在什么情况下，while可以比while更有效率？ [英] In what situations can do-while be more efficient than while?

问题描述

vs。与do-while

在功能块为空时，同时与do-while在功能上等效 >，尽管看起来更自然：

While and do-while are functionally equivalent when the blocks are empty, although while seems more natural:

do {} while (keepLooping());
while (keepLooping()) {}

一个典型的while / do用例-同时使用空块是用compareAndSet（CAS）强制更新原子对象。例如，下面的代码将以线程安全的方式递增 a ：

One typical use case of a while/do-while with an empty block is to force an update of atomic objects with a compareAndSet (CAS). For example the code below will increment a in a thread-safe way:

int i;
AtomicInteger a = new AtomicInteger();
while (!a.compareAndSet(i = a.get(), i + 1)) {}

上下文

java.util.concurrent的多个部分使用 do {}而（。 ..）成语，用于CAS操作和 ForkJoinPool 的javadoc说明：

Several parts of java.util.concurrent use the do {} while (...) idiom for CAS operations and the javadoc of ForkJoinPool explains:

有几次发生不寻常的 do {}而（！cas ...）是强制更新CAS的最简单方法ed变量。

There are several occurrences of the unusual do {} while (!cas...) which is the simplest way to force an update of a CAS'ed variable.

因为他们承认这很不正常，所以我认为它们的意思是 best 而不是最简单的。

Since they admit it is unusual, I suppose they meant best rather than simplest.

问题

在某些情况下做{} while（！cas）比 while（！cas）{} 更有效，原因何在？ / p>

Are there situations where do {} while (!cas) can be more efficient than while (!cas) {} and for what reasons?

推荐答案

所以'do while'意味着它将在while循环中运行一次代码。然后，如果条件为真，则仅在while循环内运行代码。

So a 'do while' means it will run the code in the while loop once. Then, it only runs the code inside the while loop if the condition is true.

简单演示

boolean condition = false;

do{
  System.out.Println("this text displayed");
}while(condition == true);

输出显示的文字

正常

while(condition == true){
 System.out.Println("this text displayed");
}

输出

• *由于条件为假，因此不显示任何输出。

为什么或在何处使用do ，我没有满足您的需要，所以我无法为您提供帮助。只是确定问题/需求并使用您所知道的知识来解决问题，就像乐高玩具一样，机械式的不是障碍。

Why or where you would use do while, I havn't come accross the need to so I can't help you there. It's just a matter of identifying a problem/need, and using what you know to solve it.similar to lego-the mechanical kind not 'blocks'.

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