Multiplication of two int's gets negative

问题

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?

回答1:

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);

回答2:

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

You're overflowing.

Use long for your calculations.

回答3:

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

回答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.

来源：https://stackoverflow.com/questions/6050796/multiplication-of-two-ints-gets-negative