Java: Declaring variables with multiple generic "constraints" on them

Question

I have a bunch of classes in java that all implement an interface called IdObject (specifying a getId() method). Moreover, they also all implement Comparable<> with themselves as type parameter, so they are all comparable to themselves.

What I'd like to do is declare a list of such objects, fill it, then sort it and call getId() on them. So my code looks like this:

List<? extends IdObject & Comparable<?>> objectList = null;

if (foo) {
    objectList = new ArrayList<TypeA>();
    ...
} else if (bar) {
    objectList = new ArrayList<TypeB>();
    ...   
}

if (objectList != null) {
    Collections.sort(objectList);
    for (IdObject o : objectList) {
        System.out.println(o.getId());
    }
}

Basically my problem lies in the first line -- I want to specify two "constraints" for the type because I need the first one to make sure I can print the ID in the loop and the second one to make sure I can use Collections.sort() on the list.

The first line does not compile.

Is there a way to make this work without specifying a generic type without type parameters and using unchecked operations? I could also not find an example of this on the internet.

Greetings

Answer 1

List<? extends IdObject & Comparable<?>>

This type of multiple bounded type parameters is only possible in class and method signatures, I'm afraid.

So I guess the closest you can get is to define an interface:

public interface MyInterface extends IdObject, Comparable<MyInterface>

And declare your list like this:

List<? extends MyInterface> objectList = null;