[英]How does method reference casting work?
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. 两者都是具有相同签名的实例方法。 Why does one work and the other doesn't? 为什么一个工作而另一个不工作?
Signature of String::toUpperCase
: String toUpperCase();
String::toUpperCase
签名: String toUpperCase();
There are 3 constructs to reference a method: 有3个构造引用方法:
object::instanceMethod
Class::staticMethod
Class::instanceMethod
The line: 这条线:
Capitalizer c = String::toUpperCase; //This works
use 3'rd construct - Class::instanceMethod
. 使用3'rd构造 - Class::instanceMethod
。 In this case first parameter becomes the target of the method . 在这种情况下, 第一个参数成为方法的目标 。 This construct is equivalent (translates) to following Lambda: 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: 这条线:
c = Main::toUpperCase; //Compile error
Translates to: 翻译为:
(Main m) -> m.toUpperCase();
Which does not work with the Capitalizer
interface. 哪个不适用于Capitalizer
界面。 You could verify this by changing Capitalizer
to: 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。
You have a method which 你有一个方法
public String capitalize(String name);
Takes a String
and returns a String
. 获取String
并返回String
。 Such a method can have a number of patterns. 这种方法可以具有许多模式。
A constructor 构造函数
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。
You should change: 你应该改变:
public String toUpperCase()
to 至
public static String toUpperCase(String text)
You should read the java tutorial on method references . 您应该阅读有关方法引用的java教程。 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). 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. 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。 The method reference would invoke the method a.compareToIgnoreCase(b). 方法引用将调用方法a.compareToIgnoreCase(b)。
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