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javaintdownload-manager

Multiplication of two int's gets negative

I'm currently coding a little download manager and I get a funny output when I try to calculate the download-progress in percent. This is what i use to calculate it:

int progress = (byte_counter * 100) / size;
System.out.println("("+byte_counter+" * 100) = "+(byte_counter * 100)
  +" / "+size+" = "+progress);

byte-counter is an int (it counts the total bytes read from the InputStream) and size is the length of the downloaded file in bytes.

This works great with small downloads. But when i get to bigger files (40MB) it starts making funny things. The Output for the calculation looks like this:

[...]
(21473280 * 100) = 2147328000 / 47659008 = 45
(21474720 * 100) = 2147472000 / 47659008 = 45
(21476160 * 100) = -2147351296 / 47659008 = -45
(21477600 * 100) = -2147207296 / 47659008 = -45
[...]

I don't know why, but the calculation gets negative. Since an normal Integer should be fine with numbers till 231-1, this shouldn't be the problems root. But what am I missing?

Solution

  • See http://en.wikipedia.org/wiki/Arithmetic_overflow

    To fix in java, try using a long instead.

    int progress = (int) ((byte_counter * 100L) / size);

    or reverse order of operations

    int progress = (int) (((float) byte_counter) / size) * 100);