递归类型界限的使用 [英] Uses of recursive type bounds

问题描述

我的一个朋友在Java API(

A friend of mine found this tidbit in the Java API (https://docs.oracle.com/javase/7/docs/api/java/lang/Enum.html),

Class Enum<E extends Enum<E>>

并阅读以下文章 https://docs.oracle. com/javase/tutorial/java/generics/genTypes.html 我可以理解上述行在语法上的含义，但是从给出的示例中，我无法找出Enum类以外的用例(查看源代码)

and by reading the following article https://docs.oracle.com/javase/tutorial/java/generics/genTypes.html I could understand what the aforementioned line entailed syntactically but from the examples given I could not figure out a use case for this beyond the Enum class (reviewed the source).

我想详细了解上述可能提出解决方案的可能问题.

I'd like to learn more about possible problems where the above may present a solution.

推荐答案

例如，允许子类使用自己的类型很有用

It's for example useful to to allow subclasses to use their own type

想象一个像

class Node {
    Node next;
}

如果您扩展该类，则会陷入Node.

if you extend that class, you're stuck with Node.

class SpecialNode extends Node {
    void foo() {
        // euwww
        SpecialNode nextNode = (SpecialNode) this.next;
    }
}

您也不能像这样定义它

class Node<T> {
    T next;
}

因为这将允许T使用任何内容.您确实想要一个extends Node的T，或者您不能在不强制转换的情况下将Node中的T用作Node.不过，它适用于子班.

because that would allow anything for T. You really want a T that extends Node or you can no longer use T as Node from within Node without casting. It would work for child classes though.

通过使用递归边界，如

class Node<T extends Node<T>> {
    T next;
}

您将T限制为您自己或您自己的子类，然后您就可以这样做

You limit T to yourself or subclasses of yourself which then allows you to do

class SpecialNode extends Node<SpecialNode> {
    void foo() {
        SpecialNode nextNode = this.next; // type-safe!
    }
}

这样，父类和子类都可以完全抽象安全地访问其抽象级别上的所有内容.

That way both parent and child class can access everything on their abstraction level fully typesafe.

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