Basic Inheritance (extends)

Part of the Object Oriented Programming section of Coddy's Java journey — lesson 20 of 87.

Inheritance is one of the core pillars of OOP. It allows a class to acquire the properties and methods of another class, promoting code reuse and establishing a natural hierarchy between classes.

In Java, you create an inheritance relationship using the extends keyword. The class that inherits is called the subclass (or child class), and the class being inherited from is the superclass (or parent class).

public class Animal {
    private String name;
    
    public Animal(String name) {
        this.name = name;
    }
    
    public void eat() {
        System.out.println(name + " is eating");
    }
    
    public String getName() {
        return name;
    }
}

public class Dog extends Animal {
    public Dog(String name) {
        super(name);  // Calls parent constructor
    }
    
    public void bark() {
        System.out.println(getName() + " says woof!");
    }
}

The Dog class automatically inherits the eat() method and getName() method from Animal. It can also define its own methods like bark():

Dog myDog = new Dog("Buddy");
myDog.eat();   // Inherited: "Buddy is eating"
myDog.bark();  // Own method: "Buddy says woof!"

Think of inheritance as an "is-a" relationship. A Dog is an Animal, so it makes sense for Dog to extend Animal. This relationship lets you build specialized classes that share common functionality while adding their own unique behaviors.

Challenge

Easy

Let's build a simple vehicle system that demonstrates inheritance in action. You'll create a parent class that holds common vehicle properties, and a child class that extends it with specialized behavior.

You'll organize your code across three files:

Vehicle.java: Create the parent class that represents any vehicle. It should have:
- A private field for the vehicle's brand (String)
- A constructor that accepts the brand name and uses super() isn't needed here since Vehicle is the parent
- A method getBrand() that returns the brand
- A method start() that prints: [brand] is starting
Motorcycle.java: Create a subclass that extends Vehicle. A motorcycle is a vehicle, so this "is-a" relationship makes inheritance appropriate. Your Motorcycle should:
- Use extends to inherit from Vehicle
- Have a constructor that takes the brand and passes it to the parent using super(brand)
- Add its own method wheelie() that prints: [brand] is doing a wheelie!
Main.java: Bring everything together by creating a Motorcycle object and demonstrating both inherited and unique behaviors. You'll receive one input: the brand name. Print two lines by calling:
- The inherited start() method
- The Motorcycle's own wheelie() method

You will receive one input: the brand name (String) for your motorcycle.

Notice how your Motorcycle automatically has access to start() and getBrand() without rewriting them - that's the power of inheritance!

Cheat sheet

Use the extends keyword to create an inheritance relationship between classes:

public class Animal {
    private String name;
    
    public Animal(String name) {
        this.name = name;
    }
    
    public void eat() {
        System.out.println(name + " is eating");
    }
    
    public String getName() {
        return name;
    }
}

public class Dog extends Animal {
    public Dog(String name) {
        super(name);  // Calls parent constructor
    }
    
    public void bark() {
        System.out.println(getName() + " says woof!");
    }
}

The subclass (child) inherits all methods from the superclass (parent) and can add its own:

Dog myDog = new Dog("Buddy");
myDog.eat();   // Inherited method
myDog.bark();  // Own method

Use super() in the subclass constructor to call the parent class constructor.

Inheritance represents an "is-a" relationship between classes.

Try it yourself

import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String brand = scanner.nextLine();
        
        // TODO: Create a Motorcycle object with the brand
        
        // TODO: Call the inherited start() method
        
        // TODO: Call the wheelie() method
    }
}