Java嵌套的通配符通用不会编译 [英] Java nested wildcard generic won't compile

问题描述

我在Java泛型中遇到了有界嵌套通配符的问题。

以下是一个常见的情况：

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

这是标准的Java泛型，工作正常。 b

但是，如果通配符嵌套，它将不再有效：

  public void doSomething（Map< String，Set< ;? extends Number> map）{} 
 $ b public void callDoSomething（）{
 Map< String，Set< Integer>> map = new HashMap< String，Set< Integer>>（）; 
 doSomething（地图）; 
} 
  

这会导致编译器错误。

我尝试了各种各样的强制转换和通配符排列，但是我无法使其工作。我不记得之前看到过这个问题，而且我已经与仿制药合作了多年。我只是太累了，错过了一些明显的东西？

解决方案

所以问题是， doSomething 可以实现为：

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

您需要决定您的实际意思。

可能类似于：

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

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?

解决方案

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) {}

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