Java 未绑定通配符泛型 [英] Java unbound wildcard generics

问题描述

在 Bar 类中使用通配符类型的泛型比完全跳过它们有什么优势吗?

Are there any advantages of using wildcard-type generics in the Bar class over completely skipping them?

public class Foo<T> {}

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

推荐答案

优势多多.

• 它们不会像使用原始类型那样产生编译器警告

• 它们提供了更多的类型安全性.例如，考虑一下 Foo 是否是 List.如果你使用 List 而不是 List，你可以这样做:

• They won't produce compiler warnings like using the raw type would

• They give more type safety. For example, consider if Foo was List instead. If you used List instead of List<?>, you could do this:

myBar.getFoo("numbers").add("some string");

即使列表仅应该包含Number.如果您返回一个 List，那么您将无法向其中添加 任何内容(除了 null)，因为列表未知.

even if the list was only supposed to contain Numbers. 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.

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