Why is long 2500000000 overflowing?

Question

My understanding is that a long in java has a max value of 2^63-1. Any ideas why when storing 50000^2=2500000000 in a long I get -1794967296 considering 2500000000 is less than 2^63-1? Thanks

    long l = 50000 * 50000;
    System.out.println(l);

I expect the output to be 2500000000, but the actual output is -1794967296.

that's not long overflow, it's int overflow as you're doing int multiplication. You're using int literals, not long literals. — Hovercraft Full Of Eels, May 07 '19 at 01:18
`50000` is `int` literal not `long` so you are doing `int` * `int` which is also `int` (regardless of values used and received, that is why overflow happens). Try with `50000L*50000L`. — Pshemo, May 07 '19 at 01:19
ok. that worked. How do I do that for variables? eg: long l = 50000; long n = l * l; — yonSon, May 07 '19 at 01:22
@Yoni, that example works as expected, `long n = l * l;` produces `2500000000`, demo https://repl.it/@downshift/HumiliatingAltruisticAggregator — chickity china chinese chicken, May 07 '19 at 01:27
ok I used Long.valueOf(n) in the end. Thanks fr all the help. much appreciated — yonSon, May 07 '19 at 01:29

Behrang · Answer 1 · 2019-08-17T07:06:24.173

According to the Java Language Specification:

If an integer multiplication overflows, then the result is the low-order bits 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.

You are multiplying two integers that overflow, then store the result in a long, which won't help.

You either have to explicitly use long literals:

 long l = 50000L * 50000L;
 long l = 50000L * 50000; // this also works

or cast to long before multiplying:

 long l = ((long) 50000) * ((long)50000);
 long l = ((long) 50000) * 50000; // this also works

See the spec to see why it is enough to only explicitly use one long literal or one casting to long.

Why is long 2500000000 overflowing?

1 Answers1