Question

Given the interface:

public interface BasedOnOther<T, U extends BasedList<T>> {

    public T getOther();

    public void staticStatisfied(final U list);

}

The BasedOnOther<T, U extends BasedList<T>> looks very ugly in my use-cases. It is because the T type parameter is already defined in the BasedList<T> part, so the “uglyness” comes from that T needs to be typed twice.

Problem: is it possible to let the Java compiler infer the generic T type from BasedList<T> in a generic class/interface definition?

Ultimately, I’d like to use the interface like:

class X implements BasedOnOther<Y> {
    public SomeType getOther() { ... }
    public void staticStatisfied(final Y list) { ... }
} // Does not compile, due to invalid parameter count.

Where Y extends BasedList<SomeType>.

Instead:

class X implements BasedOnOther<SomeType, Y> {
    public SomeType getOther() { ... }
    public void staticStatisfied(final Y list) { ... }
}

Where Y extends BasedList<SomeType>.

Update: ColinD suggested

public interface BasedOnOther<T> {
    public T getOther();
    public void staticSatisfied(BasedList<T> list);
}

It is impossible to create an implementation such as:

public class X implements BasedOnOther<SomeType> {
    public SomeType getOther() { ... }
    public void staticStatisfied(MemoryModel list);
} // Does not compile, as it does not implement the interface.

Where MemoryModel extends BasedList<SomeType>, which is needed (as it provides other methods).

Answer 1

It looks as if you don’t actually need the type parameter U extends BasedList<T>, if you don’t actually need to do anything in the class that requires some specific subclass/implementation of BasedList<T>. The interface could just be:

public interface BasedOnOther<T> {
  public T getOther();
  public void staticSatisfied(BasedList<T> list);
}

Edit: Based on your update, I don’t think there’s any way you can do this. I think you’ll have to either just go with your original declaration or make some intermediate type that specifies T, like:

public interface BasedOnSomeType<U extends BasedList<SomeType>>
         extends BasedOnOther<SomeType, U>
{
}

public class X implements BasedOnSomeType<MemoryModel> { ... }

That seems like kind of a waste though, and I don’t really think the original declaration looks that bad.