方法参考投射如何工作？ [英] How does method reference casting work?

问题描述

public class Main {
    interface Capitalizer {
        public String capitalize(String name);
    }

    public String toUpperCase() {
        return "ALLCAPS";
    }

    public static void main(String[] args) {
        Capitalizer c = String::toUpperCase; //This works
        c = Main::toUpperCase; //Compile error
    }
}

两者都是具有相同签名的实例方法。为什么一个工作而另一个不工作？

Both are instance methods with same signature. Why does one work and the other doesn't?

签名 String :: toUpperCase ： String toUpperCase（）;

推荐答案

有3种结构可供参考方法：

There are 3 constructs to reference a method:

1. object :: instanceMethod


2. Class :: staticMethod


3. Class :: instanceMethod

1. object::instanceMethod
2. Class::staticMethod
3. Class::instanceMethod

该行：

Capitalizer c = String::toUpperCase; //This works

使用3'rd构造 - Class :: instanceMethod 。在这种情况下，第一个参数成为方法的目标。此构造与以下Lambda等效（翻译）：

use 3'rd construct - Class::instanceMethod. In this case first parameter becomes the target of the method. This construct is equivalent (translates) to following Lambda:

Capitalizer = (String x) -> x.toUpperCase();

此Lambda表达式有效，因为Lambda 获取 String 作为参数并返回字符串结果 - 根据 Capitalizer 界面的要求。

This Lambda expression works because Lambda gets String as parameter and returns String result - as required by Capitalizer interface.

该行：

c = Main::toUpperCase; //Compile error

转换为：

(Main m) ->  m.toUpperCase();

哪个不适用于 Capitalizer 界面。您可以通过将 Capitalizer 更改为：

Which does not work with the Capitalizer interface. You could verify this by changing Capitalizer to:

interface Capitalizer {
    public String capitalize(Main name);
}

此更改后 Main :: toUpperCase 将编译。

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