Java: Dealing with multiple complex interfaces without duplicate code

Question

I am working with a Java API which requires me to implement a number of fairly large interfaces. However generally there is only one or two details that actually vary between implementations, so I made some abstract base classes to provide most of the implementation.

However now Ive come across cases where I need to extend from some other class and/or implement multiple such interfaces, and so I am unable to extend my abstract base classes.

In C++ I was able to use multiple inheritance, and also use some other tricks like below. However Java doesn't allow multiple inheritance, or generics to be used in this way.

class MyClass : public HelperForInterfaceA, public HelperForInterfaceB {...};
class template<class BASE> MyHelper : public BASE {...};

The best idea I have right now is to have the concrete implementation of my abstract helper class as a field, and then make all the interface methods forward to that field instance, with the field having a reference to the main object to implement the last details if needed.

class MyClass extends A implements IB, IC {
    private static class B extends BAbstractHelper {
        private A a;
        public B(A a, int size) {
            super(size);
            this.a = a;
        }

        @Override
        public boolean foo(int x, int y) {
            return a.foo(x, y);
        }
    }
    private static class C extends CAbstractHelper {
        ...
    }


    private B b;
    private C c;
    private int range;

    @Override
    public boolean foo(int x, int y) {
        return x*x + y*y <= range*range;
    }

    @Override
    public float bar(float x, int y, String s) {
        return b.bar(x,y,s);
    }

    ...
}

However this seems a bit of a pain, with a lot of wrapper methods required. Is there a better way of dealing with this?

Answer 1

A Java best practice is to prefer composition over inheritance. Keeps things simpler but creates more boilerplate code. You have the right idea to create delegate methods. A good Java IDE can generate all the boilerplate delegate methods code for you. Other languages on the JVM like Scala (with traits) make this easier.