Multiplication of two int's gets negative

Question

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:

Answer 1

2^31-1 = 2147483647  < 21476160 * 100 = 2147616000

Answer 2

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);

Answer 3

21476160 * 100 = 2 147 616 000 is greater than 2 147 483 647, the max int.

You're overflowing.

Use long for your calculations.

Answer 4

You should use a long -- 2147760000 in binary is 10000000 00000100 00110111 10000000 and since the most significant bit is 1 it is interpreted as a negative number.