使用下限通配符（Java）的“意外令牌”

Question

I have something along the lines of:

interface Foo<T> {
    //... lines [0,45]...

/*line 46*/ <R, X super T&R> List<X> weave(R value);
    //...
}

But IntelliJ is reporting:

1. Error:(46, 18) java: > expected
2. Error:(46, 19) java: illegal start of type
3. Error:(46, 26) java: '(' expected
4. Error:(46, 28) java: < identifier > expected
5. Error:(46, 29) java: 'l' expected
6. Error:(46, 43) java: < identifier > expected

What's the problem? Am I not allowed to bind a name to a lower bound? Or am I only allowed to use a R&X expression in an upper bound?

Changing it to

interface Foo<T> {
    //... lines [0,45]...

/*line 46*/ <R> List<? super T&R> weave(R value);
    //...
}

yields

1. Error(46, 31) java: > expected
2. Error(46, 32) java: '(' expected
3. Error(46, 33) java: illegal start of type

Answer 1

By my reading of the specification, super can only be used with a wildcard and can't be captured into a type variable; see JLS 4.5.1 . Similarly, & is only valid in type variables , not type arguments , and type variables can't use super .

After having thought about it, here's my explanation: The reason for a type variable is to eliminate explicit casting to improve type safety. When you declare a type parameter to be super Foo , you're saying that it's okay for that parameter to be any superclass of Foo . This means that it could be anything up to and including Object , and so you have no safe way to presume anything about the objects whose type satisfies that bound, and so there's no information whatsoever contained within a named type variable; you just wildcard it and can call hashCode() or toString() , but nothing type-specific.