Core Java Interview Questions

1.What is the most important feature of Java?

A.Java is a platform independent language.

2.What do you mean by platform independence?

A.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).

3.What is a JVM?

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

4.Are JVMs platform independent?

A.JVMs are not platform independent. JVMs are platform specific run time implementation provided by the vendor.

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

A.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.

6.What is a pointer and does Java support pointers?

A.Pointer is a reference handle to a memory location. Improper handling of pointers leads to memory leaks and reliability issues hence Java doesnt support the usage of pointers.

7.What is the base class of all classes?

A.java.lang.Object

8.Does Java support multiple inheritance?

A.Java doesnt support multiple inheritance.

9.Is Java a pure object oriented language?

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

10.Are arrays primitive data types?

A.In Java, Arrays are objects.

11.What is difference between Path and Classpath?

A.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.

12.What are local variables?

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

13.What are instance variables?

A.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.

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

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

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

A.Main() method doesnt return anything hence declared void.

16.Why is the main() method declared static?

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

17.What is the arguement of main() method?

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

18.Can a main() method be overloaded?

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

19.Can a main() method be declared final?

A.Yes. Any inheriting class will not be able to have its own default main() method.

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

A.No. It doesnt matter but void should always come before main().

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

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

22.What is a package?

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

23.Which package is imported by default?

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

24.Can a class declared as private be accessed outside its package?

A.Not possible.

25.Can a class be declared as protected?

A.A class cant be declared as protected. only methods can be declared as protected.

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

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

27.What is the purpose of declaring a variable as final?

A.A final variables value cant be changed. final variables should be initialized before using them.

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

A.A method declared as final cant be overridden. A sub-class cant have the same method signature with a different implementation.

29.I dont want my class to be inherited by any other class. What should i do?

A.You should declared your class as final. But you cant define your class as final, if it is an abstract class. A class declared as final cant be extended by any other class.

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

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

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

A.final is a modifier which can be applied to a class or a method or a variable. final class cant be inherited, final method cant be overridden and final variable cant 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.

32.When will you define a method as static?

A.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.

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

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

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

A.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. What is the importance of static variable?

A.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.

36.Can we declare a static variable inside a method?

A.Static varaibles are class level variables and they cant be declared inside a method. If declared, the class will not compile.

37.What is an Abstract Class and what is its purpose?

A.A Class which doesnt provide complete implementation is defined as an abstract class. Abstract classes enforce abstraction.

38.Can a abstract class be declared final?

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

39.What is use of a abstract variable?

A.Variables cant be declared as abstract. only classes and methods can be declared as abstract.

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

A.Not possible. Abstract classes cant be instantiated.

41.Can a abstract class be defined without any abstract methods?

A.Yes its possible. This is basically to avoid instance creation of the class.

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?

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

43.Can a method inside a Interface be declared as final?

A.No not possible. Doing so will result in compilation error. public and abstract are the only applicable modifiers for method declaration in an interface.

44.Can an Interface implement another Interface?

A.Intefaces doesnt provide implementation hence a interface cannot implement another interface.

45.Can an Interface extend another Interface?

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

46.Can an Interface extend another Interface?

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

47.Can a Class extend more than one Class?

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

48.Why is an Interface be able to extend more than one Interface but a Class cant extend more than one Class?

A.Basically Java doesnt allow multiple inheritance, so a Class is restricted to extend only one Class. But an Interface is a pure abstraction model and doesnt have inheritance hierarchy like classes(do remember that the base class of all classes is Object). So an Interface is allowed to extend more than one Interface.

49.Can an Interface be final?

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

50.Can a class be defined inside an Interface?

A.Yes its possible.

51.Can an Interface be defined inside a class?

A.Yes its possible.

52.What is a Marker Interface?

A.An Interface which doesnt have any declaration inside but still enforces a mechanism.

53.Which object oriented Concept is achieved by using overloading and overriding?

A.Which object oriented Concept is achieved by using overloading and overriding?

54.Which object oriented Concept is achieved by using overloading and overriding?

A.

55.Which object oriented Concept is achieved by using overloading and overriding?

A.Polymorphism.

56.Why does Java not support operator overloading?

A.Operator overloading makes the code very difficult to read and maintain. To maintain code simplicity, Java doesnt support operator overloading.

57.Can we define private and protected modifiers for variables in interfaces?

A.No.

58.What is Externalizable?

A.Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in).

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

A.Only public and abstract modifiers are allowed for methods in interfaces.

60.What is a local, member and a class variable?

A.Variables declared within a method are "local" variables. Variables declared within the class i.e not within any methods are "member" variables (global variables). Variables declared within the class i.e not within any methods and are defined as "static" are class variables.

61.What is an abstract method?

A.An abstract method is a method whose implementation is deferred to a subclass.

62.What value does read() return when it has reached the end of a file?

A.The read() method returns -1 when it has reached the end of a file.

63.Can a Byte object be cast to a double value?

A.No, an object cannot be cast to a primitive value.

64.What is an objects lock and which objects have locks?

A.An objects lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the objects lock. All objects and classes have locks. A classes lock is acquired on the classes Class object.