1. Contents
  2. 8. Interfaces and Polymorphism
  3. 8.1. Introduction

8.1. Introduction

The third pillar of Object-Oriented Programming that we’ll explore is polymorphism.

8.1.1. Polymorphism

Polymorphism is an object-oriented mechanism that allows for objects of different types to be used in the same way.

We’ve already encountered polymorphism made possible by inheritance when talking about casting with our Cat and HouseCat classes. In that case, we stored an object of type HouseCat in its compatible type, Cat.

Let’s take a closer look at how polymorphism might work in our cat-centric application.

Example

Suppose we had a CatOwner class like the one below:

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public class CatOwner
{
   private Cat pet;

   public CatOwner(Cat pet) {
      this.pet = pet;
   }

   public void feedTheCat() {

      // ...code to prepare the cat's meal...

      pet.eat();
   }
}

The method feedTheCat uses the field pet, which is of type Cat. Since a HouseCat is a Cat via inheritance, it is perfectly acceptable to use an instance of HouseCat to fill the pet field.

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HouseCat suki = new HouseCat("Suki", 12);
CatOwner Annie = new CatOwner (suki);

Annie.feedTheCat();

Similarly, feedTheCat can accept Tiger instances as well. This is because the only thing that the method requires is that the input parameter has the methods defined within Cat, and via inheritance, both of the subclasses satisfy this requirement.

In addition to using classes to code in a polymorphic way, we can use interfaces.