Java nested wildcard generic won't compile

问题:

I have a problem with bounded nested wildcards in Java generics.

Here's a common case:

public void doSomething(Set<? extends Number> set) {}

public void callDoSomething() {
    Set<Integer> set = new HashSet<Integer>();
    doSomething(set);
}

This is standard Java generics, works fine.

However if the wildcard becomes nested, it no longer works:

public void doSomething(Map<String, Set<? extends Number>> map) {}

public void callDoSomething() {
    Map<String, Set<Integer>> map = new HashMap<String, Set<Integer>>();
    doSomething(map);
}

This leads to a compiler error.

I've tried a variety of casts and wildcard permutations, but I'm unable to get this working. I don't recall seeing this issue before, and I've worked with generics for years. Am I just too tired and missing something obvious?

回答1:

So the problem is, doSomething could be implemented as:

public void doSomething(Map<String, Set<? extends Number>> map) {
    Set<Float> set = ...;
    map.put("xyz", set);
}

You need to decide what you actually mean.

Probably something like:

public void doSomething(Map<String, ? extends Set<? extends Number>> map) {}

回答2:

this will work for you:

public void doSomething(Map<String, ? extends Set<? extends Number>> map) {}

回答3:

To make code to work Create HashMap as:

Map<String, Set<? extents Number>> map = new HashMap<String, Set<? extents Number>>();