Java Interview Questions

Some of the Java Interview Questions for the fresher and Experienced ( Set - 3 )

1. What is package in Java? What is its advantage ?

Ans: This is the basic question might be asked in the interview. You just need to give the exact and proper definition in simple words.
" Package is the storage directory of java files. It is a way by which java classes get organized and placed in well structured directory. "

Advantages of packages:

1. It is going to organize the group of java classes in a well directory structure.
2. It reduces the naming conflict between the java classes.
3. It gives the hierarchy directory structure to store java classes.
4. It also increases the maintenance issue of java classes stored in different packages.
5. It increases the readability of java classes.

2. What is the difference between equals() method " == " operator ?

Ans: This is one of the commonly asked question in an interview.This can be answered with the following points:-

1. equals() is a method where as '==' is an operator.
2. equals() method can be used to compare the string literals where as '==' operator is used to compare the primitive types.
3. equals() is used to compare the content of the String object where as '==' is used to compare the memory location of the objects, whether that two object belong to same memory location or not.

3. What is the Garbage Collector in Java?

Ans: Garbage Collector in java is used to de-allocate the memory of the unreferenced object remain in the heap memory.

4. What do you understand by immutability in Java?

Ans: This is one of the famous and mostly asked question in an interview.  This can be explained in simple terms as " the object which cannot be modified once created is considered as immutable object in java , even if we try to modify the existing object then it will create another object rather modifying to the existing one ".

In java String is the immutable class.

5. What are Wrapper classes in Java?

Ans: In java for every primitive type there is a class which is called as the Wrapper classes. By the use of wrapper class we can represent the primitive types as an object.

6. What is Exception in Java ? How to handle the exception in Java?

Ans: Exception raised in the program will stops the normal flow of program unconditionally but it won't terminate the program that means exceptions can be handled.

There are two ways by which exception raised in the program :
1. By using the try-catch block.
2. Using the 'throws' keyword (let caller or JVM will handle )

7. What is error in Java? Tell some errors you have experienced while writing programs?

Ans: Error totally terminates the flow of execution of the program and hence control will never back to the program from where it get terminated. Error can't be handled and can't be recovered.

Interviewer may ask you about the error you have experienced while writing the codes if you going to an interview as a experienced guy. So you just need to mention some of the common errors we you have got like :
a) NoSuchDefinitionError
b) OutOfMemoryError....etc

8. What is the difference between Error and Exception in Java?

Ans: This is the common freshers interview question.
Error vs Exception:
1. Error terminates the normal flow of execution of program whereas it won't terminate the program.
2. Error can't be handled whereas Exception can be handled.

9. What is inheritance in Java? What are its types?

Ans: Inheritance is one of the important Oops features. Inheritance is the mechanism by which a new class is derived from existing class. By inheritance a relation is being developed between classes hierarchically.

According to Oops there are 5 types of inheritance :-

a) Simple Inheritance
b) Multiple Inheritance
c) Multi-level Inheritance
d) Hierarchical Inheritance
e) Hybrid Inheritance.

10. Is other overloaded main() method will be invoked by the JVM or not ?

Ans: No , other overloaded main() will not be invoked by JVM as it will only look for the main() method signature i.e public static void main(String[] arg) .
We are responsible to call all other overloaded method.

11. What do you understand by 'import' statement in Java?

Ans: 'import' is one of the keyword in java, which is used in java to import the packages. Import statement must be placed after package statement in java programs. Import statement will look alike:-

import java.lang.*; // one way ( will import all the classes within the java.lang package)
import java.lang.Object ; // another way ( importing specific class )

Note: second approach is the good practice always.

12. What is thread in Java?

Ans: This is one of the commonly and basic question to be asked in the interview. Thread is the smallest unit of process. It is the independent path of execution within a program. More than one thread can execute concurrently within a program which creates multi-threaded environment and hence enhance the performance.

13. Is constructor can be inherited in Java?

Ans: No , constructor cannot be inherited .

14. Is it possible to make main() as final?

Ans: Yes, it is very much possible to make main() as final.

15. Can you make methods inside an interface as static and why ?

Ans: No, method inside the interface can't be made static as because static method cannot be override in the sub-class.

16. What is finally block?

Ans: finally is a block in java to be executed at least once and it must be written either with try or with try-catch block. Finally block will have all the codes for releasing all the resources allocated while executing the java program.

17. What is singleton class in Java?

Ans: Singleton in java means the object for which will be created at the time of class loading and the same will be returned always, so singleton class will have only one object created and the same will be used every-time.

18. How many objects are created in below written codes?
String s="java";
String s1="java";

Ans: Only one object will be created.

19. Difference between StringBuilder and StringBuffer?
Ans: This is one of the most asked question in both fresher's and experienced interview.

StringBuilder :

a) All the methods in the StringBuilder are non-synchronized.
b) It is not a thread safe.

StringBuffer:

a) All the methods in the StringBuffer are synchronized.
b) It is thread safe.

20. What is the purpose of using toString() in Java?

Ans: toString() method in java will return the string representation of an object that is,

getClass().getName() + "@" + Integer.toHexString(hashCode());  

Like this toString() is overridden in different class and they have their own representation.

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Struts 1 index

1. Introduction to Struts 1

2. What happen at container Start-up

3. ActionServlet in Struts 1 ( Request Process )

4. About execute() method in Struts 1

5. Configuring ActionServlet in Struts 1

6. Configuring Action class in web.xml

7. Struts Request Processing Flow

8. Struts 1 Example using Eclipse

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Java Interview questions index

1. Java Interview Questions ( Set - 1 )

2. Java Interview Questions ( Set - 2 )

3. Java Interview Questions ( Set - 3 )

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Java Interview Questions

Some of the java Interview Questions for the fresher and Experienced ( Set - 2 )

1. What will happen if we try to modify the value of a final variable ?

Ans: Final variable can't be modified , if we try to do so then a compile time exception will be thrown i.e java.lang.IllegalAccessException.

2. What is Oops Concept ?

Ans: Oops is considered as Object Oriented Feature. As java is Object- Oriented language so it supports the following Oops features:-

a) Encapsulation
b) Inheritance
c) Polymorphism
d) Message Passing
e) Data Abstraction
f) Data Hiding

3. What is static variable ?where it is useful?

Ans: Static variable is a class variable which is at the class scope and not at the object scope. It is declared using the 'static' keyword with the variable name..........Click here to know more 


Memory allocation for the static are done only once at the time of class loading , so it is useful only if there is a common information which will be shared among all the object created for that class. 

4. What is instance variable ?

Ans: Instance variable are at the object level which means which is at the Object level scope. Each object can have a their own copy of instance variable. It is initialized by the constructor calling i.e while creating the object. 

5. What is polymorphism? How many types of Polymorphism is there in Java?

Ans: Polymorphism means 'many forms'. In java Polymorphism defines as the 'more than one methods having same name but perform different functions' . 

Basically there are two types of polymorphism in java :- 

a) Dynamic Polymorphism
b) Static Polymorphism

6. What is the difference between static method and instance method?

Ans: Static method :
a) It can be called with the class name.
b) It has only stack. 

Instance method :
a) It will be called with the Object only.
b) Separate stack will be created for each object calling instance method.  

7. Is it possible to access static content from non-static area? why ?

Ans: No, its not possible to access static content from non-static content. It is because non-static content can be accessible only if we are creating object and there are not guarantee for always creating the object. 

8. Difference between class level variable and local variable?

Ans: 
Class level variable :-

a) It is at the class scope or Object scope.
b) It can be static or instance. 
c) Memory will be allocated at the time of class loading if class level variable is static.
d) Memory will be allocated at the time of creating the object if the class level variable is instance.
e) It consume the space in heap memory. 

Local Variable:-

a) It is at the method scope,
b) It can't be static. 
c) Memory will be allocated only if method will be called.
d) It consumes the space in stack. 

9. Difference between dynamic and static Polymorphism ?

Ans: 
Dynamic Polymorphism:

a) It is otherwise called as overriding. 
b) Method to be called will be decided at the time of execution. 
c) It will be achieved by dynamic dispatch. 
d) It will have one method at the super class and another at the sub-class.

Static Polymorphism:

a) It is otherwise called as overloading.
b) Method to be called will be decided at the compile time only.
c) It will be achieved by calling respective overloaded method by passing requirement information.
d) It will have all the methods at the same class.

10. Why main() method is declared as static?

Ans: main() method is declared as static resulting there will not be any need to creating object for the JVM to call main() method. 

11. What is Dynamic Dispatch?

Ans: Dynamic dispatch means assigning sub-class object to the super-class reference variable. By this we can approach for dynamic polymorphism.

For ex: 
A--> super class
B----> Sub-class

A a = new B() ;---> Dynamic dispatch 

12. What is an Abstract class?
Ans:  A class declared with the abstract keyword is called as Abstract class. 
It has following features:-

a) An abstract class cannot be instantiated but can be subclassed. 
b) It can have both instance and static variables.
c) It can also have both abstract and non-abstract methods.
...............Click here to know more .

13. What is a marker interface and what is its usage?
Ans: Marker interface is an interface without any method declaration. Its is a empty interface like Serializable, Cloneable etc interfaces. 

It will be used just to indicate JVM that to perform for which that marker is designed.
for ex Cloneable marker interface has been designed to indicate JVM that there is a requirement of creating the clone of the object of the class implementing Cloneable interface.

14. What is the difference between Abstract and Interface?
Ans: Click here to know all the difference 

15. What is an anonymous class and its use?
Ans: An anonymous class is a class without any name and hence we can create only one object of an anonymous class.......Click here to know more about Anonymous class .

Use of Anonymous class:

a) It provide security to the application over network as we cannot create object of the anonymous explicitly. 
b) It can be used if we have to create only a single object to operate. 

16. Can we write constructor inside anonymous class and why ?
Ans: No , we cannot create/write constructor for the anonymous class as it has no name and constructor is same as the class name. 

17. What is the default package imported in Java class automatically ?
Ans: Default package which is going to be imported for every single java class is java.lang.* pakage. 

18. Name some of the methods from Object class?
Ans: Some of the common methods from Object class are:-
a) toString()
b) clone()
c) wait()
d) notify()
e) notifyAll()
e) hashCode()
f) equals()
g) finalize()
h) getClass()

19. What is hashcode?
Ans: HashCode is the integer representation of the object. Actual object get modified resulting some integer value is called as hashcode. Two different object can have same hashcode. 

20. What is cloning?
Ans: Cloning is process of creating proxy or dummy object of the actual object, which covers all the features of the actual/original object. Default cloning is done using clone() method. 

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Java Interview Questions

Some of the java Interview Questions for the fresher and Experienced ( Set - 1 )

1. What is Java , speak something about Java ?

Ans: Basically few points has to be remembered regarding java :

a) Java is high level language.
b) Java is Compiled and Interpreted language.
c) Java is a Object-Oriented Language but not a pure Object-Oriented.
d) Java is Platform-Independent Language which makes language as Portable.
........ Click here to see more

If interviewer ask you to speak something about java then without any hesitation ,confidently go on explaining from very basic of java till OOPS concept. Still interview won't let you stop speaking then start explaining threading feature of java. But mostly they will let you cover all the basic things of java and then they start asking their next level of question.
You should speak about following points->
i> What is Java?
ii> OOPS features of Java.
iii> Explain in brief about each Oops features.

2. Is Java Platform Independent ? If yes then justify your answer ?

Ans: Yes , java is platform independent. As we know that java is compiled + interpreted language and .class file is the result of compilation and this file is going to be interpreted by JVM so we need JVM to execute this .class file and thus JVM has to be present and designed for each operating System like Windows , Unix etc.So due to the portability to .class file from a machine to another machine makes java platform independent.


3. What are the command for compiling and executing Java program ?

Ans: javac is a command for compiling java file will result in .class file
        java is a command for executing .class file.

4. What is Classpath in Java ?

Ans: Classpath in java means setting path for the .class files. Classpath is a command in java used for setting path for the .class file. Let suppose we have a .class file in a directory and we are currently at another location, now that .class file which is at different location so can be from another location but we need to set the path for the .class file using classpath command.
like this way, set classpath='......directorylocaltion....';

5. Who is the super class of all the Java classes ?

Ans: Object class is super class all the classes.

6. How many keywords does Java have , name few which you have used in your program?

Ans: Java has 50 keywords. Some of the commonly used keywords are :-

public    private     protected    static    final    new    synchronized    class   int   float   long  
byte       double   default     abstract      interface   for   if     import   package   instanceof  
native     return   super    this   strictfp    switch   throw    throws   try   catch   void   while  
transient   volatile   void    implements    goto      finally   enum   do   continue   const  
break    assert   else   short.
null   true   false  ( These 3 words are Reserved word ).

7. What is "this" in Java and what is its use ?

Ans: Few points about this has to be remembered :

a) At very first this is a keyword in java.
b) this points to the current object.
c) Although 'this' is not a variable so it cannot hold any other reference.
...........Click here to know more

Some of the used of "this" keyword -
a) 'this' is the simplest way to use instead of current object reference.
b) As it always points to the current object , so we can perform any task using 'this' for the current object.

8. What is "super" in Java and for what purpose super is being used in Java programs ?

Ans:  Few points about this has to be remembered :

a) At first 'super' is a keyword in java.
b) 'super' refers to the inherited member from super class to its sub-class.
...........Click here to know more

9. How "this" is different from "super" ?

Ans: Point of differences :

a) 'this' refers to the current object where as 'super' refers to the inherited member of the super class.
b) this() is used for constructor chaining where as super() is used for calling super class default constructor.

10. Is Java is pure-object oriented language and why ?

Ans: No java is not a pure-Objected Oriented Language.
Basically there are two strong reason by which we can conclude and say that java is not a pure- Object Oriented Language, they are-

a) Use of static will not involve any Object creation.
b) Use of primitive type. Although respective Wrapper class is there but still primitive is being used for many purposes.

11. Is Java supporting multiple inheritance and why ?

Ans: No java is not supporting Multiple inheritance directly but approached towards performing multiple inheritance using interfaces. There would lead to some issues if java is supporting multiple inheritance directly :
a) Memory wastage problem
b) Ambiguity Problem in calling methods.

12. Is pointer supported by Java and why?

Ans: Yes Pointer are supported by java but is fully abstract from the developer. It is internally implemented by the java vendor. It is not given at the developer level as because to make java secured. As we know that pointer directly goes to the memory address so there may have chance of security threat if pointer is allowed to implement in java.

13. What is the use of  "native" keyword in Java ?

Ans: 'native' is a keyword in java and are used with the method, which says, that method is implemented using some low level language. So if we have to implement something in low level language then we can use the 'native' keyword.

14. What is Constructor ?

Ans: Constructor is like a method but with no return type. Constructor has same name as the class name. It is used to initialize the  instance variable.
..........Click here to know more

15. What is the purpose of making main() as static ? 

Ans: Purpose of main() as static:
a) So that JVM need not to create any object to call main().
.............Clicl here to know more

16. Does Constructor return any value ? 

Ans: As constructor doesn't have any return type so it cannot return any value, bt we can write 'return' in the constructor as writing 'return' simply means returning the control to the caller.

17. What if we write simply "return" statement in constructor?

Ans: Nothing will happen it will simply return the control to the caller.

18. How may types of initialization blocks in java ?

Ans: There are two types of initialization block:
1. Static initialization block
2. Instance initialization block.

19. What is the use of introducing static initializing block in java ?

Ans: Static initialization block can be used for following purposes:

a) It can be used to initialize the final static variable.
b) It can be used to execute some other method before the main() method.
...............Click here to know more

20. What is the use and purpose of instance initialization block in java ?

Ans: Static initialization block can be used for following purposes:

a) It can be used to initialize the final instance variable.

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