Multiplication operation in Java is resulting in negative value

Question

Why does the below calculation produce a negative value?

long interval = 0;

interval = ((60000 * 60) * 24) * 30;

Answer 1

Every expression in there is being evaluated (at compile-time, of course; it's a constant) as int * int instead of long * long. The result overflows at some point. So just use L to make all the operand literals long:

interval = ((60000L * 60L) * 24L) * 30L;

Of course you could get away with only making some of the operands longs, but I tend to find it's easier to just change everything.

Having said all of this, if you're looking for "30 days-worth of milliseconds" it would be better to use:

long interval = TimeUnit.DAYS.toMillis(30);