How to rectify? — “both define getObjectCopy(), but with unrelated return types” — but it's *one* function

Question

I have the following interface heirarchy (with all non-relevant functions stripped out). I am getting this error when trying to compile it:

types ValidLineGettable

Answer 1

Assuming all the return values extend Copyable, have all versions of getObjectCopy() return Copyable. For example:

public interface ValidateValue<O> extends Copyable
{
     // Other functions...

     @Override
     Copyable getObjectCopy();
}

public Blammy implements ValidateValue<String>
{
    // Other functions...

     @Override
    public Copyable getObjectCopy()
    {
        SomethingThatExtendsCopyable blammy = new SomethingThatExtendsCopyable();

        return (Copyable)blammy;
    }
}

Edit

In your code above the error is caused by the fact that the "getObjectCopy" method has a different return value in the ValidateValue<String> and ValidLineGettable interfaces, but the calling signature is the same. In java, you do not get polymorphism by changing only the return value; this results in a compile error.

If you change the return value to Copyable then the TextLineValidator no longer gains value by extending both of its parent interfaces. A simpler approach is to have one interface (Copyable) and multiple classes that implement that interface, each of which returns a Copyable value which may be an instance of a class that extends (or implements) Copyable.

Answer 2

The Eclipse compiler compiles your code without errors. JavaC from JDK 7 (1.7.0_45) and JDK 8 (1.8.0-ea) also work.

I think this is a bug in the JDK, most likely one related to bug #122881 (please note this one is fixed). I also found an issue in Google Protocol Buffer that points to another bug, but I can't find that one.

You could compile it with Eclipse or JDK 7, or change the code so that it doesn't require this feature.