Java Multiple Inheritance

Question

In an attempt to fully understand how to solve Java's multiple inheritance problems I have a classic question that I need clarified.

Lets say I have class Animal this has sub classes Bird and Horse and I need to make a class Pegasus that extends from Bird and Horse since Pegasus is both a bird and a horse.

I think this is the classic diamond problem. From what I can understand the classic way to solve this is to make the Animal, Bird and Horse classes interfaces and implement Pegasus from them.

I was wondering if there was another way to solve the problem in which I can still create objects for birds and horses. If there was a way to be able to create animals also that would be great but not necessary.

Answer 1

I have a stupid idea:

public class Pegasus {
    private Horse horseFeatures; 
    private Bird birdFeatures; 

   public Pegasus(Horse horse, Bird bird) {
     this.horseFeatures = horse;
     this.birdFeatures = bird;
   }

  public void jump() {
    horseFeatures.jump();
  }

  public void fly() {
    birdFeatures.fly();
  }
}

Answer 2

May I suggest the concept of Duck-typing?

Most likely you would tend to make the Pegasus extend a Bird and a Horse interface but duck typing actually suggests that you should rather inherit behaviour. As already stated in the comments, a pegasus is not a bird but it can fly. So your Pegasus should rather inherit a Flyable-interface and lets say a Gallopable-interface.

This kind of concept is utilized in the Strategy Pattern. The given example actually shows you how a duck inherits the FlyBehaviour and QuackBehaviour and still there can be ducks, e.g. the RubberDuck, which can't fly. They could have also made the Duck extend a Bird-class but then they would have given up some flexibility, because every Duck would be able to fly, even the poor RubberDuck.

Answer 3

It is safe to keep a horse in a stable with a half door, as a horse cannot get over a half door. Therefore I setup a horse housing service that accepts any item of type horse and puts it in a stable with a half door.

So is a horse like animal that can fly even a horse?

I used to think a lot about multiple inheritance, however now that I have been programming for over 15 years, I no longer care about implementing multiple inheritance.

More often than not, when I have tried to cope with a design that pointed toward multiple inheritance, I have later come to release that I had miss understood the problem domain.

OR

If it looks like a duck and quacks like a duck but it needs batteries, you probably have the wrong abstraction.

Answer 4

You could create interfaces for animal classes (class in the biological meaning), such as public interface Equidae for horses and public interface Avialae for birds (I'm no biologist, so the terms may be wrong).

Then you can still create a

public class Bird implements Avialae {
}

and

public class Horse implements Equidae {}

and also

public class Pegasus implements Avialae, Equidae {}

---

Adding from the comments:

In order to reduce duplicate code, you could create an abstract class that contains most of the common code of the animals you want to implement.

public abstract class AbstractHorse implements Equidae {}

public class Horse extends AbstractHorse {}

public class Pegasus extends AbstractHorse implements Avialae {}

---

Update

I'd like to add one more detail. As Brian remarks, this is something the OP already knew.

However, I want to emphasize, that I suggest to bypass the "multi-inheritance" problem with interfaces and that I don't recommend to use interfaces that represent already a concrete type (such as Bird) but more a behavior (others refer to duck-typing, which is good, too, but I mean just: the biological class of birds, Avialae). I also don't recommend to use interface names starting with a capital 'I', such as IBird, which just tells nothing about why you need an interface. That's the difference to the question: construct the inheritance hierarchy using interfaces, use abstract classes when useful, implement concrete classes where needed and use delegation if appropriate.

Answer 5

To solve the problem of mutiple inheritance in Java → interface is used

J2EE (core JAVA) Notes By Mr. K.V.R Page 51

Day - 27

1. Interfaces are basically used to develop user defined data types.
2. With respect to interfaces we can achieve the concept of multiple inheritances.
3. With interfaces we can achieve the concept of polymorphism, dynamic binding and hence we can improve the performance of a JAVA program in turns of memory space and execution time.

An interface is a construct which contains the collection of purely undefined methods or an interface is a collection of purely abstract methods.

[...]

Day - 28:

Syntax-1 for reusing the features of interface(s) to class:

[abstract] class <clsname> implements <intf 1>,<intf 2>.........<intf n>
{
    variable declaration;
    method definition or declaration;
};

In the above syntax clsname represents name of the class which is inheriting the features from ‘n’ number of interfaces. ‘Implements’ is a keyword which is used to inherit the features of interface(s) to a derived class.

[...]

Syntax-2 inheriting ‘n’ number of interfaces to another interface:

interface <intf 0 name> extends <intf 1>,<intf 2>.........<intf n>
{     
    variable declaration cum initialization;
    method declaration;
};

[...]

Syntax-3:

[abstract] class <derived class name> extends <base class name> implements <intf 1>,<intf 2>.........<intf n>
{
  variable declaration;
  method definition or declaration;
};

Answer 6

As you will already be aware, multiple inheritance of classes in Java is not possible, but it's possible with interfaces. You may also want to consider using the composition design pattern.

I wrote a very comprehensive article on composition a few years ago...

https://codereview.stackexchange.com/questions/14542/multiple-inheritance-and-composition-with-java-and-c-updated