Generics issue - inferred type does not conform to declared bound(s)

Question

We currently have one class that won't compile with openjdk 6. The following shortened class reproduces this error:

public class Copy implements ICopy {        
    public <A,B extends List<A>,C extends Map<A,B>> A method(A a, B b) {
        A something = null;
        return something;
    }

    @Override
    public <A,B extends List<A>> A method2(A a, B b) {
        return method(a,b);
    }    
}

The interface is simple, it just shows, that we can't change the signature of the method2:

public interface ICopy {    
    public <A,B extends List<A>> A method2(A a, B b);
}

It compiles with oracles java versions and openjdk 7 - but fails with openjdk 6, and, unfortunatly, that's the version that we have to use to compile the (original) code.

The error message is

Copy.java:15: invalid inferred types for C; inferred type does not conform to declared bound(s)
inferred: java.util.Map<A,B>
bound(s): java.util.Map<A,B>
                return method(a,b);
                             ^

All I need are some ideas how to change the implementation of method2 so that it would compile. I wouldn't even care about compiler warnings...

Answer 1

I would try specifying the bounds explicitly, like this:

return this.<A,B,Map<A,B>>method(a,b);

However, more generally, I don't see the need for the type parameter C at all. None of method's arguments or its return type references C at all, so if you can change the signature of method, I would just omit that type parameter entirely.