Java - 逗号运算符，用于循环声明 [英] Java - comma operator outside for loop declaration

问题描述

（int i = 1，j = 15; j>< p> 10; i ++，j--）{
//做一些整洁的
}

<但是有些文章似乎提示逗号操作符可以在for循环声明之外使用，例如

  int j = 2，k = 4; 
 int x; 
 //带逗号运算符的赋值语句
 x = j + 1，k; 
  

来源： http://www.cs.umd.edu/~clin/MoreJava/ControlFlow/comma.html

或

  int x =（expression）？ （i ++，2）：3; 
  

来源： https://stackoverflow.com/a/12047433/1084813

这对于代码迷惑竞赛来说是一个很好的窍门，或者让我的同事迷惑不解，但是这两个例子都不会编译（Java 1.6，Eclipse Juno），错误是赋值的左边必须是变量。我试着看编译器设置，看是否可以禁止不良代码，但没有运气。

有什么错？逗号运算符是以后更改的旧规范的一部分吗？是谁写的这些例子使用不同的Java设置，允许这个人吗？
解决方案

什么是错的？

一些在C中工作的技巧在Java中不起作用。

逗号运算符是旧规范的一部分，后来发生了变化？

在Java AFAIK中。

使用不同的Java设置来编写这些示例的人员是允许的吗？

假设Java就像C或C ++一样是一个常见的错误，因为它是相似的。在SO上编码错误的很大一部分是由于人们试图用Java编写C ++，当它不能达到他们期望的时候感到困惑。

BTW：犯了同样的错误，假设C ++就像Java一样，因为我的C ++知识并不是最新的。

---

Java，但可能不是C。

您可以使用所有货币符号，或者有几个看起来相同的货币符号。

例如

  if（⁀‿⁀==⁀⁔⁀||¢+¢==₡）
  

您可以使用不可见和c对的字符，这些字符在打印时会颠倒其余行的顺序。 ;）

这个程序编译并运行并打印所有可以在Java标识符中使用的奇数字符

<$ p $ （charch= 0;ch< Character.MAX_VALUE;ch++）
if（Character.isJavaIdentifierPart（ch）& &！Character.isJavaIdentifierStart（ch））
System.out.printf（％04x<％s>％n，（int）ch，+ch ）;

这使得它变得简单。

http：// vanillajava .blogspot.co.uk / 2012/08 / uses-for-special-characters-in-java-code.html

http://vanillajava.blogspot.co.uk/2012/09/hidden-code.html

I know I can use the comma operator like this

for (int i = 1, j = 15; j>10; i++, j--) {
    // do something neat
}

but some articles seem to suggest that the comma operator can be used outside of the for loop declaration, for example

int j = 2, k = 4 ;
int x ;
// Assignment statement with comma operator
x = j + 1, k ;

source: http://www.cs.umd.edu/~clin/MoreJava/ControlFlow/comma.html

or

int x = (expression) ? (i++,2) : 3;

source: https://stackoverflow.com/a/12047433/1084813

This would be a neat trick for a code obfuscation contest or to confuse my colleagues, but neither of the examples will compile (Java 1.6, Eclipse Juno), the error is "The left-hand side of an assignment must be a variable". I tried looking at the compiler settings to see whether it could be forbidden to prevent bad code, but without luck.

What's wrong? Was the comma operator a part of an older specification which later changed? Are the people that wrote those examples using a different Java setup that allows this?

解决方案

What's wrong?

Some of the tricks which work in C don't work in Java.

Was the comma operator a part of an older specification which later changed?

This never worked in Java AFAIK.

Are the people that wrote those examples using a different Java setup that allows this?

Its a common mistake to assume Java is just like C or C++ because it is similar. A good portion of coding mistakes on SO are due to people trying to write C++ in Java and getting confused when it doesn't do what they expect.

BTW: I have made the same mistake assuming C++ is just like Java as my knowledge of C++ is not current.

---

However some tricks which working Java but perhaps not C.

You can use all currency symbols or which there are a few which look at most the same.

e.g.

if( ⁀ ‿ ⁀ == ⁀ ⁔ ⁀ || ¢ + ￠== ₡)

You can use character which are invisible and c couple which reverse the order the rest of the line when printed. ;)

This program compiles and runs and prints all the odd characters you can use in Java identifiers

for (char c‮h = 0; c‮h < Character.MAX_VALUE; c‮h++)
    if (Character.isJavaIdentifierPart(c‮h) && !Character.isJavaIdentifierStart(c‮h))
        System.out.printf("%04x <%s>%n", (int) c‮h, "" + c‮h);

which makes its almost too easy.

http://vanillajava.blogspot.co.uk/2012/08/uses-for-special-characters-in-java-code.html

http://vanillajava.blogspot.co.uk/2012/09/hidden-code.html

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