我如何在语法上正确地实现具有类型的接口<K extends Comparable<K> ,V&gt;?

Question

K and V are guarantee to be either String or Integer. I've use generics quite a few times, but never <K extends Comparable<K>, V> , so I am struggling right now.

Answer 1

When you implement a generic interface, you should always specify the type arguments. You can get away with not specifying them, but then you are using raw types, which you shouldn't .

When you do specify them, you also need to specify them in a valid way.

First option is wrong because of two reasons:

• When you specify the type arguments, you never rewrite the bounds on that argument, so writing K extends Comparable<K> is wrong
• K and V are not defined anywhere.

Second option is wrong because you rewrote the bounds on K . This option differs from the first option in that K and V are defined here - K and V are the type arguments of Son .

Third option compiles, but it uses raw types, which kind of defeats the purpose of using generics in the first place.

Ideally, you would write something like:

class Son<K extends Comparable<K>, V> implements Father<K, V> {

}

Note how I did not rewrite the bounds, and defined K and V as the type arguments for Son .

Alternatively, specify a concrete type:

class Son implements Father<String, Integer> {

}