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

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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 intersection 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.

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