If you are extending any abstract class that have abstract method, you must either provide the implementation of the method or make this class abstract.

Java Programming Language Polymorphism in Java (Article) Polymorphism in Java (Program)

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Program:

abstract class Parent{
  abstract void age();
}
abstract class Child extends Parent{
abstract void age();
	}

class Child1 extends Child{
	void age(){
			System.out.println("age 22");
		}
		public static void main(String args[]){
		 Parent obj = new Child1();
		 obj.age();
	}
}

Output:

age 22
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