什么是闭包？ java有闭包吗？ [英] What is a closure? Does java have closures?

问题描述

我正在阅读面向对象的Javascript，并发现了闭包的概念。我不太明白为什么和什么时候使用。其他语言如Java也有闭包？我基本上想了解如何知道闭包的概念可以帮助我改进我的编码。

I was reading Object Oriented Javascript and found the concept of closures. I didn't quite understand why and when it is used. Do other languages like Java also have closures? I basically want to understand how knowing the concept of closures can help me improve my coding.

推荐答案

闭包是一个第一类函数带有绑定变量。

A closure is a first class function with bound variables.

大致意思是：

• 闭包作为其他函数的参数


• 闭包存储在创建时存在的词汇作用域中的一些变量的值

Java最初对闭包没有句法支持（这些在Java 8中引入），尽管使用匿名内部类来模拟它们是相当常见的做法。这里有一个例子：

Java initially didn't have syntactic support for closures (these were introduced in Java 8), although it was fairly common practice to simulate them using anonymous inner classes. Here's an example:

import java.util.Arrays;
import java.util.Comparator;

public class StupidComparator { 
    public static void main(String[] args) {
        // this is a value used (bound) by the inner class
        // note that it needs to be "final"
        final int numberToCompareTo=10;

        // this is an inner class that acts like a closure and uses one bound value
        Comparator<Integer> comp=new Comparator<Integer>() {
            public int compare(Integer a, Integer b) {
                int result=0;
                if (a<numberToCompareTo) result=result-1;
                if (b<numberToCompareTo) result=result+1;
                return result;
            }
        };

        Integer[] array=new Integer[] {1,10, 5 , 15, 6 , 20, 21, 3, 7};

        // this is a function call that takes the inner class "closure" as a parameter
        Arrays.sort(array,comp);

        for (int i:array) System.out.println(i);
    }
}

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