1) First, remember left-shift only operates on integer types.

2) Second, remember most if not all integer types use 2's complement, where leading bit represents sign.

Now the dark side - Multiplication by 2 can move the original number between positive and negative universes.

In this context, Left shift is simpler to understand than multiplication. Left shift by 1 is equivalent to (xxx *2). Confirmed on Sun.

Q: how about multiplication by 4 or 3?

%%A: I feel multiplication by even numbers tend to give the same flipping problem.

Sun says -

If an integer multiplication overflows, then the result is the low-order bits (possibly with a leading '1') of the mathematical product as represented in some sufficiently large two's-complement format. As a result, if overflow occurs, then the sign of the result may not be the same as the sign of the mathematical product of the two operand values.

## Sunday, February 6, 2011

### dark side of left shift operator - java

Labels: javaMisc

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