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Java OOPS Interview Questions

1 . What is the difference between abstraction and encapsulation?

    • Abstraction focuses on the outside view of an object (i.e. the interface) Encapsulation (information hiding) prevents clients from seeing it’s inside view, where the behavior of the abstraction is implemented.
    • Abstraction solves the problem in the design side while Encapsulation is the Implementation.
    • Encapsulation is the deliverables of Abstraction. Encapsulation barely talks about grouping up your abstraction to suit the developer needs.

2 . How does Java implement polymorphism?

  • (Inheritance, Overloading and Overriding are used to achieve Polymorphism in java). Polymorphism manifests itself in Java in the form of multiple methods having the same name.

    • In some cases, multiple methods have the same name, but different formal argument lists (overloaded methods).
    • In other cases, multiple methods have the same name, same return type, and same formal argument list (overridden methods).

3 . Explain the different forms of Polymorphism.

  • There are two types of polymorphism one is Compile time polymorphism and the other is run time polymorphism. Compile time polymorphism is method overloading. Runtime time polymorphism is done using inheritance and interface. Note: From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java:

    • Method overloading
    • Method overriding through inheritance
    • Method overriding through the Java interface

4 . What is runtime polymorphism or dynamic method dispatch?

  • In Java, runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass. The determination of the method to be called is based on the object being referred to by the reference variable.

5 . What is Dynamic Binding?

  • Binding refers to the linking of a procedure call to the code to be executed in response to the call. Dynamic binding (also known as late binding) means that the code associated with a given procedure call is not known until the time of the call at run-time. It is associated with polymorphism and inheritance.

6 . What is method overloading?

  • Method Overloading means to have two or more methods with same name in the same class with different arguments. The benefit of method overloading is that it allows you to implement methods that support the same semantic operation but differ by argument number or type. Note:

    • Overloaded methods MUST change the argument list
    • Overloaded methods CAN change the return type
    • Overloaded methods CAN change the access modifier
    • Overloaded methods CAN declare new or broader checked exceptions
    • A method can be overloaded in the same class or in a subclass

7 . What is method overriding?

  • Method overriding occurs when sub class declares a method that has the same type arguments as a method declared by one of its superclass. The key benefit of overriding is the ability to define behavior that’s specific to a particular subclass type. Note:

    • The overriding method cannot have a more restrictive access modifier than the method being overridden (Ex: You can’t override a method marked public and make it protected).
    • You cannot override a method marked final
    • You cannot override a method marked static

8 . What are the differences between method overloading and method overriding?

  • Method overriding is when a child class redefines the same method as a parent class, with the same parameters.
    For example, the standard Java class java.util.LinkedHashSet extends java.util.HashSet. The method add() is overridden in LinkedHashSet. If you have a variable that is of type HashSet, and you call its add() method, it will call the appropriate implementation of add(), based on whether it is a HashSet or a LinkedHashSet. This is called polymorphism.

    Method overloading is defining several methods in the same class, that accept different numbers and types of parameters. In this case, the actual method called is decided at compile-time, based on the number and types of arguments. For instance, the method System.out.println() is overloaded, so that you can pass ints as well as Strings, and it will call. a different version of the method.

    Overriding is an example of run time polymorphism. The JVM does not know which version of method would be called until the type of reference will be passed to the reference variable. It is also called Dynamic Method Dispatch.
    Overloading is an example of compile time polymorphism.

9 . Can overloaded methods be override too?

  • Yes, derived classes still can override the overloaded methods. Polymorphism can still happen. Compiler will not binding the method calls since it is overloaded, because it might be overridden now or in the future.

10 . Is it possible to override the main method?

  • NO, because main is a static method. A static method can't be overridden in Java.