Java · Lesson 10 of 15

Inheritance and Polymorphism

extends, super, overriding and treating subclasses as their parent type.

Intermediate
15 min read
3 objectives

Before this lessonLesson 9: Methods and Overloading

What you will learn

Extend a class
Override methods
Use polymorphism

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Inheritance lets a class reuse and extend another. The child (subclass) gets the parent's (superclass's) fields and methods and can add or replace behaviour. Model an is-a relationship: a Dog is an Animal.

extends and super

class Animal {
    protected String name;
    Animal(String name) { this.name = name; }
    String sound() { return "..."; }
    public String toString() { return name + " says " + sound(); }
}

class Dog extends Animal {
    Dog(String name) { super(name); }   // call the parent constructor
    @Override
    String sound() { return "Woof"; }
}

public class Main {
    public static void main(String[] args) {
        System.out.println(new Dog("Rex"));
        System.out.println(new Animal("Thing"));
    }
}

Output

Rex says Woof
Thing says ...

Polymorphism

A variable of the parent type can hold any subclass. Java calls the actual object's version of an overridden method at runtime.

class Animal { String sound() { return "..."; } }
class Dog extends Animal { String sound() { return "Woof"; } }
class Cat extends Animal { String sound() { return "Meow"; } }

public class Main {
    public static void main(String[] args) {
        Animal[] zoo = { new Dog(), new Cat(), new Animal() };
        for (Animal a : zoo) {
            System.out.println(a.sound());
        }
    }
}

Output

Woof
Meow
...

Rules worth remembering

A class can extend only one class (single inheritance).
Add @Override so the compiler catches typos in method names.
final on a class or method forbids extending or overriding it.
Every class extends Object, which is where toString, equals and hashCode come from.

Up next · Lesson 11Interfaces and Abstract ClassesContracts, default methods and when to choose an interface or an abstract class.