What is abstract class in Java?

A class with ‘abstract’ modifier is called as abstract class in Java. Abstract class in Java may or may not contain abstract; it is not mandatory for abstract class to have abstract methods.

In Java you cannot create instance of ‘abstract’ class and so if you want to use ‘abstract’ class then you must extend it as subclass/child class for another class which is not abstract.

If you know only know the partial implementation of class then you can declare it as ‘abstract’. Abstract class may or may not contain abstract method and non abstract methods i.e. you may or may not have methods with implementation (non-abstract methods) or methods without implementation (abstract methods) in an abstract class.

Main use of abstract class is to force the provider of subclass to implement unimplemented methods of abstract parent class.

Writing the abstract class:

        i.            A class containing at least one abstract must be declared abstract.

      ii.            Instance variables, instance methods, constructors and instance blocks are allowed inside the abstract block.

    iii.            Static variables, static methods and static blocks are allowed inside the abstract block.

     iv.            You cannot use ‘static’, ’final’ modifiers with abstract class declaration.

       v.            ‘private’, ’protected’ modifiers cannot be used with top level abstract class declaration.