Java 泛型 - ArrayList 初始化 [英] Java generics - ArrayList initialization

问题描述

众所周知，arraylist init.应该是这样

It is known that arraylist init. should be like this

ArrayList<A> a = new ArrayList<A>();
ArrayList<Integer> a = new ArrayList<Number>(); // compile-time error

那么，为什么java允许这些?

so, why does java allow these ?

1. ArrayList<? extends Object> a1 = new ArrayList<Object>();
2. ArrayList<?> a2 = new ArrayList<Integer>();

那么，如果它们是正确的，为什么不允许这些?

then, if they are correct why doesn't allow these ?

1. a1.add(3);
2. a2.add(3);

编译器消息是:ArrayList 类型中的方法 add(int, capture#1-of ? extends Object) 不适用于参数 (int)

the compiler message is : The method add(int, capture#1-of ? extends Object) in the type ArrayList is not applicable for the arguments (int)

更一般的

  1. a1.add(null e);
  2. a2.add(? e);

我读到了这个，但很高兴收到你的来信.谢谢

I read about this but it is good to hear from you. thanks

另一个有趣的地方是:

 ArrayList<ArrayList<?>> a = new ArrayList<ArrayList<?>>(); // correct
 ArrayList<?> a = new ArrayList<?>(); // wrong. I know it's reason but I have some 
question in my mind that mentioned above 

推荐答案

您不能将 List 分配给 List 类型的引用因为 List 允许 Integer 以外的数字类型.如果您被允许这样做，则将允许以下内容:

You can't assign a List<Number> to a reference of type List<Integer> because List<Number> allows types of numbers other than Integer. If you were allowed to do that, the following would be allowed:

List<Number> numbers = new ArrayList<Number>();
numbers.add(1.1); // add a double
List<Integer> ints = numbers;
Integer fail = ints.get(0); // ClassCastException!

类型 List 保证它包含的任何内容都是 Integer.这就是为什么您可以在不进行强制转换的情况下从中获取 Integer 的原因.如您所见，如果编译器允许将其他类型的 List(例如 Number)分配给 List，则保证将被打破.

The type List<Integer> is making a guarantee that anything it contains will be an Integer. That's why you're allowed to get an Integer out of it without casting. As you can see, if the compiler allowed a List of another type such as Number to be assigned to a List<Integer> that guarantee would be broken.

将 List 分配给类型的引用，例如 List 或 List 是合法的，因为 ? 的意思是给定类型的一些未知子类型"(其中类型是 Object 在只有 ? 和 Number 在 ? extends Number 的情况下).

Assigning a List<Integer> to a reference of a type such as List<?> or List<? extends Number> is legal because the ? means "some unknown subtype of the given type" (where the type is Object in the case of just ? and Number in the case of ? extends Number).

由于 ? 表示您不知道List 将接受什么特定类型的对象，因此添加任何内容都是不合法的但是 null 给它.但是，您可以从它检索任何对象，这是使用 的目的吗?扩展 X 有界通配符类型.请注意，对于 ?super X 有界通配符类型...一个 List 是一些未知类型的列表，它至少是 Integer 的超类型".虽然你不知道它是什么类型的 List(可能是 List, List, List