Java未绑定的通配符泛型

Question

Are there any advantages of using wildcard-type generics in the Bar class over completely skipping them? 在Bar类中使用通配符类型泛型相比完全跳过它们有什么优势吗？

public class Foo<T> {}

public interface Bar {
    public void addFoo(Foo<?> foo);
    public Foo<?> getFoo(String name);
}

Answer 1

There are multiple advantages. 有许多优点。

They won't produce compiler warnings like using the raw type would 它们不会产生像使用原始类型那样的编译器警告
They give more type safety. 它们提供更多类型的安全性 For example, consider if Foo was List instead. 例如，考虑Foo为List 。 If you used List instead of List<?> , you could do this: 如果您使用List而不是List<?> ，则可以执行以下操作：
```
 myBar.getFoo("numbers").add("some string"); 
```
even if the list was only supposed to contain Number s. 即使列表只应该包含Number秒。 If you returned a List<?> , then you would not be able to add anything to it (except null ) since the type of list is not known. 如果您返回了List<?> ，那么您将无法向其添加任何内容（ null除外），因为列表类型未知。
They document something completely different than a raw type, namely that Foo is typed on some unknown but specific type . 它们记录的内容与原始类型完全不同，即Foo是在某种未知但特定类型上键入的 。