Java 8功能接口分配上下文 [英] Java 8 Functional interface assignment context

问题描述

问题在于功能接口的分配上下文-

Question is regarding assignment context of functional interface-

Predicate<String> p = String::isEmpty;

在String类中的isEmpty方法声明为-public boolean isEmpty(){}时工作正常.

Works fine where the isEmpty method declaration in String class is - public boolean isEmpty(){}.

如果我尝试在自定义类中声明相同的内容-

If I try to declare same in custom class like -

class Test{
   public boolean isEmpty(){
       ...
   }
}

并进行相同的分配-

Predicate<String> p = Test::isEmpty;

这将是编译错误-

Test类型未定义适用的isEmpty(String) 这里

The type Test does not define isEmpty(String) that is applicable here

Predicate<T>表示一个参数的谓词(布尔值函数)，函数方法为boolean test(T t){}.

And Predicate<T> represents a predicate (boolean-valued function) of one argument and functional method is boolean test(T t){}.

有什么解释吗?而且我想念什么吗?

Any explanation? And am I missing anything?

推荐答案

您应该具有:

Predicate<Test> p = Test::isEmpty;

而不是

Predicate<String> p = Test::isEmpty;

class Test {
    public boolean isEmpty(){
           ...
    }
}

那为什么要有Predicate<String>?

请参见方法引用指南.这里的情况是第三种情况对特定类型任意对象的实例方法的引用".

See the tutorial of method references. What you have here is the 3rd case "Reference to an instance method of an arbitrary object of a particular type".

拥有

Predicate<String> p = String::isEmpty();
String s = "";

调用p.test(s);与callign s.isEmpty();相同，因此这就是为什么不能给String作为参数来调用Test中的方法的原因.

Calling p.test(s); is the same as callign s.isEmpty(); so that's why you cannot give as argument a String to call a method from Test.

如果String和Test都将使用方法boolean isEmpty()实现接口Empty，然后具有Predicate<Empty>，则有可能具有通用的Predicate.然后p.test(string)和p.test(test)都可以工作；否则，Java不会具有强类型，并且不支持鸭子类型.

It would have been possible to have a common Predicate if both String and Test would implement an interface Empty with the method boolean isEmpty() and then to have Predicate<Empty>. Then both p.test(string) and p.test(test) would work; otherwise it won't, Java has strong typing and does not support duck typing.

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