Java 循环效率(“for"与“foreach") [英] Java loop efficiency ("for" vs. "foreach")

问题描述

(问那些熟悉 JVM 编译和优化技巧的人的问题... :-)

(A question for those who know well the JVM compilation and optimization tricks... :-)

是否有任何for"和foreach"模式明显优于另一个?

Is there any of the "for" and "foreach" patterns clearly superior to the other?

考虑以下两个例子:

public void forLoop(String[] text)
{
    if (text != null)
    {
        for (int i=0; i<text.length; i++)
        {
            // Do something with text[i]
        }
    }
}

public void foreachLoop(String[] text)
{
    if (text != null)
    {
        for (String s : text)
        {
            // Do something with s, exactly as with text[i]
        }
    }
}

forLoop 比 foreachLoop 快还是慢?

假设在这两种情况下 text 数组都不需要任何健全性检查，是否有明显的赢家或仍然太接近而无法拨打电话?

Assuming that in both cases the text array did not need any do sanity checks, is there a clear winner or still too close to make a call?

如某些答案中所述，数组的性能应该相同，而foreach"模式对于抽象数据类型(如列表)可能稍好一些.另请参阅此答案 讨论主题.

As noted in some of the answers, the performance should be identical for arrays, whereas the "foreach" pattern could be slightly better for Abstract Data Types like a List. See also this answer which discusses the subject.

推荐答案

来自 JLS 第 14.14.2 节:

否则，表达式必须有一个数组类型，T[].让 L1 ... Lm 是紧接在增强的 for 语句之前的标签序列(可能是空的).那么增强的for语句的含义由以下基本的for语句给出:

Otherwise, the Expression necessarily has an array type, T[]. Let L1 ... Lm be the (possibly empty) sequence of labels immediately preceding the enhanced for statement. Then the meaning of the enhanced for statement is given by the following basic for statement:

T[] a = Expression;
L1: L2: ... Lm:
for (int i = 0; i < a.length; i++) {
        VariableModifiersopt Type Identifier = a[i];
        Statement
}

换句话说，我希望它们最终被编译成相同的代码.

In other words, I'd expect them to end up being compiled to the same code.

绝对有一个明显的赢家:增强的 for 循环更具可读性.这应该是您的主要关注点 - 只有当您证明最易读的表单没有达到您想要的效果时，您才应该考虑对这类事情进行微优化.

There's definitely a clear winner: the enhanced for loop is more readable. That should be your primary concern - you should only even consider micro-optimizing this sort of thing when you've proved that the most readable form doesn't perform as well as you want.

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