Java开发网 - 小心用j=j++语句

Topic: 小心用j=j++语句

1.小心用j=j++语句	Copy to clipboard
Posted by: awrong Posted on: 2006-08-25 01:06 看到一个例子： int j=0; for(int i=0;i<100;i++) j=j++; System.out.println(j); 对于不同的程序语言，其运算结果会不一样，在java中运行j值为0。对于初学java者，或者你以前学过其它程序语言的，一不小心可能会犯这样的错误。

2.Re:小心j=j++语句 [Re: awrong]	Copy to clipboard
Posted by: why Posted on: 2006-08-25 01:57 I haven't seen anyone code something like j = j++ in real life. I don't think any developer would do so.

3.Re:小心j=j++语句 [Re: awrong]

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Posted by: ranchgirl
Posted on: 2006-08-25 05:47


int j=0;
for(int i=0;i<100;i++)
  j=j++;
System.out.println(j);

No matter what language you are using, you should never, ever write such garbage code!!!!!! Period!

1) In C/C++, this is undefined!!! you should not change a variable value twice in one statement. Read here
http://www.research.att.com/~bs/bs_faq2.html#this

2) In Java, it is defined according to the left-to-right-evaluation rule, however, you should never, ever write such garbage code either.

Read here!
http://bobcat.webappcabaret.net/javachina/faq/07.htm#lef_Q2

Thanks!

4.Re:小心j=j++语句 [Re: ranchgirl]

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Posted by: zcjl
Posted on: 2006-08-25 09:58

关心这个问题没错，不过不应该只是提出这个问题来，而应该自己尝试去了解bytecode和jvm，然后分析产生这个现象的原因
甚至，你也可以比较c/c++编译器实现原理的不同，以至于在c/c++中，这段代码的结果是不一样的

btw，这个话题在cjd已经出现得够多了，请先搜索一下
记得ranchgirl同学已经多次重申never, ever write such garbage code
^_^

5.Re:小心用j=j++语句 [Re: awrong]	Copy to clipboard
Posted by: lwfllr Posted on: 2006-09-04 20:19 呃~~ 刚开始还真搞错了~~ 后来想想才发现取值>赋值后J还是0~~ 郁闷~~

6.Re:小心用j=j++语句 [Re: awrong]	Copy to clipboard
Posted by: qingsure Posted on: 2006-09-04 21:18 果然，差点犯错。谢谢提醒！

7.Re:小心用j=j++语句 [Re: awrong]	Copy to clipboard
Posted by: leange Posted on: 2006-09-05 21:11 掌握运算先后顺序后也不会那么难理解吧？？认真看书！！

8.Re:小心用j=j++语句 [Re: awrong]	Copy to clipboard
Posted by: 紫杉 Posted on: 2006-10-11 09:15 顶.讨论的不错.

9.Re:小心用j=j++语句 [Re: awrong]	Copy to clipboard
Posted by: cxp108 Posted on: 2006-10-12 08:45 这是一元运算符的计算顺序问题。 ++这个运算符优先顺序永远是最低的。这个例子可能有点极端，但这个问题新手还是要多留意。