Java 嵌套通配符泛型无法编译 [英] Java nested wildcard generic won't compile

问题描述

我在 Java 泛型中遇到有界嵌套通配符的问题.

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

这是一个常见的案例:

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.

This is standard Java generics, works fine.

但是，如果通配符嵌套，则不再有效:

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);
}

这会导致编译器错误.

我尝试了各种类型转换和通配符排列，但无法正常工作.我不记得以前见过这个问题，而且我已经使用泛型多年了.我是不是太累了，错过了一些明显的东西?

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?

推荐答案

所以问题是，doSomething 可以实现为:

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);
}

你需要决定你真正的意思.

You need to decide what you actually mean.

大概是这样的:

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

这篇关于Java 嵌套通配符泛型无法编译的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！