为什么我们不能做 Listmylist = ArrayList();

假设我们可以.那么这个程序应该没问题:

Suppose we could. Then this program would have to be fine:

ArrayList<Banana> bananas = new ArrayList<Banana>();
List<Fruit> fruit = bananas;
fruit.add(new Apple());

Banana banana = bananas.get(0);

这显然不是类型安全的 - 您最终在香蕉集合中得到了一个苹果.

That's clearly not type safe - you've ended up with an apple in the collection of bananas.

你可以做的是:

List<? extends Fruit> fruit = new ArrayList<Banana>();

这是安全的，因为编译器不会让您尝试将添加到水果列表中.它知道这是一个某种水果的列表，所以你可以这样写:

this is safe, because the compiler won't then let you try to add to the list of fruit. It knows that it's a list of some kind of fruit, so you could write:

Fruit firstFruit = fruit.get(0);

但它不知道它的列表具体是哪种水果，并确保您不会做错事.

but it doesn't know what exact kind of fruit it's a list of, and make sure you can't do the wrong thing.

请参阅 Java 泛型常见问题解答另一种解释.