Toys

Recall our toy class hierarchy (now with volume!).

What is wrong with this design?

public class LoudToy {

    private int volume;

    public LoudToy(int volume) {
        this.volume = volume;
    }

    public int getVolume() {
        return volume;
    }

    public void setVolume(int volume) {
        this.volume = volume;
    }

    public void makeNoise() {
        // This should be overridden in subclasses.
        System.out.println("Generic noise.");
    }

}

public class ToySheep extends LoudToy {

    public ToySheep() {
        super(3);
    }

    @Override
    public void makeNoise() {
        System.out.println("Baaa.");
    }
}

public class ToyRobot extends LoudToy {

    private int chargeLevel;

    public ToyRobot() {
        super(10);
        chargeLevel = 5;
    }

    @Override
    public void makeNoise() {
        System.out.println("Beep Beep!");
    }

    public void recharge() {
        chargeLevel = 10;
        System.out.println("Charged up!");
    }
}

Improvement…

public abstract class LoudToy {

    private int volume;

    public LoudToy(int volume) {
        this.volume = volume;
    }

    public int getVolume() {
        return volume;
    }

    public void setVolume(int volume) {
        this.volume = volume;
    }

    public abstract void makeNoise();
    
}

public class ToySheep extends LoudToy {

    public ToySheep() {
        super(3);
    }

    @Override
    public void makeNoise() {
        System.out.println("Baaa.");
    }
}

public class ToyRobot extends LoudToy {

    private int chargeLevel;

    public ToyRobot() {
        super(10);
        chargeLevel = 5;
    }

    @Override
    public void makeNoise() {
        System.out.println("Beep Beep!");
    }

    public void recharge() {
        chargeLevel = 10;
        System.out.println("Charged up!");
    }
}

Two Key Benefits

• Prevent code duplication: common functionality can be inherited from the superclass.

• Polymorphism: we can write code that will work with any descendent.

  public class PlayUtils {
  
      public static void playWithLoudToy(LoudToy toy) {
          System.out.println("This is a fun loud toy!");
          toy.makeNoise();
      }
  }

Cell Phone

A cell phone is not a toy, but it does share some functionality…

public class CellPhone {
    private int chargeLevel;
    private int volume;

    public CellPhone(int chargeLevel, int volume) {
        this.chargeLevel = chargeLevel;
        this.volume = volume;
    }

    public int getVolume() {
        return volume;
    }

    public void setVolume(int volume) {
        this.volume = volume;
    }

    public void makeCall() {
        System.out.println("Riing... Hello.");
    }

    public void recharge() {
        chargeLevel = 10;
        System.out.println("All charged up!");
    }

}

Will this compile?

    public static void muteAll(ArrayList<LoudToy> toys) {
        for (LoudToy current : toys) {
            current.setVolume(0);
        }
    }

    public static void main(String[] args) {
        ArrayList<LoudToy> items = new ArrayList<>();
        
        ToySheep sheep = new ToySheep();
        CellPhone phone = new CellPhone(3, 4);
        
        items.add(sheep);
        items.add(phone);
        
        muteAll(items);
    }

• No! A CellPhone is not a LoudToy.
• We want to be able to interact with CellPhones and LoudToys based on their common functionality, but they don’t belong in the same class hierarchy.

Interfaces

/**
 * Any class that has an adjustable volume may 
 * implement this interface.
 */
public interface Audible
{
    int MAX_VOLUME = 10;

    int getVolume();

    void setVolume(int volume);
}

public class LoudToy implements Audible {


}

public class CellPhone implements Audible {


}

Now, this will work:

    public static void muteAll(ArrayList<Audible> items) {
        for (Audible current : items) {
            current.setVolume(0);
        }
    }

    public static void main(String[] args) {
        ArrayList<Audible> items = new ArrayList<>();
        
        ToySheep sheep = new ToySheep();
        CellPhone phone = new CellPhone(3, 4);
        
        items.add(sheep);
        items.add(phone);
        muteAll(items);
    }

Multiple Implementation

Java classes can only extend one class, but they can implement multiple interfaces:

/**
 * Any class is rechargable may implement this interface.
 */
public interface Rechargeable
{
    void recharge();
}

public class ToyRobot extends LoudToy implements Rechargeable {

 
}

public class CellPhone implements Audible, Rechargeable {


}

UML