Method Overriding

Overview

Method overriding allows a subclass to provide a specific implementation of a method that is already provided by its base class.

Rules for Method Overriding

Covariance

Covariance preserves the ordering of types.

The return type in an overriden method should be the same or a sub-type of the return type defined in the super-class.

public class AnimalShelter {
    public Animal getAnimalForAdoption() {
        // do something
    }
}

public class CatShelter extends AnimalShelter {
    @Override
    public Cat getAnimalForAdoption() {
        // do something
    }
}

Contravariance

Contravariance reverses the ordering of types.

In the context of OOP, this involves methods in a subclass to have arguments wider than the arguments passed in its parent class.

public class AnimalShelter {
    public Animal putAnimal(Animal someAnimal) {
        // do something
    }
}

public class CatShelter extends AnimalShelter {
    public Cat putAnimal(Object someAnimal) {
        // do something
    }
}

Note that Java does not recognise this as overriding. It instead sees it as an unrelated method.