<?> 和有什么区别?和 <?扩展对象>在 Java 泛型中? [英] What's the difference between <?> and <? extends Object> in Java Generics?

问题描述

我曾见过通配符表示任何对象 - 但最近看到了使用:

I've seen the wildcard used before to mean any object - but recently saw a use of:

<? extends Object>

既然所有的对象都扩展了对象，那么这两种用法是同义词吗?

Since all objects extend Object, are these two usages synonymous?

推荐答案

和 是同义词，正如您所期望的那样.

<?> and <? extends Object> are synonymous, as you'd expect.

在某些情况下，extends Object 实际上并不是多余的泛型.例如，<T extends Object &Foo> 将导致 T 在擦除时变成 Object，而使用 它将变成 Foo 下擦除.(如果您试图保持与使用 Object 的前泛型 API 的兼容性，这可能很重要.)

There are a few cases with generics where extends Object is not actually redundant. For example, <T extends Object & Foo> will cause T to become Object under erasure, whereas with <T extends Foo> it will become Foo under erasure. (This can matter if you're trying to retain compatibility with a pre-generics API that used Object.)

来源:http://download.oracle.com/javase/教程/额外/泛型/convert.html;它解释了为什么 JDK 的 java.util.Collections 类有一个带有这个签名的方法:

Source: http://download.oracle.com/javase/tutorial/extra/generics/convert.html; it explains why the JDK's java.util.Collections class has a method with this signature:

public static <T extends Object & Comparable<? super T>> T max(
    Collection<? extends T> coll
)

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