Java - for 循环声明之外的逗号运算符 [英] Java - comma operator outside for loop declaration

问题描述

我知道我可以像这样使用逗号运算符

I know I can use the comma operator like this

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

但有些文章似乎建议逗号运算符可以在for循环声明之外使用，例如

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 ;

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

这对于代码混淆竞赛或混淆我的同事来说是一个巧妙的技巧，但两个示例都无法编译(Java 1.6，Eclipse Juno)，错误是赋值的左侧必须是一个多变的".我尝试查看编译器设置，看看是否可以禁止以防止不良代码，但没有运气.

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.

怎么了?逗号运算符是否是后来更改的旧规范的一部分?编写这些示例的人是否使用允许这样做的不同 Java 设置?

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?

推荐答案

怎么了?

某些在 C 中有效的技巧在 Java 中无效.

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

逗号运算符是否是后来更改的旧规范的一部分?

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

这在 Java AFAIK 中从未奏效.

This never worked in Java AFAIK.

编写这些示例的人是否使用允许这样做的不同 Java 设置?

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

认为 Java 就像 C 或 C++ 是一个常见的错误，因为它们很相似.SO 上的很大一部分编码错误是由于人们试图用 Java 编写 C++ 并且在它没有达到他们的预期时感到困惑.

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.

顺便说一句:我犯了同样的错误，认为 C++ 就像 Java，因为我对 C++ 的了解不是最新的.

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

但是有些技巧可以在 Java 中运行，但可能不能在 C 中运行.

However some tricks which working Java but perhaps not C.

您可以使用所有货币符号，也可以使用几个看起来最多相同的货币符号.

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

例如

if( ⁀ ‿ ⁀ == ⁀ ⁔ ⁀ || ¢ + ￠== ₡)

您可以使用不可见的字符和 c 对，它们在打印时颠倒行的其余部分的顺序.;)

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

这个程序编译、运行并打印所有可以在 Java 标识符中使用的奇数字符

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);

这让它几乎太容易了.

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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