[英]Java nested wildcard generic won't compile
I have a problem with bounded nested wildcards in Java generics. 我在Java泛型中遇到了有界嵌套通配符的问题。
Here's a common case: 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);
}
This is standard Java generics, works fine. 这是标准的Java泛型,工作正常。
However if the wildcard becomes nested, it no longer works: 但是,如果通配符变为嵌套,则它不再起作用:
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. 这会导致编译器错误。
I've tried a variety of casts and wildcard permutations, but I'm unable to get this working. 我尝试了各种演员表和通配符排列,但我无法使其正常工作。 I don't recall seeing this issue before, and I've worked with generics for years.
我不记得以前看过这个问题了,多年来我一直在使用仿制药。 Am I just too tired and missing something obvious?
我是否太累了,错过了一些明显的东西?
So the problem is, doSomething
could be implemented as: 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);
}
You need to decide what you actually mean. 你需要决定你的意思。
Probably something like: 可能是这样的:
public void doSomething(Map<String, ? extends Set<? extends Number>> map) {}
这对你有用:
public void doSomething(Map<String, ? extends Set<? extends Number>> map) {}
使代码工作创建HashMap为:
Map<String, Set<? extents Number>> map = new HashMap<String, Set<? extents Number>>();
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