JAVA Interview Questions

By Bhavik Thummar

From GEC, Modasa

Email : bhavikthummar9797@yahoo.com

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JAVA Interview Questions

By Bhavik Thummar

From GEC, Modasa 5th Sem I.T

bhavikthummar9797@yahoo.com

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Computer Engineering

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JAVA Interview Questions

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1. What is Java?

A high-level programming language developed by Sun Microsystems.

Java was originally called OAK, and was designed for handheld devices

and set-top boxes.

Oak was unsuccessful so in 1995 Sun changed the name to Java and

modified the language to take advantage of the burgeoning World

Wide Web.

2. What are the main features of java?

The main features of Java are

Compiled and Interpreted

Object oriented

Robust and secure

Type safe

High Performance.

3. What are pass by reference and passby value?

Pass By Reference means the passing the address itself rather than

passing the value.

Passby Value means passing a copy of the value to be passed.

4. What is the Java API?

The Java API is a large collection of ready-made software components

that provide many useful capabilities, such as graphical user interface

(GUI) widgets.

5. What is the Java Virtual Machine (JVM)?

The Java Virtual Machine is software that can be ported onto various

hardware-based platforms.

6. What is variables and then types?

Variables is an identifier that denotes a storage location used to store

a data values.unlike constants that remain unchanged during the

execution of a program, a variable may takes different values at

different times during the execution of the program.

Instance variables

Class variables

Local variable

Parameters

7. What is dot operator?

The dot operator(.) is used to access the instance variables and

methods of class objects.It is also used to access classes and sub-

packages from a package.

Examples :

Person1.age ---------> Reference to the variable age

8. Define strings?

Strings represent a sequence of characters.

The easiest way to represent a sequence of characters in java is by

using a character array.

9. What is serialization?

Serialization is the process of converting a objects into a stream of

bytes.

10. What are different types of access modifiers?

Access specifiers are keywords that determine the type of access to

the member a class.

Public

Protected

Private

Default

11. What is an abstract class?

Abstract class is a class which contain one or more abstract methods,

which has to be implemented by sub classes.

An abstract class can contain no abstract methods also i.e. abstract

class may contain concrete methods.

12. What are class variables

Class variables are global to a class and belong to the entire set of

objects that class creates. Only one memory location is created for

each variable.

13. What is the Collection interface?

The Collection interface provides support for the implementation of a

mathematical bag - an unordered collection of objects that may

contain duplicates.

14. What must a class do to implement an interface?

The class must provide all of the methods in the interface and identify

the interface in its implements clause.

15. What is the Collections API?

The Collections API is a set of classes and interfaces that support

operations on collections of objects.

16. What is an array?

Array is a group of related data items that share a common name.For

instance, we can define an array name salary to represent a set of

salaries of a group of employees.

Examples : salary[500]

17. What is a list iterator?

The List and Set collections provide iterators, which are objects that

allow going over all the elements of a collection in sequence.

The java.util.Iterator interface provides for one-way traversal and

java.util.ListIterator is an iterator for lists that allows the programmer

to traverse the list in either direction (i.e. forward and or backward)

and modify the list during iteration.

18. What is the main difference between a String and a StringBuffer

class?

a) 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.

b) 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

synchronized,which makes it slightly faster at the cost of not being

thread-safe.

19. When to use serialization?

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.

20. What is the main difference between shallow cloning and deep

cloning of objects?

Java supports shallow cloning of objects by default when a class

implements the java.lang.Cloneable interface.

Deep cloning through serialization is faster to develop and easier to

maintain but carries a performance overhead.

21. What are wrapper classes?

Primitive data types may be converted into object types by using the

wrapper classes contained in the java.lang package.

Exampes : int, float, long, char, double

22. What is the difference between an instance variable and a static

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 are initialized.

Instance variables are non-static and there is one occurrence of an

instance variable in each class instance.Also known as a member

variable or a field.

23. Where and how can you use a private constructor?

Private constructor is used if you do not want other classes to

instantiate the object and to prevent subclassing.The instantiation is

done by a public static method (i.e. a static factory method) within

the same class.

24. What is type casting?

Type casting means treating a variable of one type as though it is

another type.

Examples :

int m = 5;

byte n =i;

25. What is a user defined exception?

User defined exceptions may be implemented by defining a new

exception class by extending the Exception class.

26. What is an instanceof operator?

Instanceof is an object reference operator and returns true if the

object on the left-hand side is an instance of the glass given to the

right hand side.This operator allows to determine whether the object

belongs to a particular class or not.

27. What are runtime exceptions?

Runtime exceptions are those exceptions that are thrown at runtime

because of either wrong input data or because of wrong business logic

etc. These are not checked by the compiler at compile time.

28. What is the difference between an interface and an abstract

class?

An abstract class may contain code in method bodies, which is not

allowed in an interface. With abstract classes, you have to inherit your

class from it and Java does not allow multiple inheritance. On the

other hand, you can implement multiple interfaces in your class.

29. What is a package?

A package is a namespace that organizes a set of related classes and

interfaces.

The classes contained in the packages of other programs can be easily

reused.Packages, classes can be unique compared with classes in

other packages.

That is, two classes in two different packages can have the same

name.

30. Why do threads block on I/O?

Threads block on i/o (that is enters the waiting state) so that other

threads may execute while the i/o Operation is performed.

31. What is the List interface?

The List interface provides support for ordered collections of objects.

32. What is the Vector class?

The Vector class provides the capability to implement a growable

array of objects.

33. What is the base class of all classes?

java.lang.Object

34. What is the importance of static variable?

Static variables are class level variables where all objects of the class

refer to the same variable. If one object changes the value then the

change gets reflected in all the objects.

35. What is the difference between a while statement and a do

while statement?

A while statement checks at the beginning of a loop to see whether

the next loop iteration should occur.

A do while statement checks at the end of a loop to see whether the

next iteration of a loop should occur.

The do while-statement will always execute the body of a loop at

least once.

36. Describe life cycle of thread?

A thread is similiar to a program that has a single flow of control.A

thread is a execution in a program. The life cycle of threads are:

Newborn state

Runnable state

Running state

Blocked state(Wating State)

Dead state

37. What is an Applets?

Applets are small java programs that are primarily used in Internet

computing.

They can be transported over the internet from one computer to

another and run using the Applet Viewer or any web browser that

supports java.

38. What are wrapped classes?

Wrapped classes are classes that allow primitive types to be accessed

as objects.

Examples : Integer, Float, String, Character, Boolean, Long, Double

39. What is the difference between an if statement and a switch

statement?

The if statement is used to select among two alternatives.

It uses a boolean expression to decide which alternative should be

executed.

The switch statement is used to select among multiple alternatives.

It uses an int expression to determine which alternative should be

executed.

40. What modifiers are allowed for methods in an Interface?

Only public and abstract modifiers are allowed for methods in

interfaces.

41. What is the difference between method overriding and

overloading?

Overriding is a method with the same name and arguments as in a

parent, whereas overloading is the same method name but different

arguments.

42. What do you mean by polymorphism?

Polymorphism is the ability of an object to take on many forms.

The most common use of polymorphism in OOP occurs when a parent

class reference is used to refer to a child class object..

Simple means: IS-A

43. What is the difference between an abstract class and an

interface?

Abstract class Interface - Have executable methods and abstract

methods.Can only subclass one abstract class

Interface - Have no implementation code. All methods are abstract.A

class can implement any number of interfaces.

44. What is the purpose of finalization?

The purpose of finalization is to give an unreachable object the

opportunity to perform any cleanup processing before the object

is garbage collected.

45. What is the difference between a break statement and a

continue statement?

Break statement results in the termination of the statement to

which it applies (switch, for, do, or while).

A Continue statement is used to end the current loop iteration and

return control to the loop statement.

46. When is a method said to be overloaded and when is a method

said to be overridden?

Overloading deals with multiple methods in the same class with the

same name but different method signatures.

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.

47. How is final different from finally and finalize()?

Final - constant declaration.

The finally block always executes when the try block exits, except

System.exit(0) call.

finalize() is a method of Object class which will be executed by the

JVM just before garbage collecting object to give a final chance for

resource releasing activity.

48. What is Byte Code?

All Java programs are compiled into class files that contain bytecodes.

These byte codes can be run in any platform and hence java is said to

be platform independent.

49. What is the difference between error and an exception?

Exception means When a method encounters an abnormal condition

(an exception condition) that it can’t handle itself, it may throw an

exception. ssError means system doesn’t handle.

For example:Overflow,Out of memory.

50. What if the main method is declared as private?

When a method is declared as private, the program compiles properly

but it will give runtime error Main method not "public".

51. What type of parameter passing does Java support?

In Java the arguments are always passed by value.

52. What is singleton?

It is one of the design pattern.

This falls in the creational pattern of the design pattern.

There will be only one instance for that entire JVM.

You can achieve this by having the private constructor in the class.

53. What is Locale?

A Locale object represents a specific geographical, political, or cultural

region.

54. What is the difference between constructors and normal

methods?

Constructors must have the same name as the class and can not

return a value.

They are only called once while regular methods could be called many

times and it can return a value or can be void.

55. What is the difference between static and non-static variables?

A static variable is associated with the class as a whole rather than

with specific instances of a class.

Non-static variables take on unique values with each object instance.

56. What is the difference between java and c++?

Java is a true object - oriented language while c++ is basically c with

object-oriented extension.

C++ supports multiple inheritance but Java provides interfaces in case

of multiple inheritance.

Java does not support operator overloading.

Java does not have template classes as in c++.

Java does not use pointers.