Expressive Puzzlers - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 17

rather than sign extension. The resulting int value is 65535 , which is just what the program prints.
Although there is a simple rule describing the sign extension behavior of widening primitive
conversions between signed and unsigned integral types, it is best not to write programs that depend
on it. If you are doing a widening conversion to or from a char , which is the only unsigned integral
type, it is best to make your intentions explicit.
If you are converting from a char value c to a wider type and you don't want sign extension,
consider using a bit mask for clarity, even though it isn't required:
int i = c & 0xffff;
Alternatively, write a comment describing the behavior of the conversion:
int i = c; // Sign extension is not performed
If you are converting from a char value c to a wider integral type and you want sign extension, cast
the char to a short , which is the same width as a char but signed. Given the subtlety of this code,
you should also write a comment:
int i = (short) c; // Cast causes sign extension
If you are converting from a byte value b to a char and you don't want sign extension, you must use
a bit mask to suppress it. This is a common idiom, so no comment is necessary:
char c = (char) (b & 0xff);
If you are converting from a byte to a char and you want sign extension, write a comment:
char c = (char) b; // Sign extension is performed
The lesson is simple: If you can't tell what a program does by looking at it, it probably doesn't
do what you want . Strive for clarity. Although a simple rule describes the sign extension behavior
of widening conversions involving signed and unsigned integral types, most programmers don't
know it. If your program depends on it, make your intentions clear.
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