Adrian Petrescu Adrian Petrescu - 1 month ago 11
Java Question

Why can't you have multiple interfaces in a bounded wildcard generic?

I know there's all sorts of counter-intuitive properties of Java's generic types. Here's one in particular that I don't understand, and which I'm hoping someone can explain to me. When specifying a type parameter for a class or interface, you can bound it so that it must implement multiple interfaces with

public class Foo<T extends InterfaceA & InterfaceB>
. However, if you're instantiating an actual object, this doesn't work anymore.
List<? extends InterfaceA>
is fine, but
List<? extends InterfaceA & InterfaceB>
fails to compile. Consider the following complete snippet:

import java.util.List;

public class Test {

static interface A {
public int getSomething();
}

static interface B {
public int getSomethingElse();
}

static class AandB implements A, B {
public int getSomething() { return 1; }
public int getSomethingElse() { return 2; }
}

// Notice the multiple bounds here. This works.
static class AandBList<T extends A & B> {
List<T> list;

public List<T> getList() { return list; }
}

public static void main(String [] args) {
AandBList<AandB> foo = new AandBList<AandB>(); // This works fine!
foo.getList().add(new AandB());
List<? extends A> bar = new LinkedList<AandB>(); // This is fine too
// This last one fails to compile!
List<? extends A & B> foobar = new LinkedList<AandB>();
}
}


It seems the semantics of
bar
should be well-defined -- I can't think of any loss of type-safety by allowing an intersection of two types rather than just one. I'm sure there's an explanation though. Does anyone know what it is?

Answer

Interestingly, interface java.lang.reflect.WildcardType looks like it supports both upper bounds and lower bounds for a wildcard arg; and each can contain multiple bounds

Type[] getUpperBounds();
Type[] getLowerBounds();

This is way beyond what the language allows. There's a hidden comment in the source code

// one or many? Up to language spec; currently only one, but this API
// allows for generalization.

The author of the interface seems to consider that this is an accidental limitation.

The canned answer to your question is, generics is already too complicated as it is; adding more complexity might prove to be the last straw.

To allow a wildcard to have multiple upper bounds, one has to scan through the spec and make sure the entire system still works.

One trouble I know would be in the type inference. The current inference rules simply can't deal with intercection types. There's no rule to reduce a constraint A&B << C. If we reduced it to

    A<<C 
  or
    A<<B

any current inference engine has to go through major overhaul to allow such bifurcation. But the real serious problem is, this allows multiple solutions, but there's no justification to prefer one over another.

However, inference is not essential to type safety; we can simply refuse to infer in this case, and ask programmer to explicitly fill in type arguments. Therefore, difficulty in inference is not a strong argument against intercection types.