是列表<狗>List<Animal> 的子类?为什么 Java 泛型不是隐式多态的? [英] Is List<Dog> a subclass of List<Animal>? Why are Java generics not implicitly polymorphic?

问题描述

我对 Java 泛型如何处理继承/多态感到有些困惑.

I'm a bit confused about how Java generics handle inheritance / polymorphism.

假设以下层次结构 -

动物(父母)

狗 - 猫(儿童)

所以假设我有一个方法doSomething(Listanimals).根据继承和多态性的所有规则，我会假设 List<Dog> is 一个 List 和一个 List<Cat> is List<Animal> - 所以任何一个都可以传递给这个方法.不是这样.如果我想实现这种行为，我必须通过说 doSomething(List animals) 明确告诉该方法接受 Animal 的任何子类的列表.

So suppose I have a method doSomething(List<Animal> animals). By all the rules of inheritance and polymorphism, I would assume that a List<Dog> is a List<Animal> and a List<Cat> is a List<Animal> - and so either one could be passed to this method. Not so. If I want to achieve this behavior, I have to explicitly tell the method to accept a list of any subclass of Animal by saying doSomething(List<? extends Animal> animals).

我了解这是 Java 的行为.我的问题是为什么?为什么多态性一般是隐含的，而泛型却必须指定?

I understand that this is Java's behavior. My question is why? Why is polymorphism generally implicit, but when it comes to generics it must be specified?

推荐答案

不，List 不是 List代码>.考虑一下你可以用 List<Animal> 做什么 - 你可以向其中添加 any 动物......包括一只猫.现在，你能合乎逻辑地将一只猫添加到一窝小狗中吗?绝对不是.

No, a List<Dog> is not a List<Animal>. Consider what you can do with a List<Animal> - you can add any animal to it... including a cat. Now, can you logically add a cat to a litter of puppies? Absolutely not.

// Illegal code - because otherwise life would be Bad
List<Dog> dogs = new ArrayList<Dog>(); // ArrayList implements List
List<Animal> animals = dogs; // Awooga awooga
animals.add(new Cat());
Dog dog = dogs.get(0); // This should be safe, right?

突然间你有一只非常困惑的猫.

Suddenly you have a very confused cat.

现在，您不能将 Cat 添加到 List 因为你不知道它是一个 List<Cat>.您可以检索一个值并知道它将是一个 Animal，但您不能添加任意动物.List - 在这种情况下，您可以安全地添加一个 Animal 到它，但是您不知道可以从中检索到什么，因为它可能是一个 列出<对象>.

Now, you can't add a Cat to a List<? extends Animal> because you don't know it's a List<Cat>. You can retrieve a value and know that it will be an Animal, but you can't add arbitrary animals. The reverse is true for List<? super Animal> - in that case you can add an Animal to it safely, but you don't know anything about what might be retrieved from it, because it could be a List<Object>.

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