Inheritance

Creating a parent class

If we want to create a set of classes that share some common functionality, we should create a parent class. This parent class will contain the shared functionality, and the child classes will inherit from it.

public class SmartDevice {
    protected boolean isOn;

    public SmartDevice() {
        this.isOn = false;
    }

    public void togglePower() {
        this.isOn = !this.isOn;
    }
}

INFO

The protected access modifier means that isOn is accessible by any subclasses of this parent class.

If we changed it to private, then subclasses would be unable to access it (but they would still be able to manipulate it through the parent class' public methods).

Inheriting from a parent class

To inherit from a parent class, we use the extends keyword, and then call the parent class's constructor in the child class's constructor using super().

SmartCamera.java

public class SmartCamera extends SmartDevice {
    private String location;
    private int batteryLife;

    public SmartCamera(String location) {
        super(); // Call the parent class constructor
        this.location = location;
        this.batteryLife = 100;
    }
}

Main.java

public class Main {
    public static void main(String[] args) {
        SmartCamera poolCam = new SmartCamera("Pool House");
        poolCam.togglePower();
        System.out.println(poolCam.isOn()) // true
    }
}

Notice that the SmartCamera class has togglePower() and isOn(), even though we didn't define them in the SmartCamera class. This is because SmartCamera inherits from SmartDevice.

TIP

You can pass arguments to the super() method.

class A {
  protected int a;
  public A(int a) {
    this.a = a;
  }
}

class B extends A {
  private int b;
  public B(int a, int b) {
    super(a); // pass a to the parent constructor
    this.b = b;
  }
}

Now B instance = new B(1, 2); will have instance.a // 1 and instance.b // 2.

Abstract classes

It is possible to create an instance of SmartDevice directly:

SmartDevice device = new SmartDevice();

If this isn't desirable, we can make the SmartDevice class into an abstract class.

public abstract class SmartDevice {
    protected boolean isOn;

    public SmartDevice() {
        this.isOn = false;
    }

    public void togglePower() {
        this.isOn = !this.isOn;
    }
}

Now, attempting to instantiate SmartDevice directly will result in an error:

SmartDevice device = new SmartDevice(); // Error