How to extends class for taking methods from other two classes

Question

I have the following classes:

class Vehicle {
    public void Move() {
        System.out.println("Move");
    }
}

class Bike extends Vehicle {
    public void RideOnBackWheel() {}
}

class VehicleWithEngine extends Vehicle {
    public void EngineStart() {}
    public void EngineStop() {}
}

Now, I have to create class Motorbike with the methods from Bike and VehicleWithEngine, but it doesn't works:

class Motorbike extends Bike, VehicleWithEngine {
    private boolean engine = false;

    @Override
    public void EngineStart() {
        engine = true;
        System.out.println("Engine started.");
    }

    @Override
    public void EngineStop() {
        engine = false;
        System.out.println("Engine stopped.");
    }

    @Override
    public void RideOnBackWheel() {
        if(engine) System.out.println("1 wheel");
    }

    @Override
    public void Move() {
        if(engine) System.out.println("Motorbike.move()");
    }
}

Thanks for any help.

You can extend only one `Class` in Java and implement multiple `Interfaces` http://csis.pace.edu/~bergin/patterns/multipleinheritance.html — GOXR3PLUS, Jan 17 '17 at 23:15
you could make a Motorbike class that uses composition, whereby the class contains a Bike and VehicleWithEngine — RAZ_Muh_Taz, Jan 17 '17 at 23:18

score 4 · Accepted Answer · 2017-01-17T23:22:56.870

In java you can't inherit from two classes. You must change your Vehicle, Bike and VehicleWithEngine classes to interfaces using implements keyword instead od extends in Motorbike class.

interface Vehicle
{
    default public void Move()
    {
        System.out.println("Move");
    }
}

interface Bike extends Vehicle
{
    public void RideOnBackWheel();
}

interface VehicleWithEngine extends Vehicle
{
    public void EngineStart();
    public void EngineStop();
}

class Motorbike implements Bike, VehicleWithEngine
{
    private boolean engine = false;

    @Override
    public void EngineStart()
    {
        engine = true;
        System.out.println("Engine started.");
    }

    @Override
    public void EngineStop()
    {
        engine = false;
        System.out.println("Engine stopped.");
    }

    @Override
    public void RideOnBackWheel()
    {
        if(engine) { System.out.println("1 wheel"); }
    }

    @Override
    public void Move()
    {
        if(engine) { System.out.println("Motorbike.move()"); }
    }
}

score 1 · Answer 2 · answered Jan 17 '17 at 23:20

It looks like Bike and VehicleWithEngine don't actually have implementations. If so, you can turn them into interfaces and have Motorbike implement both:

interface Bike extends Vehicle {
    void RideOnBackWheel();
}

interface VehicleWithEngine extends Vehicle {
    void EngineStart();
    void EngineStop();
}

class Motorbike implements Bike, VehicleWithEngine {
    // implementations
}

If Bike and Vehicle have stateless implementations (they don't need to directly access member variables), you can turn them into default methods, for example:

interface Bike extends Vehicle {
    default void RideOnBackWheel() {
        // implementation
    }
}

Otherwise it's not possible to extend from two classes. See the linked duplicate.

How to extends class for taking methods from other two classes

2 Answers2