6.4. Multiple Inheritance

Multiple inheritance

The example below illustrates multiple inheritance, a situation where a type immediately specializes two or more types, that is, hovercraft would specialize land vehicle and water vehicle.

The counterparts of types in Java are classes and interfaces.

As we have previously seen, Java supports single inheritance through class extension, in which one class directly inherits accessible fields and methods from another class by extending that class.

Java does not support multiple inheritance through class extension, which is only possible through interfaces.

The code below does not compile!

public class Vehicle {
  void accelerate(int targetSpeed) {
    System.out.println("Vehicle: Accelerate to " + targetSpeed);
  }
}

public class WaterVehicle extends Vehicle {
  void turn(int radius) {
    System.out.println("WaterVehicle: Turning " + radius);
  }
}

public class LandVehicle extends Vehicle {
  void turn(int radius) {
    System.out.println("LandVehicle: Turning " + radius);
  }
}

// THIS WON'T COMPILE BECAUSE MULTIPLE CLASS INHERITANCE IS FORBIDDEN
public class Hovercraft extends LandVehicle, WaterVehicle {
  void accelerateAndTurn(int targetSpeed, int radius) {
    accelerate(targetSpeed);
    turn(radius);
  }
}

What if these were interfaces?

public interface Vehicle {
  void accelerate(int targetSpeed);
}

public interface WaterVehicle extends Vehicle {
  void turn(int radius);
}

public interface LandVehicle extends Vehicle {
  void turn(int radius);
}

public class Hovercraft implements LandVehicle, WaterVehicle {
  @Override
  public void accelerate(int targetSpeed) {
    System.out.println("Hovercraft: Accelerate to " + targetSpeed);
  }

  @Override
  public void turn(int radius) {
    System.out.println("Hovercraft: Turning " + radius);
  }
  
  void accelerateAndTurn(int targetSpeed, int radius) {
    accelerate(targetSpeed);
    turn(radius);
  }
}

What if we had default methods?

public interface Vehicle {
  void accelerate(int targetSpeed);
}

public interface WaterVehicle extends Vehicle {
  default void turn(int radius) {
    System.out.println("WaterVehicle: Turning " + radius);
  }
}

public interface LandVehicle extends Vehicle {
  default void turn(int radius) {
    System.out.println("LandVehicle: Turning " + radius);
  }
}

public class Hovercraft implements LandVehicle, WaterVehicle {
  @Override
  public void accelerate(int targetSpeed) {
    System.out.println("Hovercraft: Accelerate to " + targetSpeed);
  }

  void accelerateAndTurn(int targetSpeed, int radius) {
    accelerate(targetSpeed);
    turn(radius);
  }

  @Override
  public void turn(int radius) {
    LandVehicle.super.turn(radius);
  }
}