Core Java Interview Questions

Core Java Interview Questions

1. What is a JVM?

JVM is Java Virtual Machine which is a run time environment for the compiled java class files.

2. Does Java support multiple inheritance?

Java doesn't support multiple inheritance.

3. What is the most important feature of Java?

Java is a platform independent language.

4. What is difference between Path and Classpath?

Path and Classpath are operating system level environment variales. Path is used define where the system can find the executables(.exe) files and classpath is used to specify the location .class files.

5. What are instance variables?

Instance variables are those which are defined at the class level. Instance variables need not be initialized before using them as they are automatically initialized to their default values.

6. What is a pointer and does Java support pointers?
Pointer is a reference handle to a memory location. Improper handling of pointers leads to memory leaks and reliability issues hence Java doesn't support the usage of pointers.

7. What is the return type of the main() method?

Main() method doesn't return anything hence declared void.

8. What are local variables?

Local variables are those which are declared within a block of code like methods. Local variables should be initialized before accessing them.

9. Is Java a pure object oriented language?

Java uses primitive data types and hence is not a pure object oriented language.

10. Is JVM platform independent?

JVM's are not platform independent. JVM's are platform specific run time implementation provided by the vendor.
Java Standard Edition
11. Can a main() method be overloaded?

Yes. You can have any number of main() methods with different method signature and implementation in the class.

12. What is the base class of all classes?
 

java.lang.Object

13. What do you mean by platform independence?
 

Platform independence means that we can write and compile the java code in one platform (eg Windows) and can execute the class in any other supported platform eg (Linux,Solaris,etc).

14. Are arrays primitive data types?
 

In Java, Arrays are objects.

15. What is the difference between a JDK and a JVM?
 

JDK is Java Development Kit which is for development purpose and it includes execution environment also. But JVM is purely a run time environment and hence you will not be able to compile your source files using a JVM.

16. Does the order of public and static declaration matter in main() method?
 

No. It doesn't matter but void should always come before main().

17. What is the impact of declaring a method as final?

A method declared as final can't be overridden. A sub-class can't have the same method signature with a different implementation.

18. Can a class be declared as protected?

A class can't be declared as protected. only methods can be declared as protected.

19. How to define a constant variable in Java?


The variable should be declared as static and final. So only one copy of the variable exists for all instances of the class and the value can't be changed also.
static final int PI = 2.14; is an example for constant.

20. Which package is imported by default?

java.lang package is imported by default even without a package declaration.

21. What is the argument of main() method?

main() method accepts an array of String object as argument.

22. Can a source file contain more than one class declaration?
 

Yes a single source file can contain any number of Class declarations but only one of the class can be declared as public.

23. What is the access scope of a protected method?

A protected method can be accessed by the classes within the same package or by the sub classes of the class in any package.

24. Can a class declared as private be accessed outside it's package?

Not possible.

25. Why is the main() method declared static?
main() method is called by the JVM even before the instantiation of the class hence it is declared as static.

26. What is the purpose of declaring a variable as final?
A final variable's value can't be changed. final variables should be initialized before using them.

27. What is a package?
Package is a collection of related classes and interfaces. package declaration should be first statement in a java class.

28. Can a main() method be declared final?
Yes. Any inheriting class will not be able to have it's own default main() method.

29. I don't want my class to be inherited by any other class. What should i do?
 

You should declared your class as final. But you can't define your class as final, if it is an abstract class. A class declared as final can't be extended by any other class.

30. Should a main() method be compulsorily declared in all java classes?
 

No not required. main() method should be defined only if the source class is a java application.

31. When will you define a method as static?
When a method needs to be accessed even before the creation of the object of the class then we should declare the method as static.

32. 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.

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

final is a modifier which can be applied to a class or a method or a variable. final class can't be inherited,final method can't be overridden and final variable can't be changed. 

finally is an exception handling code section which gets executed whether an exception is raised or not by the try block code segment.
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.

34. I want to print "Hello" even before main() is executed. How will you achieve that?


Print the statement inside a static block of code. Static blocks get executed when the class gets loaded into the memory and even before the creation of an object. Hence it will be executed before the main() method. And it will be executed only once.

35. Can we declare a static variable inside a method?
Static variables are class level variables and they can't be declared inside a method. If declared, the class will not compile.

36. Can a abstract class be defined without any abstract methods?
Yes it's possible. This is basically to avoid instance creation of the class.

37. Can you give few examples of final classes defined in Java API?

java.lang.String, java.lang.Math are final classes.

38. Can a abstract class be declared final?

Not possible. An abstract class without being inherited is of no use and hence will result in compile time error.

39. Can you create an object of an abstract class?

Not possible. Abstract classes can't be instantiated.

40. What are the restriction imposed on a static method or a static block of code?

A static method should not refer to instance variables without creating an instance and cannot use "this" operator to refer the instance.

41. Can a Class extend more than one Class?

Not possible. A Class can extend only one class but can implement any number of Interfaces.

42. Class C implements Interface I containing method m1 and m2 declarations. Class C has provided implementation for method m2. Can i create an object of Class C?

No not possible. Class C should provide implementation for all the methods in the Interface I. Since Class C didn't provide implementation for m1 method, it has to be declared as abstract. Abstract classes can't be instantiated.

43. Can a class be defined inside an Interface?

Yes it's possible.

44. Can an Interface extend another Interface?

Yes an Interface can inherit another Interface, for that matter an Interface can extend more than one Interface.

45. What is use of a abstract variable?
Variables can't be declared as abstract. only classes and methods can be declared as abstract.

46. Can an Interface be defined inside a class?
Yes it's possible.

47. What is an Abstract Class and what is it's purpose?

A Class which doesn't provide complete implementation is defined as an abstract class. Abstract classes enforce abstraction.

48. Can an Interface be final?

Not possible. Doing so so will result in compilation error.

49. Can an Interface implement another Interface?

Interfaces doesn't provide implementation hence a interface cannot implement another interface.

50. Why does Java not support operator overloading?

Operator overloading makes the code very difficult to read and maintain. To maintain code simplicity, Java doesn't support operator overloading.


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7 comments:

Good answers with explanation. Thanks

Java does supports Multiple Inheritance but not Multilevel. Please correct the question no 2.
Thanks for writing good content by the way.

thanks for the hard work taken to do this. really helpfull

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