts DC
Design Patterns DP
Concurrency Issues CI
Performance Issues PI
Memory Issues MI
Exception Handling EH
Security SE
Scalability Issues SI
Coding1 CO
Popular Questions: Q01, Q04, Q07, Q08, Q13, Q16, Q17, Q18, Q19, Q25, Q27, Q29, Q32, Q34, Q40, Q45, Q46, Q50, Q51, Q53, Q54,
Q55, Q63,Q64, Q66, Q67
1 Unlike other key areas, the CO is not always shown against the question but shown above the actual subsection of relevance within a
question.
KEY
ARE
AS
Java
11
Java – Language Fundamentals
Q 01: Give a few reasons for using Java? LF DC
A 01: Java is a fun language. Let’s look at some of the reasons:
Built-in support for multi-threading, socket communication, and memory management (automatic garbage
collection).www.allwalkin.blogspot.com
Object Oriented (OO).
Better portability than other languages across operating systems.
Supports Web based applications (Applet, Servlet, and JSP), distributed applications (sockets, RMI. EJB etc)
and network protocols (HTTP, JRMP etc) with the help of extensive standardised APIs (Application Program
Interfaces).
Q 02: What is the main difference between the Java platform and the other software platforms? LF
A 02: Java platform is a software-only platform, which runs on top of other hardware-based platforms like UNIX, NT etc.
The Java platform has 2 components:
Java Virtual Machine (JVM) – ‘JVM’ is a software that can be ported onto various hardware platforms. Byte
codes are the machine language of the JVM.
Java Application Programming Interface (Java API) -
Q 03: What is the difference between C++ and Java? LF
A 03: Both C++ and Java use similar syntax and are Object Oriented, but:
Java does not support pointers. Pointers are inherently tricky to use and troublesome.
Java does not support multiple inheritances because it causes more problems than it solves. Instead Java
supports multiple interface inheritance, which allows an object to inherit many method signatures from
different interfaces with the condition that the inheriting object must implement those inherited methods. The
multiple interface inheritance also allows an object to behave polymorphically on those methods. [Refer Q 8
and Q 10 in Java section.]
Java does not support destructors but rather adds a finalize() method. Finalize methods are invoked by the
garbage collector prior to reclaiming the memory occupied by the object, which has the finalize() method. This
means you do not know when the objects are going to be finalized. Avoid using finalize() method to
release non-memory resources like file handles, sockets, database connections etc because Java has only
a finite number of these resources and you do not know when the garbage collection is going to kick in to
release these resources through the finalize() method.
Java does not include structures or unions because the traditional data structures are implemented as an
object oriented framework (Java collection framework – Refer Q14, Q15 in Java section).
Java
12
All the code in Java program is encapsulated within classes therefore Java does not have global variables or
functions.
C++ requires explicit memory management, while Java includes automatic garbage collection. [Refer Q32 in
Java section].
Q 04: Explain Java class loaders? Explain dynamic class loading? LFwww.allwalkin.blogspot.com
A 04: Class loaders are hierarchical. Classes are introduced into the JVM as they are referenced by name in a class that
is already running in the JVM. So how is the very first class loaded? The very first class is specially loaded with
the help of static main() method declared in your class. All the subsequently loaded classes are loaded by the
classes, which are already loaded and running. A class loader creates a namespace. All JVMs include at least one
class loader that is embedded within the JVM called the primordial (or bootstrap) class loader. Now let’s look at
non-primordial class loaders. The JVM has hooks in it to allow user defined class loaders to be used in place of
primordial class loader. Let us look at the class loaders created by the JVM.
CLASS LOADER reloadable? Explanation
Bootstrap
(primordial)
No Loads JDK internal classes, java.* packages. (as defined in the sun.boot.class.path
system property, typically loads rt.jar and i18n.jar)
Extensions No Loads jar files from JDK extensions directory (as defined in the java.ext.dirs system
property – usually lib/ext directory of the JRE)
System No Loads classes from system classpath (as defined by the java.class.path property, which
is set by the CLASSPATH environment variable or –classpath or –cp command line
options)
Bootstrap
(primordial)
(rt.jar, i18.jar)
Extensions
(lib/ext)
System
(-classpath)
Sibling1
classloader
Sibling2
classloader
JVM class loaders
Classes loaded by Bootstrap class loader have no visibility into classes
loaded by its descendants (ie Extensions and Systems class loaders).
The classes loaded by system class loader have visibility into classes loaded
by its parents (ie Extensions and Bootstrap class loaders).
If there were any sibling class loaders they cannot see classes loaded by
each other. They can only see the classes loaded by their parent class
loader. For example Sibling1 class loader cannot see classes loaded by
Sibling2 class loader
Both Sibling1 and Sibling2 class loaders have visibilty into classes loaded
by their parent class loaders (eg: System, Extensions, and Bootstrap)
Class loaders are hierarchical and use a delegation model when loading a class. Class loaders request their
parent to load the class first before attempting to load it themselves. When a class loader loads a class, the child
class loaders in the hierarchy will never reload the class again. Hence uniqueness is maintained. Classes loaded
by a child class loader have visibility into classes loaded by its parents up the hierarchy but the reverse is not true
as explained in the above diagram.
Important: Two objects loaded by different class loaders are never equal even if they carry the same values, which mean a
class is uniquely identified in the context of the associated class loader. This applies to singletons too, where each class
loader will have its own singleton. [Refer Q45 in Java section for singleton design pattern]
Explain static vs dynamic class loading?
Static class loading Dynamic class loading
Classes are statically loaded with Java’s
“new” operator.
class MyClass {
public static void main(String args[]) {
Car c = new Car();
}
}
Dynamic loading is a technique for programmatically invoking the functions of a
class loader at run time. Let us look at how to load classes dynamically.
Class.forName (String className); //static method which returns a Class
The above static method returns the class object associated with the class
name. The string className can be supplied dynamically at run time. Unlike the
static loading, the dynamic loading will decide whether to load the class Car or
the class Jeep at runtime based on a properties file and/or other runtime
Java
13
conditions. Once the class is dynamically loaded the following method returns an
instance of the loaded class. It’s just like creating a class object with no
arguments.
class.newInstance (); //A non-static method, which creates an instance of a
class (ie creates an object).
Jeep myJeep = null ;
//myClassName should be read from a properties file or Constants interface.
//stay away from hard coding values in your program. CO
String myClassName = "au.com.Jeep" ;
Class vehicleClass = Class.forName(myClassName) ;
myJeep = (Jeep) vehicleClass.newInstance();
myJeep.setFuelCapacity(50);
A NoClassDefFoundException is
thrown if a class is referenced with
Java’s “new” operator (i.e. static loading)
but the runtime system cannot find the
referenced class.
A ClassNotFoundException is thrown when an application tries to load in a
class through its string name using the following methods but no definition for the
class with the specified name could be found:
The forName(..) method in class - Class.
The findSystemClass(..) method in class - ClassLoader.
The loadClass(..) method in class - ClassLoader.
What are “static initializers” or “static blocks with no function names”? When a class is loaded, all blocks
that are declared static and don’t have function name (ie static initializers) are executed even before the
constructors are executed. As the name suggests they are typically used to initialize static fields. CO
public class StaticInitilaizer {
public static final int A = 5;
public static final int B;
//Static initializer block, which is executed only once when the class is loaded.
static {
if(A == 5)
B = 10;
else
B = 5;
}
public StaticInitilaizer(){} // constructor is called only after static initializer block
}
The following code gives an Output of A=5, B=10.
public class Test {
System.out.println("A =" + StaticInitilaizer.A + ", B =" + StaticInitilaizer.B);
}
Q 05: What are the advantages of Object Oriented Programming Languages (OOPL)? DC
A 05: The Object Oriented Programming Languages directly represent the real life objects like Car, Jeep, Account,
Customer etc. The features of the OO programming languages like polymorphism, inheritance and
encapsulation make it powerful. [Tip: remember pie which, stands for Polymorphism, Inheritance and
Encapsulation are the 3 pillars of OOPL]
Q 06: How does the Object Oriented approach improve software development? DC
A 06: The key benefits are:
Re-use of previous work: using implementation inheritance and object composition.
Real mapping to the problem domain: Objects map to real world and represent vehicles, customers,
products etc: with encapsulation.
Modular Architecture: Objects, systems, frameworks etc are the building blocks of larger systems.
Java
14
The increased quality and reduced development time are the by-products of the key benefits discussed above.
If 90% of the new application consists of proven existing components then only the remaining 10% of the code
have to be tested from scratch.
Q 07: How do you express an ‘is a’ relationship and a ‘has a’ relationship or explain inheritance and composition? What
is the difference between composition and aggregation? DC
A 07: The ‘is a’ relationship is expressed with inheritance and ‘has a’ relationship is expressed with composition. Both
inheritance and composition allow you to place sub-objects inside your new class. Two of the main techniques for
code reuse are class inheritance and object composition.
Inheritance [ is a ] Vs Composition [ has a ]
Building
Bathroom
House
class Building{
.......
}
class House extends Building{
.........
}
is a [House is a Building]
class House {
Bathroom room = new Bathroom() ;
....
public void getTotMirrors(){
room.getNoMirrors();
....
}
}
is a has a [House has a Bathroom]
has a
Inheritance is uni-directional. For example House is a Building. But Building is not a House. Inheritance uses
extends key word. Composition: is used when House has a Bathroom. It is incorrect to say House is a
Bathroom. Composition simply means using instance variables that refer to other objects. The class House will
have an instance variable, which refers to a Bathroom object.
Which one to use? The guide is that inheritance should be only used when subclass ‘is a’ superclass.
Don’t use inheritance just to get code reuse. If there is no ‘is a’ relationship then use composition for code
reuse. Overuse of implementation inheritance (uses the “extends” key word) can break all the subclasses, if
the superclass is modified.
Do not use inheritance just to get polymorphism. If there is no ‘is a’ relationship and all you want is
polymorphism then use interface inheritance with composition, which gives you code reuse (Refer Q8 in
Java section for interface inheritance).
What is the difference between aggregation and composition?
Aggregation Composition
Aggregation is an association in which one class
belongs to a collection. This is a part of a whole
relationship where a part can exist without a whole.
For example a line item is a whole and product is a
part. If a line item is deleted then corresponding
product need not be deleted. So aggregation has a
weaker relationship.
Composition is an association in which one class belongs to a
collection. This is a part of a whole relationship where a part
cannot exist without a whole. If a whole is deleted then all parts are
deleted. For example An order is a whole and line items are parts.
If an order deleted then all corresponding line items for that order
should be deleted. So composition has a stronger relationship.
Q 08: What do you mean by polymorphism, inheritance, encapsulation, and dynamic binding? DC
A 08: Polymorphism – means the ability of a single variable of a given type to be used to reference objects of different
types, and automatically call the method that is specific to the type of object the variable references. In a nutshell,
polymorphism is a bottom-up method call. The benefit of polymorphism is that it is very easy to add new classes
of derived objects without breaking the calling code (i.e. getTotArea() in the sample code shown below) that
uses the polymorphic classes or interfaces. When you send a message to an object even though you don’t know
what specific type it is, and the right thing happens, that’s called polymorphism. The process used by objectoriented
programming languages to implement polymorphism is called dynamic binding. Let us look at some
sample code to demonstrate polymorphism: CO
Java
15
+area() : double
<
Shape
+area() : double
Circle
+area() : double
HalfCircle
+area() : double
Square
//client or calling code
double dim = 5.0; //ie 5 meters radius or width
List listShapes = new ArrayList(20);
Shape s = new Circle();
listShapes.add(s); //add circle
s = new Square();
listShapes.add(s); //add square
getTotArea (listShapes,dim); //returns 78.5+25.0=103.5
//Later on, if you decide to add a half circle then define
//a HalfCircle class, which extends Circle and then provide an
//area(). method but your called method getTotArea(...) remains
//same.
s = new HalfCircle();
listShapes.add(s); //add HalfCircle
getTotArea (listShapes,dim); //returns 78.5+25.0+39.25=142.75
/** called method: method which adds up areas of various
** shapes supplied to it.
**/
public double getTotArea(List listShapes, double dim){
Iterator it = listShapes.iterator();
double totalArea = 0.0;
//loop through different shapes
while(it.hasNext()) {
Shape s = (Shape) it.next();
totalArea += s.area(dim); //polymorphic method call
}
return totalArea ;
}
Sample code:
For example: given a base
class/interface Shape,
polymorphism allows the
programmer to define
different area(double
dim1) methods for any
number of derived classes
such as Circle, Square etc.
No matter what shape an
object is, applying the area
method to it will return the
right results.
Later on HalfCicle can be
added without breaking
your called code i.e.
method getTotalArea(...)
Depending on what the
shape is, appropriate
area(double dim) method
gets called and calculated.
Circle area is 78.5sqm
Square area is 25sqm
HalfCircle area is 39.25
sqm
+area() : double
Circle
+area() : double
HalfCircle
+area() : double
Square
+area() : double
«interface»
Shape
Inheritance – is the inclusion of behaviour (i.e. methods) and state (i.e. variables) of a base class in a derived
class so that they are accessible in that derived class. The key benefit of Inheritance is that it provides the formal
mechanism for code reuse. Any shared piece of business logic can be moved from the derived class into the
base class as part of refactoring process to improve maintainability of your code by avoiding code duplication. The
existing class is called the superclass and the derived class is called the subclass. Inheritance can also be
defined as the process whereby one object acquires characteristics from one or more other objects the same way
children acquire characteristics from their parents.
There are two types of inheritances:
1. Implementation inheritance (aka class inheritance): You can extend an applications’ functionality by reusing
functionality in the parent class by inheriting all or some of the operations already implemented. In Java, you can
only inherit from one superclass. Implementation inheritance promotes reusability but improper use of class
inheritance can cause programming nightmares by breaking encapsulation and making future changes a problem.
With implementation inheritance, the subclass becomes tightly coupled with the superclass. This will make the
design fragile because if you want to change the superclass, you must know all the details of the subclasses to
avoid breaking them. So when using implementation inheritance, make sure that the subclasses depend only
on the behaviour of the superclass, not on the actual implementation. For example in the above diagram the
subclasses should only be concerned about the behaviour known as area() but not how it is implemented.
2. Interface inheritance (aka type inheritance): This is also known as subtyping. Interfaces provide a mechanism
for specifying a relationship between otherwise unrelated classes, typically by specifying a set of common
methods each implementing class must contain. Interface inheritance promotes the design concept of program to
interfaces not to implementations. This also reduces the coupling or implementation dependencies between
systems. In Java, you can implement any number of interfaces. This is more flexible than implementation
inheritance because it won’t lock you into specific implementations which make subclasses difficult to maintain. So
care should be taken not to break the implementing classes by modifying the interfaces.
Which one to use? Prefer interface inheritance to implementation inheritance because it promotes the design
concept of coding to an interface and reduces coupling. Interface inheritance can achieve code reuse with the
help of object composition. If you look at Gang of Four (GoF) design patterns, you can see that it favours
interface inheritance to implementation inheritance. CO
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16
Implementation inheritance Interface inheritance
Let’s assume that savings account and term deposit
account have a similar behaviour in terms of depositing
and withdrawing money, so we will get the super class to
implement this behaviour and get the subclasses to reuse
this behaviour. But saving account and term deposit
account have specific behaviour in calculating the interest.
public abstract class Account {
public void deposit(double amount) {
//deposit logic
}
public void withdraw(double amount) {
//withdraw logic
}
public abstract double calculateInterest(double amount);
}
public class SavingsAccount extends Account {
public double calculateInterest(double amount) {
//calculate interest for SavingsAccount
}
}
public class TermDepositAccount extends Account {
public double calculateInterest(double amount) {
//calculate interest for TermDeposit
}
}
The calling code can be defined as follows for illustration
purpose only:
public class Test {
public static void main(String[] args) {
Account acc1 = new SavingsAccount();
acc1.deposit(5.0);
acc1.withdraw(2.0);www.allwalkin.blogspot.com
Account acc2 = new TermDepositAccount();
acc2.deposit(10.0);
acc2.withdraw(3.0);
acc1.calculateInterest(500.00);
acc2.calculateInterest(500.00);
}
}
Let’s look at an interface inheritance code sample, which makes
use of composition for reusability. In the following example the
methods deposit(…) and withdraw(…) share the same piece of code
in AccountHelper class. The method calculateInterest(…) has its
specific implementation in its own class.
public interface Account {
public abstract void deposit(double amount);
public abstract void withdraw(double amount);
public abstract int getAccountType();
}
public interface SavingsAccount extends Account{
public abstract double calculateInterest(double amount);
}
public interface TermDepositAccount extends Account{
public abstract double calculateInterest(double amount);
}
The classes SavingsAccountImpl, TermDepositAccountImpl
should implement the methods declared in its interfaces. The class
AccountHelper implements the methods deposit(…) and
withdraw(…)
public class SavingsAccountImpl implements SavingsAccount{
private int accountType = 1;
//helper class which promotes code reuse through composition
AccountHelper helper = new AccountHelper();
public void deposit(double amount) {
helper.deposit(amount, getAccountType());
}
public void withdraw(double amount) {
helper.withdraw(amount, getAccountType());
}
public double calculateInterest(double amount) {
//calculate interest for SavingsAccount
}
public int getAccountType(){
return accountType;
}
}
public class TermDepositAccountImpl implements
TermDepositAccount {
private int accountType = 2;
//helper class which promotes code reuse through composition
AccountHelper helper = new AccountHelper();
public void deposit(double amount) {
helper.deposit(amount, getAccountType());
}
public void withdraw(double amount) {
helper.withdraw(amount, getAccountType());
}
public double calculateInterest(double amount) {
//calculate interest for TermDeposit
}
public int getAccountType() {
return accountType;
}
}
The calling code can be defined as follows for illustration purpose
only:
public class Test {
public static void main(String[] args) {
Java
17
Account acc1 = new SavingsAccountImpl();
acc1.deposit(5.0);
Account acc2 = new TermDepositAccountImpl();
acc2.deposit(10.0);
if (acc1.getAccountType() == 1) {
((SavingsAccount) acc1).calculateInterest(500.00);
}
if (acc2.getAccountType() == 2) {
((TermDepositAccount) acc2).calculateInterest(500.00);
}
}
}
Encapsulation – refers to keeping all the related members (variables and methods) together in an object.
Specifying members as private can hide the variables and methods. Objects should hide their inner workings from
the outside view. Good encapsulation improves code modularity by preventing objects interacting with
each other in an unexpected way, which in turn makes future development and refactoring efforts easy.
Being able to encapsulate members of a class is important for security and integrity. We can protect variables
from unacceptable values. The sample code below describes how encapsulation can be used to protect the
MyMarks object from having negative values. Any modification to member variable “vmarks” can only be carried
out through the setter method setMarks(int mark). This prevents the object “MyMarks” from having any negative
values by throwing an exception. CO
setName (String name)
String getName()
int getMarks()
setMarks(int mark)
private int vmarks;
private String name;
Member
variables are
encapsulated,
so that they
can only be
accessed via
encapsulating
methods.
Class MyMarks {
private int vmarks = 0;
private String name;
public void setMarks(int mark)
throws MarkException {
if(mark > 0)
this.vmarks = mark;
else {
throw new MarkException("No negative
Values");
}
}
public int getMarks(){
return vmarks;
}
//getters and setters for attribute name goes here.
}
Sample code
Q 09: What is design by contract? Explain the assertion construct? DC
A 09: Design by contract specifies the obligations of a calling-method and called-method to each other. Design by
contract is a valuable technique, which should be used to build well-defined interfaces. The strength of this
programming methodology is that it gets the programmer to think clearly about what a function does, what pre
and post conditions it must adhere to and also it provides documentation for the caller. Java uses the assert
statement to implement pre- and post-conditions. Java’s exceptions handling also support design by contract
especially checked exceptions (Refer Q34 in Java section for checked exceptions). In design by contract in
addition to specifying programming code to carrying out intended operations of a method the programmer also
specifies:
1. Preconditions – This is the part of the contract the calling-method must agree to. Preconditions specify the
conditions that must be true before a called method can execute. Preconditions involve the system state and the
arguments passed into the method at the time of its invocation. If a precondition fails then there is a bug in the
calling-method or calling software component.
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18
On public methods On non-public methods
Preconditions on public methods are enforced by explicit checks
that throw particular, specified exceptions. You should not use
assertion to check the parameters of the public methods but
can use for the non-public methods. Assert is inappropriate
because the method guarantees that it will always enforce the
argument checks. It must check its arguments whether or not
assertions are enabled. Further, assert construct does not throw
an exception of a specified type. It can throw only an
AssertionError.
public void setRate(int rate) {
if(rate <= 0 || rate > MAX_RATE){
throw new IllegalArgumentException(“Invalid rate ” + rate);
}
setCalculatedRate(rate);
}
You can use assertion to check the parameters of the
non-public methods.
private void setCalculatedRate(int rate) {
assert (rate > 0 && rate < MAX_RATE) : rate; //calculate the rate and set it. } Assertions can be disabled, so programs must not assume that assert construct will be always executed: //Wrong: if assertion is disabled, CarpenterJob never //Get removed assert jobsAd.remove(PilotJob); //Correct: boolean pilotJobRemoved = jobsAd.remove(PilotJob); assert pilotJobRemoved; 2. Postconditions – This is the part of the contract the called-method agrees to. What must be true after a method completes successfully. Postconditions can be used with assertions in both public and non-public methods. The postconditions involve the old system state, the new system state, the method arguments and the method’s return value. If a postcondition fails then there is a bug in the called-method or called software component. public double calcRate(int rate) { if(rate <= 0 || rate > MAX_RATE){
throw new IllegalArgumentException(“Invalid rate !!! ”);
}
//logic to calculate the rate and set it goes here
assert this.evaluate(result) < 0 : this; //this message sent to AssertionError on failure return result; } 3. Class invariants - what must be true about each instance of a class? A class invariant as an internal invariant that can specify the relationships among multiple attributes, and should be true before and after any method completes. If an invariant fails then there could be a bug in either calling-method or called-method. There is no particular mechanism for checking invariants but it is convenient to combine all the expressions required for checking invariants into a single internal method that can be called by assertions. For example if you have a class, which deals with negative integers then you define the isNegative() convenient internal method: class NegativeInteger { Integer value = new Integer (-1); //invariant //constructor public NegativeInteger(Integer int) { //constructor logic goes here assert isNegative(); } //rest of the public and non-public methods goes here. public methods should call assert isNegative(); prior to its return //convenient internal method for checking invariants. Returns true if the integer value is negative private boolean isNegative(){ return value.intValue() < 0 ; } } The isNegative() method should be true before and after any method completes, each public method and constructor should contain the following assert statement immediately prior to its return. assert isNegative(); Explain the assertion construct? The assertion statements have two forms as shown below: assert Expression1; Java 19 assert Expression1 : Expression2; Where: Expression1 is a boolean expression. If the Expression1 evaluates to false, it throws an AssertionError without any detailed message. Expression2 if the Expression1 evaluates to false throws an AssertionError with using the value of the Expression2 as the errors’ detailed message. Note: If you are using assertions (available from JDK1.4 onwards), you should supply the JVM argument to enable it by package name or class name. Java -ea[:packagename...|:classname] or Java -enableassertions[:packagename...|:classname] Java –ea:Account Q 10: What is the difference between an abstract class and an interface and when should you use them? LF DP DC A 10: In design, you want the base class to present only an interface for its derived classes. This means, you don’t want anyone to actually instantiate an object of the base class. You only want to upcast to it (implicit upcasting, which gives you polymorphic behaviour), so that its interface can be used. This is accomplished by making that class abstract using the abstract keyword. If anyone tries to make an object of an abstract class, the compiler prevents it. The interface keyword takes this concept of an abstract class a step further by preventing any method or function implementation at all. You can only declare a method or function but not provide the implementation. The class, which is implementing the interface, should provide the actual implementation. The interface is a very useful and commonly used aspect in OO design, as it provides the separation of interface and implementation and enables you to: Capture similarities among unrelated classes without artificially forcing a class relationship. Declare methods that one or more classes are expected to implement. Reveal an object's programming interface without revealing its actual implementation. Model multiple interface inheritance in Java, which provides some of the benefits of full on multiple inheritances, a feature that some object-oriented languages support that allow a class to have more than one superclass. Shape Circle Square CircleOnSquare Diamond problem & use of interface No multiple inheritance in JAVA Circle Square CircleOnSquare <
ShapeIF
<
CircleIF
<
SquareIF
Multiple interface inheritance in JAVA
Abstract class Interface
Have executable methods and abstract methods. Have no implementation code. All methods are abstract.
Can only subclass one abstract class.
A class can implement any number of interfaces.
Can have instance variables, constructors and any
visibility: public, private, protected, none (aka package).
Cannot have instance variables, constructors and can have
only public and none (aka package) visibility.
When to use an abstract class?: In case where you want to use implementation inheritance then it is usually
provided by an abstract base class. Abstract classes are excellent candidates inside of application frameworks.
Abstract classes let you define some default behaviour and force subclasses to provide any specific behaviour.
Care should be taken not to overuse implementation inheritance as discussed in Q8 in Java section.
Java
20
When to use an interface?: For polymorphic interface inheritance, where the client wants to only deal with a type
and does not care about the actual implementation use interfaces. If you need to change your design frequently,
you should prefer using interface to abstract. CO Coding to an interface reduces coupling and interface
inheritance can achieve code reuse with the help of object composition. Another justification for using interfaces
is that they solve the ‘diamond problem’ of traditional multiple inheritance as shown in the figure. Java does not
support multiple inheritances. Java only supports multiple interface inheritance. Interface will solve all the
ambiguities caused by this ‘diamond problem’.
Design pattern: Strategy design pattern lets you swap new algorithms and processes into your program without
altering the objects that use them. Strategy design pattern: Refer Q11 in How would you go about… section.
Q 11: Why there are some interfaces with no defined methods (i.e. marker interfaces) in Java? LF
A 11: The interfaces with no defined methods act like markers. They just tell the compiler that the objects of the classes
implementing the interfaces with no defined methods need to be treated differently. Example Serializable (Refer
Q19 in Java section), Cloneable etc
Q 12: When is a method said to be overloaded and when is a method said to be overridden? LF CO
A 12:
Method Overloading Method Overriding
Overloading deals with multiple methods in the same class
with the same name but different method signatures.
class MyClass {
public void getInvestAmount(int rate) {…}
public void getInvestAmount(int rate, long principal)
{ … }
}
Both the above methods have the same method names
but different method signatures, which mean the methods
are overloaded.
Overriding deals with two methods, one in the parent class and
the other one in the child class and has the same name and
signatures.
class BaseClass{
public void getInvestAmount(int rate) {…}
}
class MyClass extends BaseClass {
public void getInvestAmount(int rate) { …}
}
Both the above methods have the same method names and
the signatures but the method in the subclass MyClass
overrides the method in the superclass BaseClass.
Overloading lets you define the same operation in
different ways for different data.
Overriding lets you define the same operation in different
ways for different object types.
Q 13: What is the main difference between an ArrayList and a Vector? What is the main difference between Hashmap
and Hashtable? LF DC PI CI
A 13:
Vector / Hashtable ArrayList / Hashmap
Original classes before the introduction of Collections
API. Vector & Hashtable are synchronized. Any
method that touches their contents is thread-safe.
So if you don’t need a thread safe collection, use the ArrayList or
Hashmap. Why pay the price of synchronization unnecessarily at
the expense of performance degradation.
So which is better? As a general rule, prefer ArrayList/Hashmap to Vector/Hashtable. If your application is a
multithreaded application and at least one of the threads either adds or deletes an entry into the collection
then use new Java collection API‘s external synchronization facility as shown below to temporarily synchronize
your collections as needed: CO
Map myMap = Collections.synchronizedMap (myMap);
List myList = Collections.synchronizedList (myList);
Java arrays are even faster than using an ArrayList/Vector and perhaps therefore may be preferable.
ArrayList/Vector internally uses an array with some convenient methods like add(..), remove(…) etc.
Q 14: Explain the Java Collection framework? LF DP
Java
21
A 14: The key interfaces used by the collection framework are List, Set and Map. The List and Set extends the
Collection interface. Should not confuse the Collection interface with the Collections class which is a utility class.
A Set is a collection with unique elements and prevents duplication within the collection. HashSet and TreeSet
are implementations of a Set interface. A List is a collection with an ordered sequence of elements and may
contain duplicates. ArrayList, LinkedList and Vector are implementations of a List interface.
The Collection API also supports maps, but within a hierarchy distinct from the Collection interface. A Map is an
object that maps keys to values, where the list of keys is itself a collection object. A map can contain duplicate
values, but the keys in a map must be distinct. HashMap, TreeMap and Hashtable are implementations of a Map
interface.
How to implement collection ordering? SortedSet and SortedMap interfaces maintain sorted order. The
classes, which implement the Comparable interface, impose natural order. For classes that don’t implement
comparable interface, or when one needs even more control over ordering based on multiple attributes, a
Comparator interface should be used.
Design pattern: What is an Iterator? An Iterator is a use once object to access the objects stored in a collection.
Iterator design pattern (aka Cursor) is used, which is a behavioural design pattern that provides a way to access
elements of a collection sequentially without exposing its internal representation.
JAVA collection framework
extends
ArrayList
AbstractCollection
AbstractList
AbstractSet
AbstractMap
Abstract
Sequential
List
LinkedList
Vector
TreeSet
HashSet
TreeMap
HashMap
SortedSet
Linked
HashSet
Collection
Weak
HashMap
List
Set
Map
Random
Acess
Linked
HashMap
Identity
HashMap
Comparator
Arrays
implements
Stack
asList
java.util.Collections
(Diagram sourced from: http://www.wilsonmar.com/1arrays.htm)
What are the benefits of the Java collection framework? Collection framework provides flexibility,
performance, and robustness.
Polymorphic algorithms – sorting, shuffling, reversing, binary search etc.
Set algebra - such as finding subsets, intersections, and unions between objects.
Performance - collections have much better performance compared to the older Vector and Hashtable
classes with the elimination of synchronization overheads.
Thread-safety - when synchronization is required, wrapper implementations are provided for temporarily
synchronizing existing collection objects.
Immutability - when immutability is required wrapper implementations are provided for making a collection
immutable.
Extensibility - interfaces and abstract classes provide an excellent starting point for adding functionality and
features to create specialized object collections.
Q 15: What are some of the best practices relating to Java collection? BP PI CI
A 15:
Use ArrayLists, HashMap etc as opposed to Vector, Hashtable etc, where possible to avoid any
synchronization overhead. Even better is to use just arrays where possible. If multiple threads concurrently
access a collection and at least one of the threads either adds or deletes an entry into the collection,
then the collection must be externally synchronized. This is achieved by:
Map myMap = Collections.synchronizedMap (myMap);
Java
22
List myList = Collections.synchronizedList (myList);
Set the initial capacity of a collection appropriately (e.g. ArrayList, HashMap etc). This is because collection
classes like ArrayList, HashMap etc must grow periodically to accommodate new elements. But if you have a
very large array, and you know the size in advance then you can speed things up by setting the initial size
appropriately.
For example: HashMaps/Hashtables need to be created with sufficiently large capacity to minimise
rehashing (which happens every time the table grows). HashMap has two parameters initial capacity and
load factor that affect its performance and space requirements. Higher load factor values (default load factor
of 0.75 provides a good trade off between performance and space) will reduce the space cost but will
increase the lookup cost of myMap.get(…) and myMap.put(…) methods. When the number of entries in the
HashMap exceeds the current capacity * loadfactor then the capacity of the HasMap is roughly doubled by
calling the rehash function. It is also very important not to set the initial capacity too high or load factor too
low if iteration performance or reduction in space is important.
Program in terms of interface not implementation: For example you might decide a LinkedList is the best
choice for some application, but then later decide ArrayList might be a better choice for performance reason.
CO
Use:
List list = new ArrayList(100); //program in terms of interface & set the initial capacity.
Instead of:
ArrayList list = new ArrayList();
Avoid storing unrelated or different types of objects into same collection: This is analogous to storing
items in pigeonholes without any labelling. To store items use value objects or data objects (as oppose to
storing every attribute in an ArrayList or HashMap). Provide wrapper classes around your collection API
classes like ArrayList, Hashmap etc as shown in better approach column. Also where applicable consider
using composite design pattern, where an object may represent a single object or a collection of objects.
Refer Q52 in Java section for UML diagram of a composite design pattern. CO
Avoid where possible Better approach
The code below is hard to maintain and understand by
others. Also gets more complicated as the requirements
grow in the future because we are throwing different
types of objects like Integer, String etc into a list just
based on the indices and it is easy to make mistakes
while casting the objects back during retrieval.
List myOrder = new ArrayList()
ResultSet rs = …
While (rs.hasNext()) {
List lineItem = new ArrayList();
lineItem.add (new Integer(rs.getInt(“itemId”)));
lineItem.add (rs.getString(“description”));
….
myOrder.add( lineItem);
}
return myOrder;
Example 2:
List myOrder = new ArrayList(10);
//create an order
OrderVO header = new OrderVO();
header.setOrderId(1001);
…
//add all the line items
LineItemVO line1 = new LineItemVO();
line1.setLineItemId(1);
LineItemVO line2 = new LineItemVO();
Line2.setLineItemId(2);
When storing items into a collection define value objects as shown
below: (VO is an acronym for Value Object).
public class LineItemVO {
private int itemId;
private String productName;
public int getLineItemId(){return accountId ;}
public int getAccountName(){return accountName;}
public void setLineItemId(int accountId ){
this.accountId = accountId
}
//implement other getter & setter methods
}
Now let’s define our base wrapper class, which represents an order:
public abstract class Order {
int orderId;
List lineItems = null;
public abstract int countLineItems();
public abstract boolean add(LineItemVO itemToAdd);
public abstract boolean remove(LineItemVO itemToAdd);
public abstract Iterator getIterator();
public int getOrderId(){return this.orderId; }
}
Now a specific implementation of our wrapper class:
public class OverseasOrder extends Order {
public OverseasOrder(int inOrderId) {
this.lineItems = new ArrayList(10);
this.orderId = inOrderId;
}
Java
23
List lineItems = new ArrayList();
lineItems.add(line1);
lineItems.add(line2);
//to store objects
myOrder.add(order);// index 0 is an OrderVO object
myOrder.add(lineItems);//index 1 is a List of line items
//to retrieve objects
myOrder.get(0);
myOrder.get(1);
Above approaches are bad because disparate objects
are stored in the lineItem collection in example-1 and
example-2 relies on indices to store disparate objects.
The indices based approach and storing disparate
objects are hard to maintain and understand because
indices are hardcoded and get scattered across the
code. If an index position changes for some reason, then
you will have to change every occurence, otherwise it
breaks your application.
The above coding approaches are analogous to storing
disparate items in a storage system without proper
labelling and just relying on its grid position.
public int countLineItems() { //logic to count }
public boolean add(LineItemVO itemToAdd){
…//additional logic or checks
return lineItems.add(itemToAdd);
}
public boolean remove(LineItemVO itemToAdd){
return lineItems.remove(itemToAdd);
}
public ListIterator getIterator(){ return lineItems.Iterator();}
}
Now to use:
Order myOrder = new OverseasOrder(1234) ;
LineItemVO item1 = new LineItemVO();
Item1.setItemId(1);
Item1.setProductName(“BBQ”);
LineItemVO item2 = new LineItemVO();
Item1.setItemId(2);
Item1.setProductName(“Outdoor chair”);
//to add line items to order
myOrder.add(item1);
myOrder.add(item2);
…
Q 16: When providing a user defined key class for storing objects in the Hashmaps or Hashtables, what methods do you
have to provide or override (ie method overriding)? LF PI CO
A 16: You should override the equals() and hashCode() methods from the Object class. The default implementation of
the equals() and hashcode(), which are inherited from the java.lang.Object uses an object instance’s memory
location (e.g. MyObject@6c60f2ea). This can cause problems when two instances of the car objects have the
same color but the inherited equals() will return false because it uses the memory location, which is different for
the two instances. Also the toString() method can be overridden to provide a proper string representation of your
object. Points to consider:
• If a class overrides equals(), it must override hashCode().
• If 2 objects are equal, then their hashCode values must be equal as well.
• If a field is not used in equals(), then it must not be used in hashCode().
• If it is accessed often, hashCode() is a candidate for caching to enhance performance.
Note: Java 1.5 introdces enumerated constants, which improves readability and maintainability of your code. Java
programming language enums are more powerful than their counterparts in other languages. E.g. A class like
Weather can be built on top of simple enum type Season and the class Weather can be made immutable, and only
one instance of each Weather can be created, so that your Weather class does not have to override equals()
and hashCode() methods.
public class Weather {
public enum Season {WINTER, SPRING, SUMMER, FALL}
private final Season season;
private static final List
private Weather (Season season) { this.season = season;}
public Season getSeason () { return season;}
static {
for (Season season : Season.values()) {
listWeather.add(new Weather(season));
}
}
public static ArrayList
public String toString(){ return season;} // takes advantage of toString() method of Season.
}
Java
24
Q 17: What is the main difference between a String and a StringBuffer class? LF PI CI CO
A 17:
String StringBuffer / StringBuilder
String is immutable: you can’t modify a string
object but can replace it by creating a new
instance. Creating a new instance is rather
expensive.
//Inefficient version using immutable String
String output = “Some text”
Int count = 100;
for(int I =0; i
}
return output;
The above code would build 99 new String
objects, of which 98 would be thrown away
immediately. Creating new objects is not
efficient.
StringBuffer is mutable: use StringBuffer or StringBuilder when you want to
modify the contents. StringBuilder was added in Java 5 and it is identical in
all respects to StringBuffer except that it is not synchronised, which makes
it slightly faster at the cost of not being thread-safe.
//More efficient version using mutable StringBuffer
StringBuffer output = new StringBuffer(110);
Output.append(“Some text”);
for(int I =0; i
}
return output.toString();
The above code creates only two new objects, the StringBuffer and the final
String that is returned. StringBuffer expands as needed, which is costly
however, so it would be better to initilise the StringBuffer with the correct size
from the start as shown.
Another important point is that creation of extra strings is not limited to ‘overloaded mathematical operators’ (“+”) but
there are several methods like conact(), trim(), substring(), and replace() in String classes that generate new
string instances. So use StringBuffer or StringBuilder for computation intensive operations, which offer better
performance.
Q 18: What is the main difference between pass-by-reference and pass-by-value? LF PI
A 18: Other languages use pass-by-reference or pass-by-pointer. But in Java no matter what type of argument you
pass the corresponding parameter (primitive variable or object reference) will get a copy of that data, which is
exactly how pass-by-value (ie copy-by-value) works.
In Java, if a calling method passes a reference of an object as an argument to the called method then the passedin
reference gets copied first and then passed to the called method. Both the original reference that was
passed-in and the copied reference will be pointing to the same object. So no matter which reference you use, you
will be always modifying the same original object, which is how the pass-by-reference works as well.
ref d
Pass-by-value for p rimitive variables vs O bject references
public void first(){
int i= 10;
int x = second(i);
//At this point
//value of i is still 10
//value of x is 11
}
public int second(int k) {
k++;
return k ;
}
i = 10
k = 10
k = 11
Copy of i
s t o re s i
co pies i
acts on k
ref
public void first(){
Car c = new Car("red")
//At this point
//color is Red
second(c);
//At this point
//color is B lue
}
public void second(Car d)
{
d.setColor(blue);
//color is blue
}
Car object
String color = red
ref c
copy of c
Prim itive variables Object references
modifies the original
object through copied
reference
modifies the copy k
but not the original.
Changes
color = b lue
If your method call involves inter-process (e.g. between two JVMs) communication, then the reference of the
calling method has a different address space to the called method sitting in a separate processs (i.e separate
JVM). Hence inter-process communication involves calling method passing objects as arguments to called method
by-value in a serialized form, which can adversely affect performance due to marshalling and unmarshalling cost.
Note: As discussed in Q69 in Enterprise section, EJB 2.x introduced local interfaces, where enterprise beans that can be used
locally within the same JVM using Java’s form of pass-by-reference, hence improving performance.
Java
25
Q 19: What is serialization? How would you exclude a field of a class from serialization or what is a transient variable?
What is the common use? LF SI PI
A 19: Serialization is a process of reading or writing an object. It is a process of saving an object’s state to a sequence of
bytes, as well as a process of rebuilding those bytes back into a live object at some future time. An object is
marked serializable by implementing the java.io.Serializable interface, which is only a marker interface -- it simply
allows the serialization mechanism to verify that the class can be persisted, typically to a file.
byte stream
write to
Serialization
File
class Car implements Serializable {
String color = null;
transient File fh = null;
}
Car Object1
Class Car implements
Serializable {
String color = null;
}
Car Object 2
deserialize serialize
deserialize
Transient variables cannot be serialized. The fields marked transient in a serializable object will not be
transmitted in the byte stream. An example would be a file handle or a database connection. Such objects are only
meaningful locally. So they should be marked as transient in a serializable class.
Serialization can adversely affect performance since it:
Depends on reflection.
Has an incredibly verbose data format.
Is very easy to send surplus data.
When to use serialization? Do not use serialization if you do not have to. A common use of serialization is to use
it to send an object over the network or if the state of an object needs to be persisted to a flat file or a database.
(Refer Q57 on Enterprise section). Deep cloning or copy can be achieved through serialization. This may be fast
to code but will have performance implications (Refer Q22 in Java section).
The objects stored in an HTTP session should be serializable to support in-memory replication of sessions to
achieve scalability (Refer Q20 in Enterprise section). Objects are passed in RMI (Remote Method Invocation)
across network using serialization (Refer Q57 in Enterprise section).
Q 20: Explain the Java I/O streaming concept and the use of the decorator design pattern in Java I/O? LF DP PI SI
A 20: Java input and output is defined in terms of an abstract concept called a “stream”, which is a sequence of data.
There are 2 kinds of streams.
Byte streams (8 bit bytes) Abstract classes are: InputStream and OutputStream
Character streams (16 bit UNICODE) Abstract classes are: Reader and Writer
Design pattern: java.io.* classes use the decorator design pattern. The decorator design pattern attaches
responsibilities to objects at runtime. Decorators are more flexible than inheritance because the inheritance
attaches responsibility to classes at compile time. The java.io.* classes use the decorator pattern to construct
different combinations of behaviour at runtime based on some basic classes.
Attaching responsibilities to classes at
compile time using subclassing.
Attaching responsibilities to objects at runtime using a decorator
design pattern.
Inheritance (aka subclassing) attaches
responsibilities to classes at compile time.
When you extend a class, each individual
changes you make to child class will affect all
instances of the child classes. Defining many
classes using inheritance to have all possible
combinations is problematic and inflexible.
By attaching responsibilities to objects at runtime, you can apply changes
to each individual object you want to change.
File file = new File(“c:/temp”);
FileInputStream fis = new FileInputStream(file);
BufferedInputStream bis = new BufferedInputStream(fis);
Decorators decorate an object by enhancing or restricting functionality of an
object it decorates. The decorators add or restrict functionality to decorated
Java
26
objects either before or after forwarding the request. At runtime the
BufferedInputStream (bis), which is a decorator (aka a wrapper around
decorated object), forwards the method call to its decorated object
FileInputStream (fis). The ‘bis’ will apply the additional functionality of
buffering around the lower level file (i.e. fis) I/O.
java.lang.Object
java.io.InputStream java.io.OutputStream java.io.Reader java.io.Writer
java.io.BefferedReader java.io.InputStreamReader
java.io.FileReader
java.io.OutputStreamWriterr
java.io.FileWriter
java.io.FileInputStream java.io.FileoutputStream
java.lang.System
java.io.* class hierachy
Note: Only a few sub classes of abstract classes
like InputStream are shown for clarity.
The New I/O (NIO): more scalabe and better performance
Java has long been not suited for developing programs that perform a lot of I/O operations. Furthermore,
commonly needed tasks such as file locking, non-blocking and asynchronous I/O operations and ability to map file
to memory were not available. Non-blocking I/O operations were achieved through work arounds such as
multithreading or using JNI. The New I/O API (aka NIO) in J2SE 1.4 has changed this situation.
A server’s ability to handle several client requests effectively depends on how it uses I/O streams. When a server
has to handle hundreds of clients simultaneously, it must be able to use I/O services concurrently. One way to
cater for this scenario in Java is to use threads but having almost one-to-one ratio of threads (100 clients will have
100 threads) is prone to enormous thread overhead and can result in performance and scalability problems
due to consumption of memory stacks and CPU context switching. To overcome this problem, a new set of
non-blocking I/O classes have been introduced to the Java platform in java.nio package. The non-blocking I/O
mechanism is built around Selectors and Channels. Channels, Buffers and Selectors are the core of the NIO.
A Channel class represents a bi-directional communication channel (similar to InputStrean and OutputStream)
between datasources such as a socket, a file, or an application component, which is capable of performing one or
more I/O operations such as reading or writing. Channels can be non-blocking, which means, no I/O operation will
wait for data to be read or written to the network. The good thing about NIO channels is that they can be
asynchronously interrupted and closed. So if a thread is blocked in an I/O operation on a channel, another thread
can interrupt that blocked thread.
Buffers hold data. Channels can fill and drain Buffers. Buffers replace the need for you to do your own buffer
management using byte arrays. There are different types of Buffers like ByteBuffer, CharBuffer, DoubleBuffer, etc.
A Selector class is responsible for multiplexing (combining multiple streams into a single stream) by allowing a
single thread to service multiple channels. Each Channel registers events with a Selector. When events arrive
from clients, the Selector demultiplexes (separating a single stream into multiple streams) them and dispatches
the events to corresponding Channels. To achieve non-blocking I/O a Channel class must work in conjunction with
a Selector class.
Design pattern: NIO uses a reactor design pattern, which demultiplexes events (separating single stream into
multiple streams) and dispatches them to registered object handlers. The reactor pattern is similar to an observer
pattern (aka publisher and subscriber design pattern), but an observer pattern handles only a single source of
events (i.e a single publisher with multiple subscribers) where a reactor pattern handles multiple event sourceswww.allwalkin.blogspot.com
(i.e. multiple publishers with multiple subscribers). The intent of an observer pattern is to define a one-to-many
depndency so that when one object (i.e the publisher) changes its state, all its dependents (i.e all its subscribers)
are notfied and updated correspondingly.
Another sought after functionality of NIO is its ability to map a file to memory. There is a specialized form of a
Buffer known as MappedByteBuffer, which represents a buffer of bytes mapped to a file. To map a file to
Java
27
MappedByteBuffer, you must first get a channel for a file. Once you get a channel then you map it to a buffer and
subsequently you can access it like any other ByteBuffer. Once you map an input file to a CharBuffer, you can do
pattern matching on the file contents. This is similar to running “grep” on a UNIX file system.
Another feature of NIO is its ability to lock and unlock files. Locks can be exclusive or shared and can be held on a
contiguous portion of a file. But file locks are subject to the control of the underlying operating system.
Q 21: How can you improve Java I/O performance? PI BP
A 21: Java applications that utilise Input/Output are excellent candidates for performance tuning. Profiling of Java
applications that handle significant volumes of data will show significant time spent in I/O operations. This means
substantial gains can be had from I/O performance tuning. Therefore, I/O efficiency should be a high priority for
developers looking to optimally increase performance.www.allwalkin.blogspot.com
The basic rules for speeding up I/O performance are
Minimise accessing the hard disk.
Minimise accessing the underlying operating system.
Minimise processing bytes and characters individually.
Let us look at some of the techniques to improve I/O performance. CO
Use buffering to minimise disk access and underlying operating system. As shown below, with buffering
large chunks of a file are read from a disk and then accessed a byte or character at a time.
Without buffering : inefficient code
try{
File f = new File("myFile.txt");
FileInputStream fis = new FileInputStream(f);
int count = 0;
int b = ;
while((b = fis.read()) != -1){
if(b== '\n') {
count++;
}
}
// fis should be closed in a finally block.
fis.close() ;
}
catch(IOException io){}
Note: fis.read() is a native method call to the
underlying system.
With Buffering: yields better performance
try{
File f = new File("myFile.txt");
FileInputStream fis = new FileInputStream(f);
BufferedInputStream bis = new BufferedInputStream(fis);
int count = 0;
int b = ;
while((b = bis.read()) != -1){
if(b== '\n') {
count++;
}
}
//bis should be closed in a finally block.
bis.close() ;
}
catch(IOException io){}
Note: bis.read() takes the next byte from the input buffer and only
rarely access the underlying operating system.
Instead of reading a character or a byte at a time, the above code with buffering can be improved further by
reading one line at a time as shown below:
FileReader fr = new FileReader(f);
BufferedReader br = new BufferedReader(fr);
While (br.readLine() != null) count++;
By default the System.out is line buffered, which means that the output buffer is flushed when a new line
character is encountered. This is required for any interactivity between an input prompt and display of output.
The line buffering can be disabled for faster I/O operation as follows:
FileOutputStream fos = new FileOutputStream(file);
BufferedOutputStream bos = new BufferedOutputStream(fos, 1024);
PrintStream ps = new PrintStream(bos,false);
System.setOut(ps);
while (someConditionIsTrue)
System.out.println(“blah…blah…”);
}
It is recommended to use logging frameworks like Log4J or apache commons logging, which uses
buffering instead of using default behaviour of System.out.println(…..) for better performance. Frameworks
like Log4J are configurable, flexible, extensible and easy to use.
Java
28
Use the NIO package, if you are using JDK 1.4 or later, which uses performance-enhancing features like
buffers to hold data, memory mapping of files, non-blocking I/O operations etc.
I/O performance can be improved by minimising the calls to the underlying operating systems. The Java
runtime itself cannot know the length of a file, querying the file system for isDirectory(), isFile(), exists() etc
must query the underlying operating system.
Where applicable caching can be used to improve performance by reading in all the lines of a file into a Java
collection class like an ArrayList or a HashMap and subsequently access the data from an in-memory
collection instead of the disk.
Q 22: What is the main difference between shallow cloning and deep cloning of objects? DC LF MI PI
A 22: The default behaviour of an object’s clone() method automatically yields a shallow copy. So to achieve a deep
copy the classes must be edited or adjusted.
Shallow copy: If a shallow copy is performed on obj-1 as shown in fig-2 then it is copied but its contained objects
are not. The contained objects Obj-1 and Obj-2 are affected by changes to cloned Obj-2. Java supports shallow
cloning of objects by default when a class implements the java.lang.Cloneable interface.
Deep copy: If a deep copy is performed on obj-1 as shown in fig-3 then not only obj-1 has been copied but the
objects contained within it have been copied as well. Serialization can be used to achieve deep cloning. Deep
cloning through serialization is faster to develop and easier to maintain but carries a performance overhead.
Fig-2:Shallow cloning
Fig-3:Deep cloning
Shallow Vs Deep cloning
Obj-1
contained
Obj-1
contained
Obj-2
Fig-1:Original Object
contains contains
Obj-1
Cloned
Obj-2
contained
Obj-1
contained
Obj-2
Obj-1
contained
Obj-1
contained
Obj-2
Cloned
Obj-2
contained
Obj-1
contained
Obj-2
For example, invoking clone() method on a HashMap returns a shallow copy of HashMap instance, which means
the keys and values themselves are not cloned. If you want a deep copy then a simple method is to serialize
the HashMap to a ByteArrayOutputSream and then deserialize it. This creates a deep copy but does require that
all keys and values in the HashMap are Serializable. Its primary advantage is that it will deep copy any arbitrary
object graph.
List some of the methods supported by Java object class? clone(), toString(), equals(Object obj), hashCode()
refer Q16 in Java section, wait(), notify() refer Q42 in Java section, finalize() etc.
Q 23: What is the difference between an instance variable and a static variable? Give an example where you might use
a static variable? LF
A 23:
Static variable Instance variable
Class variables are called static variables. There is only one
occurrence of a class variable per JVM per class loader.
When a class is loaded the class variables (aka static
variables) are initialised.
Instance variables are non-static and there is one
occurrence of an instance variable in each class instance (ie
each object).
A static variable is used in the singleton pattern. (Refer Q45 in Java section). A static variable is used with a final
modifier to define constants.
Q 24: Give an example where you might use a static method? LF
Java
29
A 24: Static methods prove useful for creating utility classes, singleton classes and factory methods (Refer Q45,
Q46 in Java section). Utility classes are not meant to be instantiated. Improper coding of utility classes can lead to
procedural coding. java.lang.Math, java.util.Collections etc are examples of utility classes in Java.
Q 25: What are access modifiers? LF
A 25:
Modifier Used with Description
public Outer classes, interfaces,
constructors, Inner classes, methods
and field variables
A class or interface may be accessed from outside the
package. Constructors, inner classes, methods and field
variables may be accessed wherever their class is
accessed.
protected Constructors, inner classes, methods,
and field variables.
Accessed by other classes in the same package or any
subclasses of the class in which they are referred (ie same
package or different package).
private Constructors, inner classes,
methods and field variables,
Accessed only within the class in which they are declared
No modifier:
(Package by
default).
Outer classes, inner classes,
interfaces, constructors, methods, and
field variables
Accessed only from within the package in which they are
declared.
Q 26: Where and how can you use a private constructor? LF
A 26: Private constructor is used if you do not want other classes to instantiate the object. The instantiation is done by a
public static method within the same class.
Used in the singleton pattern. (Refer Q45 in Java section).
Used in the factory method pattern (Refer Q46 in Java section).
Used in utility classes e.g. StringUtils etc.
Q 27: What is a final modifier? Explain other Java modifiers? LF
A 27: A final class can’t be extended i.e. A final class may not be subclassed. A final method can’t be overridden when
its class is inherited. You can’t change value of a final variable (i.e. it is a constant).
Modifier Class Method Property
static A static inner class is just an
inner class associated with
the class, rather than with an
instance.
cannot be instantiated, are called by
classname.method, can only access static
variables
Only one instance
of the variable
exists.
abstract Cannot be instantiated, must
be a superclass, used
whenever one or more
methods are abstract.
Method is defined but contains no implementation
code (implementation code is included in the
subclass). If a method is abstract then the entire
class must be abstract.
N/A
synchronized N/A Acquires a lock on the class for static methods.
Acquires a lock on the instance for non-static
methods.
N/A
transient N/A N/A
Field should not
be serialized.
final Class cannot be inherited Method cannot be overridden Makes the variable
a constant.
native N/A Platform dependent. No body, only signature. N/A
Note: Be prepared for tricky questions on modifiers like, what is a “volatile”? Or what is a “const”? Etc. The reason it is tricky is
that Java does have these keywords “const” and “volatile” as reserved, which means you can’t name your variables with these
names but modifier “const” is not yet added in the language and the modifier “volatile” is very rarely used.
The “volatile” modifier is used on member variables that may be modified simultaneously by other threads. Since other threads
cannot see local variables, there is no need to mark local varibles as volatile. E.g. volatile int number; volatile private List
listItems = null; etc. The modifier volatile only synchronizes the variable marked as volatile whereas “synchronized” modifier
synchronizes all variables.
Java uses the final modifier to declare constants. A final variable or constant declared as “final” has a value that is immutable
and cannot be modified to refer to any other objects other than one it was initialized to refer to. So the “final” modifier applies only
to the value of the variable itself, and not to the object referenced by the variable. This is where the “const” modifier can come in
very useful if added to the Java language. A reference variable or a constant marked as “const” refers to an immutable object
that cannot be modified. The reference variable itself can be modified, if it is not marked as “final”. The “const” modifier will be
applicable only to non-primitive types. The primitive types should continue to use the modifier “final”.
Java
30
Q 28: What is the difference between final, finally and finalize() in Java? LF
A 28:
final - constant declaration. Refer Q27 in Java section.
finally - handles exception. The finally block is optional and provides a mechanism to clean up regardless of
what happens within the try block (except System.exit(0) call). Use the finally block to close files or to release
other system resources like database connections, statements etc. (Refer Q45 in Enterprise section)
finalize() - method helps in garbage collection. A method that is invoked before an object is discarded by the
garbage collector, allowing it to clean up its state. Should not be used to release non-memory resources like
file handles, sockets, database connections etc because Java has only a finite number of these resources and
you do not know when the garbage collection is going to kick in to release these non-memory resources
through the finalize() method.
Q 29: How does Java allocate stack and heap memory? Explain re-entrant, recursive and idempotent
methods/functions? MI CI
A 29: Each time an object is created in Java it goes into the area of memory known as heap. The primitive variables like
int and double are allocated in the stack, if they are local method variables and in the heap if they are member
variables (i.e. fields of a class). In Java methods local variables are pushed into stack when a method is invoked
and stack pointer is decremented when a method call is completed. In a multi-threaded application each thread
will have its own stack but will share the same heap. This is why care should be taken in your code to avoid any
concurrent access issues in the heap space. The stack is threadsafe (each thread will have its own stack) but the
heap is not threadsafe unless guarded with synchronisation through your code.
A method in stack is reentrant allowing multiple concurrent invocations that do not interfere with each other. A
function is recursive if it calls itself. Given enough stack space, recursive method calls are perfectly valid in Java
though it is tough to debug. Recursive functions are useful in removing iterations from many sorts of algorithms. All
recursive functions are re-entrant but not all re-entrant functions are recursive. Idempotent methods are methods,
which are written in such a way that repeated calls to the same method with the same arguments yield same
results. For example clustered EJBs, which are written with idempotent methods, can automatically recover from a
server failure as long as it can reach another server.
J a v a s t a c k & h e a p m e m o r y a l lo c a t io n
S ta c k
H e a p
p u b lic c la s s S ta c k R e f {
p u b l i c v o i d f i r s t ( ) {
s e c o n d ( ) ;
/ / a f te r
}
p u b l i c v o i d s e c o n d ( ) {
C a r c = n e w C a r ( ) ;
}
}
f i r s t ( )
s e c o n d ()
f i r s t ( )
s e c o n d () re f c
f i r s t ( )
C a r
1
2
3
4
R e f
p u b lic c la s s H e a p R e f {
C a r c = n e w C a r ( ) ;
p u b l i c v o i d f i r s t ( ) {
c = N e w C a r ( ) ;
}
}
c C a r
R e f
C a r
c C a r 2
1
Q 30: Explain Outer and Inner classes (or Nested classes) in Java? When will you use an Inner Class? LF
Java
31
A 30: In Java not all classes have to be defined separate from each other. You can put the definition of one class inside
the definition of another class. The inside class is called an inner class and the enclosing class is called an outer
class. So when you define an inner class, it is a member of the outer class in much the same way as other
members like attributes, methods and constructors.
Where should you use inner classes? Code without inner classes is more maintainable and readable. When
you access private data members of the outer class, the JDK compiler creates package-access member functions
in the outer class for the inner class to access the private members. This leaves a security hole. In general we
should avoid using inner classes. Use inner class only when an inner class is only relevant in the context of the
outer class and/or inner class can be made private so that only outer class can access it. Inner classes are used
primarily to implement helper classes like Iterators, Comparators etc which are used in the context of an outer
class. CO
Member inner class Anonymous inner class
public class MyStack {
private Object[] items = null;
…
public Iterator iterator() {
return new StackIterator();
}
//inner class
class StackIterator implements Iterator{
…
public boolean hasNext(){…}
}
}
public class MyStack {
private Object[] items = null;
…
public Iterator iterator() {
return new Iterator {
…
public boolean hasNext() {…}
}
}
}
Explain outer and inner classes?
Class Type Description Example + Class name
Outer
class
Package
member class
or interface
Top level class. Only type JVM
can recognize.
//package scope
class Outside{}
Outside.class
Inner
class
static nested
class or
interface
Defined within the context of the
top-level class. Must be static &
can access static members of its
containing class. No relationship
between the instances of outside
and Inside classes.
//package scope
class Outside {
static class Inside{ }
}
Outside.class ,Outside$Inside.class
Inner
class
Member class Defined within the context of
outer class, but non-static. Until
an object of Outside class has
been created you can’t create
Inside.
class Outside{
class Inside(){}
}
Outside.class , Outside$Inside.class
Inner
class
Local class Defined within a block of code.
Can use final local variables and
final method parameters. Only
visible within the block of code
that defines it.
class Outside {
void first() {
final int i = 5;
class Inside{}
}
}
Outside.class , Outside$1$Inside.class
Inner
class
Anonymous
class
Just like local class, but no
name is used. Useful when only
one instance is used in a
method. Most commonly used in
AWT event model.
class Outside{
void first() {
button.addActionListener ( new ActionListener()
{
public void actionPerformed(ActionEvent e) {
System.out.println(“The button was pressed!”);
}
});
}
}
Outside.class , Outside$1.class
Java
32
Q 31: What is type casting? Explain up casting vs down casting? When do you get ClassCastException? LF DP
A 31: Type casting means treating a variable of one type as though it is another type.
When up casting primitives as shown below from left to right, automatic conversion occurs. But if you go from
right to left, down casting or explicit casting is required. Casting in Java is safer than in C or other languages that
allow arbitrary casting. Java only lets casts occur when they make sense, such as a cast between a float and an
int. However you can't cast between an int and a String (is an object in Java).
byte short int long float double
int i = 5;
long j = i; //Right. Up casting or implicit casting
byte b1 = i; //Wrong. Compile time error “Type Mismatch”.
byte b2 = (byte) i ; //Right. Down casting or explicit casting is required.
When it comes to object references you can always cast from a subclass to a superclass because a subclass
object is also a superclass object. You can cast an object implicitly to a super class type (ie upcasting). If this
were not the case polymorphism wouldn’t be possible.
O bjec t
V e h ic le
B u s C a r
B MW
V e h ic le v 1 = n ew C a r ( ); / / R igh t .u p c a s t in g o r im p lic it ca s t in g
V e h ic le v 2 = n ew V e h ic le ( );
C a r c 0 = v 1 ; / /W ro n g . c om p ile t im e e r ro r "T ype M ism atch " .
/ / E x p lic it o r d ow n ca s t in g is re q u ir ed
C a r c 1 = (C a r)v 1 ; / / R igh t . d ow n c as tin g o r e xp lic it c a stin g .
/ / v 1 h as kn ow le d ge o f C a r d u e to lin e 1
C a r c 2 = (C a r)v 2 ; / /W ro n g. R un t im e e x c e p tio n C la s sC a s tE x c e p t ion
/ / v 2 h a s n o k n o w le d ge o f C ar .
B u s b 1 = n ew BM W ( ); / /W ro n g . com p ile t im e e r ro r "T yp e M ism atc h"
C a r c 3 = n ew BM W () ; / /R ig h t.u p c ast in g o r im p lic it c as tin g
C a r c 4 = (BM W )v 1 ; / /W ro n g . R u n tim e e x c ep tio n C la s sC a s tE x c e p t io n
O b je c t o = v 1 ; / / v 1 c a n o n ly b e u p c a st to its p a re n t o r
C a r c 5 = (C a r) v 1 ; / / v 1 c an b e d o w n c a s t to C a r d u e to lin e 1 .
U p c a s tin g v s D ow n c a s tin g O b je c ts
You can cast down the hierarchy as well but you must explicitly write the cast and the object must be a
legitimate instance of the class you are casting to. The ClassCastException is thrown to indicate that code
has attempted to cast an object to a subclass of which it is not an instance. We can deal with the problem of
incorrect casting in two ways:
Use the exception handling mechanism to catch ClassCastException.
try{
Object o = new Integer(1);
System.out.println((String) o);
}
catch(ClassCastException cce) {
logger.log(“Invalid casting, String is expected…Not an Integer”);
System.out.println(((Integer) o).toString());
}
Use the instanceof statement to guard against incorrect casting.
If(v2 instanceof Car) {
Car c2 = (Car) v2;
}
Design pattern: The “instanceof” and “typecast” constructs are shown for the illustration purpose only.
Using these constructs can be unmaintainable due to large if and elseif statements and can affect
performance if used in frequently accessed methods or loops. Look at using visitor design pattern to avoid
these constructs. (Refer Q11 in How would you go about section…).
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33
Points-to-ponder: We can also get a ClassCastException when two different class loaders load the same class because they
are treated as two different classes.
Q 32: What do you know about the Java garbage collector? When does the garbage collection occur? Explain different
types of references in Java? LF MI
A 32: Each time an object is created in Java, it goes into the area of memory known as heap. The Java heap is called
the garbage collectable heap. The garbage collection cannot be forced. The garbage collector runs in low
memory situations. When it runs, it releases the memory allocated by an unreachable object. The garbage
collector runs on a low priority daemon (background) thread. You can nicely ask the garbage collector to collect
garbage by calling System.gc() but you can’t force it.
What is an unreachable object? An object’s life has no meaning unless something has reference to it. If you
can’t reach it then you can’t ask it to do anything. Then the object becomes unreachable and the garbage collector
will figure it out. Java automatically collects all the unreachable objects periodically and releases the memory
consumed by those unreachable objects to be used by the future reachable objects.
G a r b a g e C o l l e c t io n & U n r e a c h a b l e O b je c t s
r e a c h a b l e
r e a c h a b l e
C a s e 1
C a r a = n e w C a r ( ) ;
C a r b = n e w C a r( )
1 r e f a
C a r o b je c t
2 r e f b
C a r o b je c t
C a s e 2
a = n e w C a r( ) r e a c h a b l e
r e a c h a b l e
u n r e a c h a b le
2 r e f b
C a r o b je c t
3 r e f a
C a r o b je c t
1
C a r o b je c t
C a s e 3
a = b
u n r e a c h a b le
r e a c h a b l e
u n r e a c h a b le
r e f a
C a r o b je c t
r e f b
C a r o b je c t
1
C a r o b je c t
2
3
C a s e 4
a = n u ll ;
b = n u ll ;
u n r e a c h a b le
u n r e a c h a b le
u n r e a c h a b le
C a r o b je c t
C a r o b je c t
1
C a r o b je c t
2
3
H e a p
We can use the following options with the Java command to enable tracing for garbage collection events.
-verbose:gc reports on each garbage collection event.
Explain types of references in Java? java.lang.ref package can be used to declare soft, weak and phantom
references.
Garbage Collector won’t remove a strong reference.
A soft reference will only get removed if memory is low. So it is useful for implementing caches while
avoiding memory leaks.
A weak reference will get removed on the next garbage collection cycle. Can be used for implementing
canonical maps. The java.util.WeakHashMap implements a HashMap with keys held by weak references.
A phantom reference will be finalized but the memory will not be reclaimed. Can be useful when you want to
be notified that an object is about to be collected.
Q 33: If you have a circular reference of objects, but you no longer reference it from an execution thread, will this object
be a potential candidate for garbage collection? LF MI
Java
34
A 33: Yes. Refer diagram below.
sample code
public void buildCar() {
Car c = new Car();
Engine e = new Engine();
//lets create a circular reference
c.engine = e;
e.car = c;
}
buildCar()
Stack Heap
Car
Engine
Before buildCar() returns
Stack Heap
Car
Engine
After buildCar() returns
Both the Car & Engine are not reachable
and potential candidate for Garbage
Collection.
Garbage Collecting Circular References
Q 34: Discuss the Java error handling mechanism? What is the difference between Runtime (unchecked) exceptions
and checked exceptions? What is the implication of catching all the exceptions with the type “Exception”? EH BP
A 34:
Errors: When a dynamic linking failure or some other “hard” failure in the virtual machine occurs, the virtual
machine throws an Error. Typical Java programs should not catch Errors. In addition, it’s unlikely that typical Java
programs will ever throw Errors either.
Exceptions: Most programs throw and catch objects that derive from the Exception class. Exceptions indicate
that a problem occurred but that the problem is not a serious JVM problem. An Exception class has many
subclasses. These descendants indicate various types of exceptions that can occur. For example,
NegativeArraySizeException indicates that a program attempted to create an array with a negative size. One
exception subclass has special meaning in the Java language: RuntimeException. All the exceptions except
RuntimeException are compiler checked exceptions. If a method is capable of throwing a checked exception it
must declare it in its method header or handle it in a try/catch block. Failure to do so raises a compiler error. So
checked exceptions can, at compile time, greatly reduce the occurence of unhandled exceptions surfacing at
runtime in a given application at the expense of requiring large throws declarations and encouraging use of poorlyconstructed
try/catch blocks. Checked exceptions are present in other languages like C++, C#, and Python.
Throw able a n d its subclasses
Object
Exception
Error
Throwable
IOException Runtim eException
NullPoin terException
LinkageError
Runtime Exceptions (unchecked exception)
A RuntimeException class represents exceptions that occur within the Java virtual machine (during runtime). An
example of a runtime exception is NullPointerException. The cost of checking for the runtime exception often
outweighs the benefit of catching it. Attempting to catch or specify all of them all the time would make your code
unreadable and unmaintainable. The compiler allows runtime exceptions to go uncaught and unspecified. If you
Java
35
like, you can catch these exceptions just like other exceptions. However, you do not have to declare it in your
“throws" clause or catch it in your catch clause. In addition, you can create your own RuntimeException
subclasses and this approach is probably preferred at times because checked exceptions can complicate method
signatures and can be difficult to follow.
Exception handling best practices: BP
Why is it not advisable to catch type “Exception”? CO
Exception handling in Java is polymorphic in nature. For example if you catch type Exception in your code then it
can catch or throw its descendent types like IOException as well. So if you catch the type Exception before the
type IOException then the type Exception block will catch the entire exceptions and type IOException block is
never reached. In order to catch the type IOException and handle it differently to type Exception, IOException
should be caught first (remember that you can’t have a bigger basket above a smaller basket).
The diagram below is an example for illustration only. In practice it is not recommended to catch type
“Exception”. We should only catch specific subtypes of the Exception class. Having a bigger basket (ie
Exception) will hide or cause problems. Since the RunTimeException is a subtype of Exception, catching the type
Exception will catch all the run time exceptions (like NullpointerException, ArrayIndexOut-OfBounds-Exception) as
well.
Catching Exceptions
try{}
catch(Exception ex){
//this block is reached
}
catch(IOException ioe) {
//this block is never reached
//There is a bigger basket
//above me who will catch it
//before I can.
}
try{}
catch(IOException ioe){
}
catch(Exception ex) {
}
Wrong approach
Right approach
basket
basket
basket
basket
Hint: as shown in the
figure, think of catching an
exception in a basket. You
should always have the
smaller basket above the
bigger one. Otherwise the
bigger basket will catch all
the exceptions and smaller
baskets will not catch any.
Why should you throw an exception early? CO
The exception stack trace helps you pinpoint where an exeception occurred by showing us the exact sequence of
method calls that lead to the exception. By throwing your exception early, the exception becomes more accurate
and more specific. Avoid suppressing or ignoring exceptions. Also avoid using exceptions just to get a flow control.
Instead of:
…www.allwalkin.blogspot.com
InputStream in = new FileInputStream(fileName); // assume this line throws an exception because filename == null.
…
Use the following code because you get a more accurate stack trace:
…
if(filename == null) {
throw new IllegalArgumentException(“file name is null”);
}
InputStream in = new FileInputStream(fileName);
…
Why should you catch a checked exception late in a catch {} block?
You should not try to catch the exception before your program can handle it in an appropriate manner. The natural
tendency when a compiler complains about a checked exception is to catch it so that the compiler stops reporting
Java
36
errors. The best practice is to catch the exception at the appropriate layer (e.g an exception thrown at an
integration layer can be caught at a presentation layer in a catch {} block), where your program can either
meaningfully recover from the exception and continue to execute or log the exception only once in detail, so that
user can identify the cause of the exception.
Note: Due to heavy use of checked exceptions and minimal use of unchecked exceptions, there has been a hot debate in the
Java community regarding true value of checked exceptions. Use checked exceptions when the client code can take some
useful recovery action based on information in exception. Use unchecked exception when client code cannot do anything. For
example, convert your SQLException into another checked exception if the client code can recover from it and convert your
SQLexception into an unchecked (i.e. RuntimeException) exception, if the client code cannot do anything about it.
A note on key words for error handling:
throw / throws – used to pass an exception to the method that called it.
try – block of code will be tried but may cause an exception.
catch – declares the block of code, which handles the exception.
finally – block of code, which is always executed (except System.exit(0) call) no matter what program flow, occurs when dealing
with an exception.
assert – Evaluates a conditional expression to verify the programmer’s assumption.
Q 35: What is a user defined exception? EH
A 35: User defined exceptions may be implemented by defining a new exception class by extending the Exception class.
public class MyException extends Exception {
/* class definition of constructors goes here */
public MyException() {
super();
}
public MyException (String errorMessage) {
super (errorMessage);
}
}
Throw and/or throws statement is used to signal the occurrence of an exception. Throw an exception:
throw new MyException(“I threw my own exception.”)
To declare an exception: public myMethod() throws MyException {…}
Q 36: What is the difference between processes and threads? LF MI CI
A 36: A process is an execution of a program but a thread is a single execution sequence within the process. A process
can contain multiple threads. A thread is sometimes called a lightweight process.
Process (JVM)
Stack Stack Stack
Each thread has its
own stack memory
Thread 1 Thread 2 Thread 3
method1() method1() method1()
Process vs Threads
Heap
Object1 Object
2
Single heap per process
shared by all the threads
A JVM runs in a single process and threads in a JVM share the heap belonging to that process. That is why
several threads may access the same object. Threads share the heap and have their own stack space. This is
Java
37
how one thread’s invocation of a method and its local variables are kept thread safe from other threads. But the
heap is not thread-safe and must be synchronized for thread safety.
Skipping Q36 – Q65 …………
Java – Personal
Q 66: Did you have to use any design patterns in your Java project? DP
A 66: Yes. Refer Q10 [Strategy], Q14 [Iterator], Q20 [Decorator], Q31 [Visitor], Q45 [Singleton], Q46 [Factory],
Q50 [Command], and Q54 [MVC] in Java section and Q11 in How would you go about… section. Note: Learning
of other patterns recommended (Gang of Four Design Patterns).
Resource: http://www.patterndepot.com/put/8/JavaPatterns.htm.
Why use design patterns, you may ask (Refer Q5 in Enterprise section). Design patterns are worthy of mention in
your CV and interview. Design patterns have a number of advantages:
Capture design experience from the past.
Promote reuse without having to reinvent the wheel.
Define the system structure better.
Provide a common design vocabulary.
Some advice if you are just starting on your design pattern journey:
If you are not familiar with UML, now is the time. UML is commonly used to describe patterns in pattern
catalogues, including class diagrams, sequence diagrams etc. (Refer Q106 - Q109 in Enterprise section).
When using patterns, it is important to define a naming convention. It will be much easier to manage a project
as it grows to identify exactly what role an object plays with the help of a naming convention e.g.
AccountFacilityBusinessDelegate, AccountFacilityFactory, AccountFacilityValueObject, AccountDecorator,
AccountVisitor, AccountTransferObject (or AccountFacilityVO or AccountTO).
Make a list of requirements that you will be addressing and then try to identify relevant patterns that are
applicable.
Q 67: Tell me about yourself or about some of the recent projects you have worked with? What do you consider your
most significant achievement? Why do you think you are qualified for this position? Why should we hire you and
what kind of contributions will you make?
A 67: [Hint:] Pick your recent projects and brief on it. Also is imperative that during your briefing, you demonstrate how
you applied your skills and knowledge in some of the following areas:
Design concepts and design patterns: How you understand and applied them.
Performance and memory issues: How you identified and fixed them.
Exception handling and best practices: How you understand and applied them.
Multi-threading and concurrent access: How you identified and fixed them.
Some of the questions in this section can help you prepare your answers by relating them to your current or past
work experience. For example:
Design Concepts: Refer Q5, Q6, Q7, Q8, Q9 etc
Design Patterns: Refer Q10, Q14, Q20, Q31, Q45, Q46, Q50 etc [Refer Q11 in How would you go about…?
section]
Performance issues: Refer Q21, Q63 etc
Memory issues: Refer Q32, Q64, Q65 etc
Exception Handling: Refer Q34, Q35 etc
Multi-threading (Concurrency issues): Refer Q29, Q40 etc
Demonstrating your knowledge in the above mentioned areas will improve your chances of being successful in
your Java/J2EE interviews. 90% of the interview questions are asked based on your own resume. So in my view it
Java
38
is also very beneficial to mention how you demonstrated your knowledge/skills by stepping through a recent
project on your resume.
The two other areas, which I have not mentioned in this section, which are also very vital, are transactions and
security. These two areas will be covered in the next section, which is the Enterprise section (J2EE, JDBC, EJB,
JMS, SQL, XML etc).
Even if you have not applied these skills knowingly or you have not applied them at all, just demonstrating that you
have the knowledge and an appreciation will help you improve your chances in the interviews. Also mention any
long hours worked to meet the deadline, working under pressure, fixing important issues like performance issues,
running out of memory issues etc.
Q 68: Why are you leaving your current position?
A 68: [Hint]
Do not criticize your previous employer or coworkers or sound too opportunistic.
It is fine to mention a major problem like a buy out, budget constraints, merger or liquidation.
You may also say that your chance to make a contribution is very low due to company wide changes or
looking for a more challenging senior or designer role.
Q 69: What do you like and/or dislike most about your current and/or last position?
A 69: [Hint]
The interviewer is trying to find the compatibility with the open position. So
Do not say anything like:
You dislike overtime.
You dislike management or coworkers etc.
It is safe to say:
You like challenges.
Opportunity to grow into design, architecture, performance tuning etc.
You dislike frustrating situations like identifying a memory leak problem or a complex transactional or a
concurrency issue. You want to get on top of it as soon as possible.
Q 70: How do you handle pressure? Do you like or dislike these situations?
A 70: [Hint]
These questions could mean that the open position is pressure-packed and may be out of control. Know what you
are getting into. If you do perform well under stress then give a descriptive example. High achievers tend to
perform well in pressure situations.
Q 71: What are your strengths and weaknesses? Can you describe a situation where you took initiative? Can you
describe a situation where you applied your problem solving skills?
A 71: [Hint]
Strengths:
Taking initiatives and being pro-active: You can illustrate how you took initiative to fix a transactional issue,
a performance problem or a memory leak problem.
Design skills: You can illustrate how you designed a particular application using OO concepts.
Problem solving skills: Explain how you will break a complex problem into more manageable sub-sections
and then apply brain storming and analytical skills to solve the complex problem. Illustrate how you went
about identifying a scalability issue or a memory leak problem.
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39
Communication skills: Illustrate that you can communicate effectively with all the team members, business
analysts, users, testers, stake holders etc.
Ability to work in a team environment as well as independently: Illustrate that you are technically sound
to work independently as well as have the interpersonal skills to fit into any team environment.
Hard working, honest, and conscientious etc are the adjectives to describe you.
Weaknesses:
Select a trait and come up with a solution to overcome your weakness. Stay away from personal qualities and
concentrate more on professional traits for example:
I pride myself on being an attention to detail guy but sometimes miss small details. So I am working on
applying the 80/20 principle to manage time and details. Spend 80% of my effort and time on 20% of the
tasks, which are critical and important to the task at hand.
Some times when there is a technical issue or a problem I tend to work continuously until I fix it without having
a break. But what I have noticed and am trying to practise is that taking a break away from the problem and
thinking outside the square will assist you in identifying the root cause of the problem sooner.
Q 72: What are your career goals? Where do you see yourself in 5-10 years?
A 72: [Hint] Be realistic. For example
Next 2-3 years to become a senior developer or a team lead.
Next 3-5 years to become a solution designer or an architect.
Note: For Q66 – Q72 tailor your answers to the job. Also be prepared for questions like:
What was the last Java related book or article you read? [Hint]
Mastering EJB by Ed Roman.
EJB design patterns by Floyd Marinescu.
Bitter Java by Bruce Tate.
Thinking in Java by Bruce Eckel.
Which Java related website(s) do you use to keep your knowledge up to date? [Hint]
http://www.theserverside.com
http://www.javaworld.com
http://www-136.ibm.com/developerworks/Java
http://www.precisejava.com
http://www.allapplabs.com
http://java.sun.com
http://www.martinfowler.com
http://www.ambysoft.com
What past accomplishments gave you satisfaction? What makes you want to work hard? [Hint]
Material rewards such as salary, perks, benefits etc naturally come into play but focus on your achievements or
accomplishments than on rewards.
Do you have any role models in software development? [Hint]
Scott W. Ambler, Martin Fowler, Ed Roman, Floyd Marinescu, Grady Booch etc.
Why do you want to work for us? (Research the company prior to the interview).
Java
40
Java – Key Points
Java is an object oriented (OO) language, which has built in support for multi-threading, socket communication,
automatic memory management (ie. garbage collection) and also has better portability than other languages across
operating systems.
Java class loaders are hierarchical and use a delegation model. The classes loaded by a child class loader have
visibility into classes loaded by its parents up the hierarchy but the reverse is not true.
Java does not support multiple implementation inheritance but supports multiple interface inheritance.
Polymorphism, inheritance and encapsulation are the 3 pillar of an object-oriented language.
Code reuse can be achieved through either inheritance (“is a” relationship) or object composition (“has a”
relationship). Favour object composition over inheritance.
When using implementation inheritance, make sure that the subclasses depend only on the behaviour of the
superclass, not the actual implementation. An abstract base class usually provides an implementation inheritance.
Favour interface inheritance to implementation inheritance because it promotes the deign concept of coding to
interface and reduces coupling. The interface inheritance can achieve code reuse through object composition.
Design by contract specifies the obligations of a calling-method and called-method to each other using preconditions,
post-conditions and class invariants.
When using Java collection API, prefer using ArrayList or HashMap as opposed to Vector or Hashtable to avoid any
synchronization overhead. The ArrayList or HashMap can be externally synchronized for concurrent access by
multiple threads.
Set the initial capacity of a collection appropriately and program in terms of interfaces as opposed to
implementations.
When providing a user defined key class for storing objects in HashMap, you should override equals(), and
hashCode() methods from the Object class.
String class is immutable and StringBuffer and StringBuilder classes are mutable. So it is more efficient to use a
StringBuffer or a StringBuilder as opposed to a String in a computation intensive situations (ie. in for, while loops).
Serialization is a process of writing an object to a file or a stream. Transient variables cannot be serialized.
Java I/O performance can be improved by using buffering, minimising access to the underlying hard disk and
operating systems. Use the NIO package for performance enhancing features like non-blocking I/O operation, buffers
to hold data, and memory mapping of files.
Each time an object is created in Java it goes into the area of memory known as heap. The primitive variables are
allocated in the stack if they are local method variables and in the heap if they are class member variables.
Threads share the heap spaces so it is not thread-safe and the threads have their own stack space, which is
thread-safe.
The garbage collection cannot be forced, but you can nicely ask the garbage collector to collect garbage.
There two types of exceptions checked (ie compiler checked) and unchecked (Runtime Exceptions). It is not
advisable to catch type Exception.
A process is an execution of a program (e.g. JVM process) but a thread is a single execution sequence within the
process.
Threads can be created in Java by either extending the Thread class or implementing the Runnable interface.
Java
41
In Java each object has a lock and a thread can acquire a lock by using the synchronized key word. The
synchronization key word can be applied in method level (coarse-grained lock) or block level (fine-grained lock
which offers better performance) of code.
Threads can communicate with each other using wait(), notify(), and notifyAll() methods. This communication
solves the consumer-producer problem.
Sockets are communication channels, which facilitate inter-process communication.
Swing uses the MVC paradigm to provide loose coupling and action architecture to implement a shared behaviour
between two or more user interface components.
Swing components should be accessed through an event-dispatching thread. There is a way to access the Swing
event-dispatching thread from outside event-handling or drawing code, is using SwingUtilities’ invokeLater() and
invokeAndWait() methods.
A signed applet can become a trusted applet, which can work outside the sandbox.
In Java typically memory leak occurs when an object of longer life cycle has a reference to objects of a short life
cycle.
You can improve performance in Java by :
1. Pooling your valuable resources like threads, database and socket connections.
2. Optimizing your I/O operations.
3. Minimising network overheads, calls to Date, Calendar related classes, use of “casting” or runtime type
checking like “instanceof” in frequently executed methods/loops, JNI calls, etc
4. Managing your objects efficiently by caching or recycling them without having to rely on garbage collection.
5. Using a StringBuffer as opposed to String and ArrayList or HashMap as oppose to Vector or Hashtable
6. Applying multi-threading where applicable.
7. Minimise any potential memory leaks.
Finally, very briefly familiarise yourself with some of the key design patterns like:
1. Decorator design pattern: used by Java I/O API. A popular design pattern.
2. Reactor design pattern/Observer design pattern: used by Java NIO API.
3. Visitor design pattern: to avoid instanceof and typecast constructs.
4. Factory method/abstract factory design pattern: popular pattern, which gets frequently asked in interviews.
5. Singleton pattern: popular pattern, which gets frequently asked in interviews.
6. Composite design pattern: used by GUI components and also a popular design pattern
7. MVC design pattern/architecture: used by Swing components and also a popular pattern.
8. Command pattern: used by Swing action architecture and also a popular design pattern.
9. Strategy design pattern: A popular design pattern used by AWT layout managers.
Refer Q11 in “How would you go about…” section for a detailed discussion and code samples on GOF (Gang of Four)
design patterns.
Recommended reading on design patterns:
The famous Gang of Four book: Design Patterns, Eric Gamma, Richard Helm, Ralph Johnson, and John Vlissides
(Addiso-Wesley Publishing Co., 1995; ISBN: 0201633612).
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42
Tips:
Try to find out the needs of the project in which you will be working and the needs of the people within the
project.
80% of the interview questions are based on your own resume.
Where possible briefly demonstrate how you applied your skills/knowledge in the key areas [design
concepts, transactional issues, performance issues, memeory leaks etc] as described in this book. Find the
right time to raise questions and answer those questions to show your strength.
Be honest to answer technical questions, you are not expected to remember everything (for example you
might know a few design patterns but not all of them etc). If you have not used a design pattern in question,
request the interviewer, if you could describe a different design pattern.
Do not be critical, focus on what you can do. Also try to be humourous to show your smartness.
Do not act superior.
GLOSSARY OF TERMS
43
GLOSSARY OF TERMS
TERM DESCRIPTION
ACID Atomicity, Consistency, Isolation, Duration.
aka Also known as.
AOP Aspect Oriented Progamming
API Application Program Interface
AWT Abstract Window Toolkit
BLOB Binary Large OBject
BMP Bean Managed Persistence
CGI Common Gateway Interface
CLOB Character Large OBject
CMP Container Managed Persistence
CORBA Common Object Request Broker Architecture
CRM Customer Relationships Management
CSS Cascading Style Sheets
DAO Data Access Object
DNS Domain Name Service
DOM Document Object Model
DTD Document Type Definition
EAR Enterprise ARchive
EIS Enterprise Information System
EJB Enterprise JavaBean
ERP Enterprise Resource Planning
FDD Feature Driven Development
GIF Graphic Interchange Format
GOF Gang Of Four
HQL Hibernate Query Language.
HTML Hyper Text Markup Language
HTTP Hyper Text Transfer Protocol
I/O Input/Output
IDE Integrated Development Environment
IIOP Internet Inter-ORB Protocol
IoC Inversion of Control
IP Internet Protocol
J2EE Java 2 Enterprise Edition
JAAS Java Authentication and Authorization Service
JAF JavaBeans Activation Framework
JAR Java ARchive
JAXB Java API for XML Binding
JAXP Java API for XML Parsing
JAXR Java API for XML Registries
JAX-RPC Java API for XML-based RPC
JAX-WS Java API for XML-based Web Services
JCA J2EE Connector Architecture
JDBC Java DataBase Connectivity
JDK Java Development Kit
JMS Java Messaging Service
JMX Java Management eXtensions
JNDI Java Naming and Directory Interface
JNI Java Native Interface
JRMP Java Remote Method Protocol
JSF JavaServer Faces
JSP Java Server Pages
JSTL Java Standard Tag Library
JTA Java Transaction API
JVM Java Virtual Machine
LDAP Lightweight Directory Access Protocol
MOM Message Oriented Middleware
MVC Model View Controller
NDS Novell Directory Service
NIO New I/O
O/R mapping Object to Relational mapping.
GLOSSARY OF TERMS
44
OO Object Oriented
OOP Object Oriented Programming
OOPL Object Oriented Programming Language
ORB Object Request Broker
ORM Object to Relational Mapping.
POJI Plain Old Java Interface
POJO Plain Old Java Object
RAR Resource adapter ARchive
RDBMS Relational Database Management System
RMI Remote Method Invocation
RPC Remote Procedure Call
RUP Rational Unified Process
SAAJ SOAP with attachment API for Java
SAX Simple API for XML
SOAP Simple Object Access Protocol
SQL Structured Query Language
SSL Secure Sockets Layer
TCP Transmission Control Protocolwww.allwalkin.blogspot.com
TDD Test Driven Development
UDDI Universal Description Discovery and Integration
UDP User Datagram Protocol
UI User Interface
UML Unified Modelling Language
URI Uniform Resource Identifier
URL Uniform Resource Locator
UTF
VO Value Object which is a plain Java class which has attributes or fields and corresponding getter getXXX()
and setter setXXX() methods .
WAR Web ARchive
WSDL Web Service Description Language
XHTML Extensible Hypertext Markup Language
XML Extensible Markup Language
XP Extreme Programming
XPath XML Path
XSD XML Schema Definition
XSL Extensible Style Language
XSL-FO Extensible Style Language – Formatting Objects
XSLT Extensible Style Language Transformation
RESOURCES
45
RESOURCES
Articles
Sun Java Certified Enterprise Architect by Leo Crawford on http://www.leocrawford.org.uk/work/jcea/part1/index.html.
Practical UML: A Hands-On Introduction for Developers by Randy Miller on http://bdn.borland.com/article/0,1410,31863,00.html
W3 Schools on http://www.w3schools.com/default.asp.
LDAP basics on http://publib.boulder.ibm.com/iseries/v5r2/ic2924/index.htm?info/rzahy/rzahyovrco.htm.
Java World articles on design patterns: http://www.javaworld.com/columns/jw-Java-design-patterns-index.shtml.
Web Servers vs. App Servers: Choosing Between the Two By Nelson King on
http://www.serverwatch.com/tutorials/article.php/1355131.
Follow the Chain of Responsibility by By David Geary on Java World - http://www.javaworld.com/javaworld/jw-08-2003/jw-0829-
designpatterns.html.
J2EE Design Patterns by Sue Spielman on http://www.onjava.com/pub/a/onjava/2002/01/16/patterns.html.
The New Methodology by Martin Fowler on http://www.martinfowler.com/articles/newMethodology.html.
Merlin brings nonblocking I/O to the Java platform by Aruna Kalagnanam and Balu G on
http://www.ibm.com//developerworks/Java/library/j-javaio.
Hibernate Tips and Pitfalls by Phil Zoio on http://www.realsolve.co.uk/site/tech/hib-tip-pitfall-series.php.
Hibernate Reference Documentation on http://www.hibernate.org/hib_docs/reference/en/html_single/.
Object-relation mapping without the container by Richard Hightower on http://www-128.ibm.com/developerworks/library/jhibern/?
ca=dnt515.
Object to Relational Mapping and Relationships with Hibernate by Mark Eagle on http://www.meagle.com:8080/hibernate.jsp.
Mapping Objects to Relational databases: O/R Mapping In detail by Scott W. Ambler on
http://www.agiledata.org/essays/mappingObjects.html.
I want my AOP by Ramnivas Laddad on Java World.
Websphere Application Server 5.0 for iSeries – Performance Considerations by Jill Peterson.
Dependency Injection using pico container by Subbu Ramanathan .
Websphere Application Server & Database Performance tuning by Michael S. Pallos on
http://www.bizforum.org/whitepapers/candle-5.htm.
A beginners guide to Dependency Injection by Dhananjay Nene on
http://www.theserverside.com/articles/article.tss?l=IOCBeginners.
The Spring series: Introduction to the Spring framework by Naveen Balani on http://www-
128.ibm.com/developerworks/web/library/wa-spring1.
The Spring Framework by Benoy Jose.
Inversion of Control Containersband the Dependency Injection pattern by Martin Fowler.
Migrate J2EE Applications for EJB 3.0 by Debu Panda on JavaPro.
EJB 3.0 in a nutshell by Anil Sharma on JavaWorld.
Preparing for EJB 3.0 by Mike Keith on ORACLE Technology Network.
Simplify enterprise Java development with EJB 3.0 by Michael Juntao Yuan on JavaWorld.
RESOURCES
46
J2SE: New I/O by John Zukowski on http://java.sun.com/developer/technicalArticles/releases/nio/.
High-Performance I/O arrives by Danniel F. Savarese on JavaPro.
Hibernate – Proxy Visitor Pattern by Kurtis Williams.
Best Practices for Exception Handling by Gunjan Doshi.
Three Rules for Effective Exception Handling by Jim Cushing.
LDAP and JNDI: Together forever – by Sameer Tyagi.
Introduction To LDAP – by Brad Marshall.
Java theory and practice: Decorating with dynamic proxies by Brian Goetz.
Java Dynamic Proxies: One Step from Aspect-Oriented Programming by Lara D’Abreo.
Java Design Patterns on http://www.allapplabs.com/java_design_patterns .
Software Design Patterns on http://www.dofactory.com/Patterns/Patterns.aspx .
JRun: Core Dump and Dr. Watson Errors on http://www.macromedia.com/cfusion/knowledgebase/index.cfm?id=tn_17534
www.javaworld.com articles.
http://www-128.ibm.com/developerworks/java articles.
http://www.devx.com/java articles.
www.theserverside.com/tss articles.
http://javaboutique.internet.com/articles articles.
Books
Beginning Java 2 by Ivor Horton.
Design Patterns by Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides (GoF) .
UML Distilled by Martin Fowler, Kendall Scott .
Mastering Enterprise Java Beans II by Ed Roman, Scott Ambler, Tyler Jewell, Floyd Marinescu [download for free] .
EJB Design Patterns by Floyd Marinescu [download for free] .
Sun Certified Enterprise Architect for J2EE Technology Study Guide by Mark Cade and Simon Roberts.
Professional Java Server Programming - J2EE edition by Wrox publication.
Design Patterns Java Companion by James W. Cooper (Free download: http://www.patterndepot.com/put/8/JavaPatterns.htm).
Test Driven Development – By Example, by Kent Beck.
INDEX
47
INDEX
Java
Discuss the Java error handling mechanism? What is the
difference between Runtime (unchecked) exceptions
and checked exceptions? What is the implication of
catching all the exceptions with the type “Exception”? 34
Explain Java class loaders? Explain dynamic class loading?
12
Explain Outer and Inner classes (or Nested classes) in
Java? When will you use an Inner Class? 30
Explain static vs dynamic class loading? 12
Explain the assertion construct? 18
Explain the Java Collection framework? 20
Explain the Java I/O streaming concept and the use of the
decorator design pattern in Java I/O? 25
Give a few reasons for using Java? 11
Give an example where you might use a static method? 28
How can you improve Java I/O performance? 27
How do you express an ‘is a’ relationship and a ‘has a’
relationship or explain inheritance and composition?
What is the difference between composition and
aggregation? 14
How does Java allocate stack and heap memory? Explain
re-entrant, recursive and idempotent
methods/functions? 30
How does the Object Oriented approach improve software
development? 13
If you have a circular reference of objects, but you no
longer reference it from an execution thread, will this
object be a potential candidate for garbage collection?
33
What are “static initializers” or “static blocks with no function
names”? 13
What are access modifiers? 29
What are some of the best practices relating to Java
collection? 21
What are the advantages of Object Oriented Programming
Languages (OOPL)? 13
What are the benefits of the Java collection framework? 21
What do you know about the Java garbage collector? When
does the garbage collection occur? Explain different
types of references in Java? 33
What do you mean by polymorphism, inheritance,
encapsulation, and dynamic binding? 14
What is a final modifier? Explain other Java modifiers? 29
What is a user defined exception? 36
What is design by contract? Explain the assertion
construct? 17
What is serialization? How would you exclude a field of a
class from serialization or what is a transient variable?
What is the common use? 25
What is the difference between aggregation and
composition? 14
What is the difference between an abstract class and an
interface and when should you use them? 19
What is the difference between an instance variable and a
static variable? Give an example where you might use a
static variable? 28
What is the difference between C++ and Java? 11
What is the difference between final, finally and finalize() in
Java? 30
What is the difference between processes and threads? 36
What is the main difference between a String and a
StringBuffer class? 24
What is the main difference between an ArrayList and a
Vector? What is the main difference between Hashmap
and Hashtable? 20
What is the main difference between pass-by-reference and
pass-by-value? 24
What is the main difference between shallow cloning and
deep cloning of objects? 28
What is the main difference between the Java platform and
the other software platforms? 11
What is type casting? Explain up casting vs down casting?
When do you get ClassCastException? 32
When is a method said to be overloaded and when is a
method said to be overridden? 20
When providing a user defined key class for storing objects
in the Hashmaps or Hashtables, what methods do you
have to provide or override (ie method overriding)? 23
When to use an abstract class? 19
When to use an interface? 20
Where and how can you use a private constructor? 29
Why is it not advisable to catch type “Exception”? 35
Why should you catch a checked exception late in a catch
{} block? 35
Why should you throw an exception early? 35
Why there are some interfaces with no defined methods
(i.e. marker interfaces) in Java? 20
Java/J2EE - Personal
Did you have to use any design patterns in your Java
project? 37
Do you have any role models in software development? 39
How do you handle pressure? Do you like or dislike these
situations? 38
Tell me about yourself or about some of the recent projects
you have worked with? What do you consider your most
significant achievement? Why do you think you are
qualified for this position? Why should we hire you and
what kind of contributions will you make? 37
What are your career goals? Where do you see yourself in
5-10 years? 39
What are your strengths and weaknesses? Can you
describe a situation where you took initiative? Can you
describe a situation where you applied your problem
solving skills? 38
What do you like and/or dislike most about your current
and/or last position? 38
What past accomplishments gave you satisfaction? What
makes you want to work hard? 39
What was the last Java related book or article you read? 39
Why are you leaving your current position? 38
Why do you want to work for us? 39
Key Points
Java - Key Points 40www.allwalkin.blogspot.com
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