Inheritance

Inheritance allows one class to derive from another. The subclass inherits accessible members from its superclass and can add or specialize behaviour. Java classes use single class inheritance: a class can extend one direct superclass.

Basic example#

class Animal {
    void eat() {
        System.out.println("Eating");
    }
}

class Dog extends Animal {
    void bark() {
        System.out.println("Barking");
    }
}

class Main {
    public static void main(String[] args) {
        Dog dog = new Dog();
        dog.eat();
        dog.bark();
    }
}

Dog extends Animal, so a Dog instance can use the accessible eat() method as well as its own bark() method.

Override inherited behaviour#

A subclass can override an inherited instance method to provide a specialized implementation:

class Animal {
    void makeSound() {
        System.out.println("Some sound");
    }
}

class Dog extends Animal {
    @Override
    void makeSound() {
        System.out.println("Woof");
    }
}

@Override tells the compiler that the method is intended to override an inherited method; it helps catch spelling and signature mistakes.

The super keyword#

super can refer to superclass members and invoke a superclass constructor. A constructor call such as super(name) must appear first in a constructor when used explicitly.

When to use inheritance#

Use inheritance when the relationship genuinely represents an “is-a” relationship and the subclass can be used where the superclass is expected. Do not use it only to share a few lines of code. Composition—having one object use another—can be a more flexible design.

Practice#

Create a Vehicle superclass and a Car subclass. Add one method to each class and call both methods on a Car object.