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Polymorphism

Polymorphism is another principle of OOP. You have already encountered polymorphism when you overloaded and overridden methods. In general, this is the essence of polymorphism. But the definition of polymorphism can be a bit intimidating:

But in reality, it's much simpler than it seems. Polymorphism, in simple terms, can be divided into 2 parts:

Method Overloading: what you learned in this chapter, but let's review.

Let's look at an example: we need to write a method that takes an int parameter and returns a String, as well as a method that does the same but with a long parameter. Let's take a look at the code snippet:

Main.java


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public String doStuff(int parameter) {
    //...
}

public String doStuff(long parameter) {
    //...
}

As you can see above, we've created 2 methods with the same name but can do different things. This is method overloading.

Method Overriding: You've encountered this topic before when you overridden the toString method in this chapter. But let's review it once again.

Let's look at a code snippet that will show us how we can override a method. We have a class called Airplane that inherits from the Transport class. And in the Transport class, there's a method called move which returns "This transport has started moving".

We need the airplane to "start flying" instead of "moving". To achieve this, we will override the move method in the child class:

Transport.java

Airplane.java


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public class Transport {
    // some fields

    public String move() {
        return "This transport has started moving";
    }
}

As you can see, we have overridden the method from the parent class in the child class as required.

In this way, polymorphism complements inheritance very well. Through polymorphism, we can conveniently and optimally extend our code, making it flexible.