简体繁体 English

Java泛型：了解有界通配符

[英]Java Generics : Understanding Bounded Wildcards

原文 2017-11-09 16:35:15 0 2 java/ generics/ inheritance/ wildcard

In Generics chapter, one sentence got my attention. 在“ 泛型”一章中，一句话引起了我的注意。 For the following declaration List<? extends Shape> 对于以下声明List<? extends Shape> List<? extends Shape> , it says: List<? extends Shape> ，它说：

Note: It could be Shape itself, or some subclass; 注意：它可以是Shape本身，也可以是某些子类。 it need not literally extend Shape . 它不需要从字面上扩展Shape 。

What does not literally in this context mean ? 在此上下文中的字面意思是什么？

2 个解决方案

Shape could be an interface and <? extends Shape> Shape可以是接口， <? extends Shape> <? extends Shape> could be an instance of a class that implements the Shape interface. <? extends Shape>可以是实现Shape接口的类的实例。

It means that the class does not have to extend Shape , ie be a direct subclass of Shape . 这意味着该类不必 extend Shape ，即作为直接子Shape 。 It could also be Shape itself, or it could be an indirect subclass of Shape , or Shape could be an interface which the class implements . 它也可以Shape自身，也可以是间接子Shape ，或Shape可以是一个interface ，其类implements 。 What the author is trying to convey is that the keyword extends is used out of convenience (adding a new keyword to a language is expensive) and familiarity, it does not mean that every type that can be used has to have exactly the string extends Shape in its definition. 什么是作者试图传达的是关键字extends用于进出方便（添加新的关键字的语言是昂贵的）和熟悉，这并不意味着每一个可以使用的类型都有一个完全相同的字符串extends Shape在其定义中。