Constraint generic type for enum type to implement some interface

Question

I have enum that implement MyInterface. While making other class using that enum I want to constrain the enumClz to be class that has implemented MyInterface.

So I describe signature to be "T extends Enum< T extends MyInterface>" at generic type declaration.

public <T extends Enum< T extends MyInterface>> C1( Class<T> enumClz) {
    for (T anEnumConst : enumClz.getEnumConstants()) {
        //....process
    }
}

What surprised me is the IDE say it is "unexpected bound" at "T extends MyInterface". I don't know what it means by such two word error message, Any solution about this?

And by the way, out of curious I have an odd question though not really important. Can an enum type T be equivalent to the following infinite loop

<T extends Enum< T extends Enum<T extends<....>>>> ?

@alfasin extends when used in generic bounds means "is, or extends, or implements". — Paul Bellora
– Paul Bellora, Commented Oct 29, 2013 at 4:36

Community · Accepted Answer · 2020-06-20 09:12:55Z

11

Declare the following instead:

public <T extends Enum<T> & MyInterface> C1(Class<T> enumClz)

Here, we're declaring T to have multiple upper bounds, which is possible for type parameters.

The declaration <T extends Enum<T extends MyInterface>> is invalid syntax because T must be bounded with a concrete type, but the T extends MyInterface in the type argument for Enum is trying to add more information about T when it's already been declared.

Note also that a class type must always come first when declaring multiple bounds. A declaration of <T extends MyInterface & Enum<T>> is also invalid syntax.

And by the way, out of curious I have an odd question though not really important. Can an enum type T be equivalent to the following infinite loop

<T extends Enum< T extends Enum<T extends<....>>>> ?

The declaration T extends Enum<T> is already "infinite" in that it's recursive. The same T that is being declared is given as a type argument for its upper bound - a type parameter's scope includes its own declaration.

More information:

edited Jun 20, 2020 at 9:12

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answered Oct 29, 2013 at 4:14

Paul Bellora

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