泛型:列表<?延伸动物>和 List<Animal> 一样吗? [英] Generics : List&lt;? extends Animal&gt; is same as List&lt;Animal&gt;?

问题描述

我只是想了解 Java 泛型中的 extends 关键字.

I am just trying to understand the extends keyword in Java Generics.

列表 意味着我们可以在 List 中填充任何对象，其中 IS A Animal

List<? extends Animal> means we can stuff any object in the List which IS A Animal

那么下面的意思是否相同:

then won't the following also mean the same thing:

List<Animal>

有人可以帮我知道上面两者之间的区别吗?对我来说 extends 在这里听起来有些多余.

Can someone help me know the difference between the above two? To me extends just sound redundant here.

谢谢！

推荐答案

List 是 List，但不是 List 的子类型.

List<Dog> is a subtype of List<? extends Animal>, but not a subtype of List<Animal>.

为什么 List 不是 List 的子类型?考虑以下示例:

Why is List<Dog> not a subtype of List<Animal>? Consider the following example:

void mySub(List<Animal> myList) {
    myList.add(new Cat());
}

如果允许您将 List 传递给此函数，则会出现运行时错误.

If you were allowed to pass a List<Dog> to this function, you would get a run-time error.

现在，如果我们使用 List 相反，将发生以下情况:

Now, if we use List<? extends Animal> instead, the following will happen:

void mySub(List<? extends Animal> myList) {
    myList.add(new Cat());     // compile error here
    Animal a = myList.get(0);  // works fine 
}

你可以向这个函数传递一个List，但是编译器意识到向列表中添加一些东西会给你带来麻烦.如果你使用 super 而不是 extends(允许你传递一个 List)，情况正好相反.

You could pass a List<Dog> to this function, but the compiler realizes that adding something to the list could get you into trouble. If you use super instead of extends (allowing you to pass a List<LifeForm>), it's the other way around.

void mySub(List<? super Animal> myList) {
    myList.add(new Cat());     // works fine
    Animal a = myList.get(0);  // compile error here, since the list entry could be a Plant
}

这背后的理论是协方差.

这篇关于泛型:列表<?延伸动物>和 List<Animal> 一样吗?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！