java 实现协方差_Java协方差

最新推荐文章于 2021-02-24 17:43:58 发布

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#java 实现协方差

本文讨论了Java中泛型的协变问题，通过一个具体的代码示例展示了为什么在将`List<Giraffe>`赋值给`List<Mammal>`时会出现编译错误。解释了由于类型安全的考虑，即使`Mammal`是`Giraffe`的父类，也不能直接赋值。同时，文章强调了这种类型的转换可能导致潜在的类型不匹配风险，并提供了相应的解决方案。

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 mammals = getMammals(); // compilation error

}

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

Type mismatch: cannot convert from List to List

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?

解决方案

Because getMammals could return a List, and if that was convertable to List 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 giraffes = new List();

List mammals = giraffes; // not allowed

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