How can I define a generic class that implements two interfaces?

Question

I don't think this is a duplicate of Check if a generic T implements an interface, but it may be(??).

So, I want to create a generic interface that only allows objects that implements two interfaces. Alike is a costum interface.

public interface AbstractSortedSimpleList<T extends Comparable<T>, Alike> {}

If I understand it correctly, Java now tries to create a generic interface AbstractSortedSimpleList<T,Alike>, which isnt exactly what I want to achieve. I want AbstractSortedSimpleList<T> where T has to implement both Comparable<T> and Alike.

Later, I want to make a new class

public class SortedSimpleList<T> implements AbstractSortedSimpleList<T> {}

The point here is to create a class SortedSimpleList<T> where T has to be implementing the aforementioned interfaces. But my code does not seem to work very well.

Answer 1

You can give multiple bounds to type parameter:

public interface AbstractSortedSimpleList<T extends Comparable<T> & Alike>

Then, your SortedSimpleList would be like:

class SortedSimpleList<T extends Comparable<T> & Alike> implements AbstractSortedSimpleList<T> {}

See JLS §4.4:

Every type variable declared as a type parameter has a bound. If no bound is declared for a type variable, Object is assumed. If a bound is declared, it consists of either:

• a single type variable T, or

• a class or interface type T possibly followed by interface types I1 & ... & In.

Note:

You can't have such multiple bounds for wildcards though. It's only for type parameters.

References:

• Java Generics FAQs - Type Parameter bounds

Answer 2

Use some generic bounds with the & notation

interface AbstractSortedSimpleList<T extends Comparable<T> & Alike> {

See the official Java tutorial on multiple bounds, here.