了解多重继承和接口

Question

I understand multiple inheritance and interface to an extent. Is it possible to use Multiple inheritance to achieve my requirement?

I have classes A,B,C,D which implements interface InterA, InterB, InterC, InterD respectively. I want a class ABCD which should have all the methods of A, B, C, D. Then I want the declared methods in InterA, InterB, InterC, InterD available at class ABCD.

I have implementations of methods in InterA, InterB, InterC, InterD already defined in classes A,B,C,D, which I don't want to define again in class ABCD. How can I do this in Java?

Answer 1

The purpose of an interface is to ensure that a class that implements that interface will implement all the methods declared in that interface.

Eg:

public interface Flyer {

    public void fly();
}

The above interface ensures that any Java class that implements the Flyer interface will have an implementation for the fly method, irregardless of how the flight is implemented.

Eg:

public class Bird implements Flyer {

    public void fly() {
        System.out.println("Bird is flapping");
    }
}

And another eg:

public class Eagle implements Flyer {

    public void fly() {
        System.out.println("Eagle is soaring");
    }
}

With an interface, I am assured that the fly() method will exist despite different implementations.

So if you want a class ABCD that has the behavior of 4 different interfaces, as specified in interface A, B, C and D, then you can implement:

public class ABCD implements A,B,C,D {

    // methods here
}

Usually, we use interfaces to inherit behavior, and parent/child classes to inherit a model.

How do we use the Flyer interface then?

Sample code:

public class FlyerTest {

    public static void main (String [] args) {
        Flyer flyer = new Eagle();
        flyer.fly();    // Prints out "Eagle is soaring"

        flyer = new Bird();
        flyer.fly();    // Prints out "Bird is flapping"
    }

}

As you see, as long as I have a Flyer, I am assured that the "fly" method will have an implementation. This takes away the need for me to wonder if the method actually exists.

This is why we use interfaces to inherit behavior.

Hope you have a better understanding of multiple inheritances.

To answer your core question, Object-Oriented programming is the one that helps to promote code re-use in general.

Answer 2

Since there is no multiple inheritance in Java you have to resort to aggregation :

class ABCD implements InterA, InterB, InterC, InterD
{
   A implA;
   B implB;
   C implC;
   D implD;

   ABCD(A pimplA, B pimplB, C pimplC, D pimplD)
   {
     implA = pimplA;
     implB = pimplB;
     implC = pimplC;
     implD = pimplD;
  }


  // @overidde methods from InterA as return implA ->method();
  // @overidde methods from InterB as return implB ->method();
  // @overidde methods from InterC as return implC ->method();
  // @overidde methods from InterD as return implD ->method();
}

That way you just have to create some instances of A, B, C and D and pass them at the construction of ABCD. Then ABCD just have to call their methods .

By doing that you will reuse the implementations of A, B, C and D .

Answer 3

there is no such a thing as "real" multiple inheritance in java. You cannot do sth. like public class D extends C,B,A {} All you can do is either implement multiple interfaces or you can make B extend A, C extend B and D extend C

Answer 4

There is no multiple inheritance in java. Why don't you just create objects of those classes in ABCD and use them to access the methods of those classes?