Field with multiple generics

Question

Why can't I declare a field with generics that implements multiple interfaces/classes?

class MyClass {
    private Class<? extends Enum<?> & Foo> bar;
    //..
}

interface Foo {
  // ...
}

public enum A implements Foo {
   // ...
}

Compiler error: Syntax error on token &

Your not extending interfaces. Interfaces are implemented. Your are asking for 2 superclasses, and in Java you can't extend more than 1 class (but you can implement more than 1 interface) — lateralus, Oct 16 '14 at 19:26
You've now posted three versions of the declaration, with no indication of why you want to do this or what the compiler error message is. Is this now the question you want to ask? I don't want to put effort into answering it, only for it to change again... — Jon Skeet, Oct 16 '14 at 19:26
@lateralus WRONG. `extends` is used in generic declarations as a specification of class hierarchy. — Boris the Spider, Oct 16 '14 at 19:27
If I'm not mistaken, `Class extends Enum extends Foo> & Foo>` is equivalent to `Class`, if `Foo` is an enum. — biziclop, Oct 16 '14 at 19:28
@BoristheSpider i commented one of 3\4 versions he posted with that mistake, he changed lot of versions of the same code — lateralus, Oct 16 '14 at 19:29
@lateralus your comment if fundamentally wrong. `extends` in a generic declaration has nothing to do with extending classes versus implementing interfaces. — Boris the Spider, Oct 16 '14 at 19:30
Did you mean to write `public class Bar, Foo> { ...` ? This is perfectly valid Java. — Dawood ibn Kareem, Oct 16 '14 at 19:37
@John Skeet, I just clarified the question so people are not confused with final classes, because it does not really matter which class/interface is used String/Enum/MyInterface. The use case: I have an enum that implements an interface and I want to make sure that the correct class is used. — Nutel, Oct 16 '14 at 19:37
Mostly it's that Java's type system just isn't designed for it, to be perfectly honest. — Louis Wasserman, Oct 16 '14 at 19:45
@LouisWasserman Actually, I think it is. My comment shows how to do it; assuming this is what OP is trying to ask. — Dawood ibn Kareem, Oct 16 '14 at 19:45
So what type are you expecting that field to be? Sorry, but I'm confused by your use of the word `Class` in the declaration. — Dawood ibn Kareem, Oct 16 '14 at 19:57

score 0 · Answer 1 · edited May 23 '17 at 10:25

Question answered here.

In short, and quoted from the java language specification:

4.9 Intersection Types An intersection type takes the form T1 & ... & Tn, n>0, where Ti, 1in, are type expressions. Intersection types arise in the processes of capture conversion (§5.1.10) and type inference (§15.12.2.7). It is not possible to write an intersection type directly as part of a program; no syntax supports this. The values of an intersection type are those objects that are values of all of the types Ti, for 1in.

Field with multiple generics

1 Answers1