无法理解复杂的参数化返回类型-Java

Question

I have come across a snippet similar to this in Java

public <H extends ABC<I, U>, I, U> Set<U> get(Type<H, I, U> type) {

}

I do not understand this. I only understand that it takes a parameter of the parameterized type Type<H,I,U> and that it returns a Set<U> which is the return type. But, I do not understand the part:

<H extends ABC<I,U>, I,U>

Can anybody clarify it?

Regards, Chetan

Answer 1

It means that

• The method takes three type parameters: H , I , and U
• I and U can be anything
• H must extend ABC<I, U> (or it can actually be ABC<I, U> )

So presumably you have a type ABC which has two type parameters. This is saying that, for instance, this would be valid:

Type<ABC<String, Date>, String, Date> type = new Type<>();
Set<String> set = instance.get(type);

...because there H would be ABC<String, Date> , I would be String , and U would be Date .