java - ArrayList 是否允许添加字符串？

Question

我遇到了下面的代码，一个向 List 添加元素的简单示例

    List list = new ArrayList<Integer>();
    ListIterator<Integer> litr = null;
    list.add("A");

    list.add("1");

    list.add(5);

    litr = list.listIterator();
    while(litr.hasNext()){
        System.out.println("UIterating " + litr.next());
    }

我原以为它会抛出一个 ClassCastException，但它却将其写入了控制台

A
1
5

这看起来很奇怪。当我尝试时:

 List<Integer> list = new ArrayList<Integer>();

我遇到编译时错误。

如果有人能解释如何将 String 对象添加到 ArrayList 中，我将不胜感激

Answer 1

您将新的 ArrayList 分配给了一个未类型化的列表。通用类型限制不适用于无类型列表，它可以让你在其中放入任何你想要的东西。编译器不会跟踪您的非类型化列表引用了用泛型类型声明的内容。

在任何情况下这都不会产生 ClassCastException，泛型只影响编译。在运行时

将类型放在列表变量上的情况:

List<Integer> list = new ArrayList<Integer>();

是首选，它应该生成一个编译器错误，告诉您您在集合中放入了错误的类型。

在 this article 中描述了遗留、非通用代码和通用代码如何互操作。 :

In proper generic code, Collection would always be accompanied by a type parameter. When a generic type like Collection is used without a type parameter, it's called a raw type.

Most people's first instinct is that Collection really means Collection<Object>. However, as we saw earlier, it isn't safe to pass a Collection<Part> in a place where a Collection<Object> is required. It's more accurate to say that the type Collection denotes a collection of some unknown type, just like Collection<?>.

But wait, that can't be right either! Consider the call to getParts(), which returns a Collection. This is then assigned to k, which is a Collection<Part>. If the result of the call is a Collection<?>, the assignment would be an error.

In reality, the assignment is legal, but it generates an unchecked warning. The warning is needed, because the fact is that the compiler can't guarantee its correctness. We have no way of checking the legacy code in getAssembly() to ensure that indeed the collection being returned is a collection of Parts. The type used in the code is Collection, and one could legally insert all kinds of objects into such a collection.

So, shouldn't this be an error? Theoretically speaking, yes; but practically speaking, if generic code is going to call legacy code, this has to be allowed. It's up to you, the programmer, to satisfy yourself that in this case, the assignment is safe because the contract of getAssembly() says it returns a collection of Parts, even though the type signature doesn't show this.