Java Polymorphism

Polymorphism means "many forms", and it occurs when we have many classes that are related to each other by inheritance.

Like we specified in the previous chapter, inheritance lets us inherit attributes and methods from another class. Polymorphism uses those methods to perform different tasks. This allows us to perform a single action in different ways.

For example, think of a superclass called Animal that has a method called animalSound(). Subclasses of Animal could be Pig, Dog, etc. - and they also have their own implementation of an animal sound:

Example

class Animal {
  public void animalSound() {
    System.out.println("The animal makes a sound");
  }
}
 
class Pig extends Animal {
  public void animalSound() {
    System.out.println("The pig says: wee wee");
  }
}
 
class Dog extends Animal {
  public void animalSound() {
    System.out.println("The dog says: bow wow");
  }
}

Remember from the Inheritance chapter that we use the extends keyword to inherit from a class.

Now we can create Pig and Dog objects and call the animalSound() method on both of them:

Example

class Animal {
  public void animalSound() {
    System.out.println("The animal makes a sound");
  }
}
 
class Pig extends Animal {
  public void animalSound() {
    System.out.println("The pig says: wee wee");
  }
}
 
class Dog extends Animal {
  public void animalSound() {
    System.out.println("The dog says: bow wow");
  }
}
 
class Main {
  public static void main(String[] args) {
    Animal myAnimal = new Animal();  // Create an Animal object
    Animal myPig = new Pig();  // Create a Pig object
    Animal myDog = new Dog();  // Create a Dog object
    myAnimal.animalSound();
    myPig.animalSound();
    myDog.animalSound();
  }
}

Why and When to Use Inheritance and Polymorphism?

It is useful for code reusability: reuse attributes and methods of an existing class when you create a new class.

Java Inheritance Java Inner Classess