Java协方差 [英] Java covariance

问题描述

我正在努力解决这个问题。说我有以下代码：

I'm having a hard time trying to figure this out. Say I have the following code:

class Animal { }
class Mammal extends Animal { }
class Giraffe extends Mammal { }
...
public static List<? extends Mammal> getMammals() { return ...; }
...

public static void main(String[] args) {
    List<Mammal> mammals = getMammals(); // compilation error
}

为什么分配会导致编译错误？错误类似于：

Why does the assignment result in a compilation error? The error is something like:

Type mismatch: cannot convert from List<capture#4-of ? extends Mammal> to List<Mammal>

根据我对协方差的理解， getMammals（）方法返回列表，它将始终包含 Mammal 对象，因此它应该是可分配的。我缺少什么？

According to my understanding of covariance, the getMammals() method returns a list that will always contain Mammal objects so it should be assignable. What am I missing?

推荐答案

因为 getMammals 可能会返回列表< Giraffe> ，如果可以转换为列表< Mammal> ，那么你就可以添加 Zebra 。您不能被允许将 Zebra 添加到 Giraffe 的列表中，是吗？

Because getMammals could return a List<Giraffe>, and if that was convertable to List<Mammal> then you'd be able to add a Zebra to it. You can't be allowed to add a Zebra to a list of Giraffe, can you?

class Zebra extends Mammal { }

List<Giraffe> giraffes = new List<Giraffe>();

List<Mammal> mammals = giraffes; // not allowed

mammals.add(new Zebra()); // would add a Zebra to a list of Giraffes

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