Trying to understand the basics of polymorphism in Java.

Question

Can somebody please tell me if the keyword 'extends' must be an used (in the syntax) of child classes that overide methods of their super class.

Answer 1

The word extends is used to indicate for the whole class that this class is a sub-class of another class. It is not related to whether the sub-class overrides certain methods or not, that is entirely up to the sub-class class. The sub-class may decide to override none, some, or all of the methods of the super-class. The sub-class may override only methods which are not marked as final in the super-class.

Here is a somewhat trivial example:

class A {
    // This is the super-class.
    public void myMethod() {...};
}

class B extends A { 
    // This extends above says: B is sub-class of A.
    // So this class B is the sub-class of A.
    // You can override methods of A here, like this
    public void myMethod() {...};
    // but you're not required to override them. 
}

Answer 2

Polymorphism in java is a concept by which we can perform a single action by different ways.it uses 2 concepts such as method overloading and method over riding. A method is a set of code which is referred to by name and can be called (invoked) at any point in a program simply by utilising the method's name. the method over riding concepts uses the key word 'extends'. We can extend a class by using the extends keyword in a class declaration after the class name and before the parent class.

      public class ParentClass {
       }

and we define child class like

       public class ChildClass extends ParentClass {
        }

// example of extending a class

  class B {
  int x = 0;
  void f1 () { x = x+1;}
  }

class C extends B {}

public class Test1 {
public static void main(String[] args) {
    B b = new B();
    b.f1();
    System.out.println( b.x ); // prints 1
 }
}

// example of extending a class, overwriting a method

   class B {
   int x;
   void setIt (int n) { x=n;}
   void increase () { x=x+1;}
   }

  class C extends B {
void increase () { x=x+2;}
}

public class Test2 {
public static void main(String[] args) {
    B b = new B();
    b.setIt(2);
    b.increase();
    System.out.println( b.x ); // prints 3

    C c = new C();
    c.setIt(2);
    c.increase();
    System.out.println( c.x ); // prints 4
    }
    }