Java - comma operator outside for loop declaration

问题

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

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?

回答1:

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

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