for循环中的最终计数器？ [英] A final counter in a for loop?

问题描述

我有这段代码：

    List<Runnable> r = new ArrayList<>();
    for(int i = 0; i < 10; i++) {
        r.add(new Runnable() {

            @Override
            public void run() {
                System.out.println(i);
            }
        });
    }

显然无法编译，因为 i 需要最终才能在匿名类中使用。但我不能把它作为最终决定，因为事实并非如此。你会怎么做？一个解决方案是复制它，但我认为可能有更好的方法：

It obviously does not compile because i would need to be final to be used in the anonymous class. But I can't make it final because it is not. What would you do? A solution is to duplicate it but I thought there might be a better way:

    List<Runnable> r = new ArrayList<>();
    for(int i = 0; i < 10; i++) {
        final int i_final = i;
        r.add(new Runnable() {

            @Override
            public void run() {
                System.out.println(i_final);
            }
        });
    }

编辑只是为了说清楚，我用过为了这个例子，这里有一个Runnable，问题实际上是关于匿名类，这可能是其他任何东西。

EDIT just to make it clear, I used a Runnable here for the sake of the example, the question is really about anonymous classes, which could be anything else.

推荐答案

我认为你的解决方案是最简单的方法。

I think your solution is the simplest way.

另一种选择是将内部类的创建重构为一个为你做的工厂函数，然后你的循环本身可以干得像：

Another option would be to refactor the creation of the inner class into a factory function that does it for you, then your loop itself could be something clean like:

List<Runnable> r = new ArrayList<>();
for(int i = 0; i < 10; i++) {
    r.add(generateRunnablePrinter(i));
}

工厂函数可以声明最终参数：

And the factory function could just declare a final parameter:

private Runnable generateRunnablePrinter(final int value) {
    return new Runnable() {
       public void run() {
           System.out.println(value);
       }
    };
}

我更喜欢这种重构方法，因为它保持代码更清晰，相对自我描述并且还隐藏了所有内部类管道。

I prefer this refactored approach because it keeps the code cleaner, is relatively self descriptive and also hides away all the inner class plumbing.

随机离题：如果你认为匿名内部类等同于闭包，那么 generateRunnablePrinter 实际上是一个更高阶的函数。谁说你不能用Java做函数式编程： - ）

Random digression: if you consider anonymous inner classes to be equivalent to closures, then generateRunnablePrinter is effectively a higher order function. Who said you can't do functional programming in Java :-)

这篇关于for循环中的最终计数器？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！