Java nested wildcard generic won't compile

Question

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?

Insipience

There's an explanation for this in the Java Generics FAQ: angelikalanger.com/GenericsFAQ/FAQSections/… – Wrench

Answer 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) {}

Jumada

Yes, I just independently realized this. I could have sworn I tried that yesterday, but I probably had some other subtle change. – Insipience

Answer 2

this will work for you:

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

Trackandfield

Answer 3

To make code to work Create HashMap as:

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

Sansbury