方法参考投射如何工作？

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
    }
}

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

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?

Signature of String::toUpperCase: String toUpperCase();

String :: toUpperCase的签名:String toUpperCase();

3 个解决方案

#1

There are 3 constructs to reference a method:

有3个构造引用方法:

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

The line:

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

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:

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):

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

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

此Lambda表达式起作用,因为Lambda将String作为参数并返回String结果 - 根据Capitalizer接口的要求。

The line:

c = Main::toUpperCase; //Compile error

Translates to:

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

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

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

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

After this change Main::toUpperCase will compile.

在此更改后,Main :: toUpperCase将进行编译。

#2

You have a method which

你有一个方法

public String capitalize(String name);

Takes a String and returns a String. Such a method can have a number of patterns.

获取String并返回String。这种方法可以具有许多模式。

A constructor

c = String::new; // calls new String(String)
// or
c = s -> new String(s);

A function on String which takes no arguments

String上的函数,不带参数

c = String::toLowerCase; // instance method String::toLowerCase()
// or
c = s -> s.toLowerCase();

of a method which takes a String as the only argument

将String作为唯一参数的方法

// method which takes a String, but not a Main
public static String toUpperCase(String str) { 

c = Main::toUpperCase;
// or
c = s -> toUpperCase(s);

In every case, the method referenced has to take the String.

在每种情况下,引用的方法都必须采用String。

If not you can do this instead.

如果不是,你可以这样做。

c = s -> capitalize(); // assuming Main.capitalize() is static

This tells the compiler to ignore the input.

这告诉编译器忽略输入。

#3

You should change:

你应该改变:

public String toUpperCase()

to

public static String toUpperCase(String text)

---

You should read the java tutorial on method references. The different kind of method references and there is a similar example with String::compareToIgnoreCase (Reference to an Instance Method of an Arbitrary Object of a Particular Type).

您应该阅读有关方法引用的java教程。不同类型的方法引用和String :: compareToIgnoreCase(引用特定类型的任意对象的实例方法)有类似的例子。

The equivalent lambda expression for the method reference String::compareToIgnoreCase would have the formal parameter list (String a, String b), where a and b are arbitrary names used to better describe this example. The method reference would invoke the method a.compareToIgnoreCase(b).

方法引用String :: compareToIgnoreCase的等效lambda表达式将具有形式参数列表(String a,String b),其中a和b是用于更好地描述此示例的任意名称。方法引用将调用方法a.compareToIgnoreCase(b)。