Multiple inheritance workaround in Java?

Question

interface IA {
    public void a();
}

class AB implements IA {
    @Override
    public void a() { System.out.println("a"); } // <---.

    public void b() { System.out.println("b"); }
}

class C {
    public void c() { System.out.println("c"); }
}


// My class:
class AC extends C implements IA {
    @Override
    public void a() { System.out.println("a"); } // duplicate code 
}

public class Main {
    public static void main(String[] args) {
        AC ac = new AC();
        ac.a(); // "a"
        ac.c(); // "c"
    }
}

Using duplicate code does not seem to be a good idea, how could I design my class properly?

JB Nizet · Accepted Answer · 2014-07-06 15:16:49Z

11

You can use composition and delegate, among other options:

interface IA {
    public void a ();
} 

interface IB {
    public void b ();
}

class A implements IA {
    @Override public void a () { /* code */ }
}

class B implements IB {
    @Override public void b () { /* code */ }
}

class AB implements IA, IB {
    final A a = new A();
    final B b = new B();
    @Override public void a () { a.a(); }
    @Override public void b () { b.b(); }
}

edited Jul 6, 2014 at 15:16

JB Nizet

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answered Jul 6, 2014 at 15:14

Jason C

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1 Comment

Jason C Over a year ago

@JBNizet Thanks; I've been in C++ land all day.

Nir Alfasi · Accepted Answer · 2014-07-06 15:15:08Z

3

Java 8 introduces the concept of "default methods" - but before that you have no way around it.

answered Jul 6, 2014 at 15:15

Nir Alfasi

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2 Comments

Mustafa sabir Over a year ago

Thats true but I still dont think we can call that multiple inheritance. There are still many restrictions for interfaces vs classes, like defining constructors , accessing variables other than local variables etc. Its sort of a compromise between mutiple inheritance and no multiple inheritance. But yes it definitely adds the problem of refrencing same method defined in both the interfaces.

Nir Alfasi Over a year ago

@Mustafasabir agreed (and for the record, I never considered it as multiple inheritance)

Alex - GlassEditor.com · Accepted Answer · 2014-07-06 15:14:35Z

0

You could make a a default method:

interface IA {
    public default void a(){ System.out.println("a"); }
}

answered Jul 6, 2014 at 15:14

Alex - GlassEditor.com

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1 Comment

Jason C Over a year ago

This is a quick way but I really personally dislike that default methods were added to Java; the potential for making a mess when used improperly is very high. If you do this be sure to make sure you maintain a clear grasp on your code structure while developing.

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