两个int的乘法变为负数 [英] Multiplication of two int's gets negative

问题描述

我正在编写一个小的下载管理器，当我尝试以百分比计算下载进度时，我得到一个有趣的输出。这是我用来计算它的：

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);

字节计数器是一个int（它计算从 InputStream 中读取的总字节数， size 是下载文件的长度（以字节为单位）。

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.

这适用于小型下载。但是当我获得更大的文件（40MB）时，它开始制作有趣的东西。计算的输出如下所示：

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
[...]

我不知道为什么，但计算变得消极。 因为正常的整数应该没有数字，直到2 ³¹ -1，这不应该是问题根。但是我错过了什么？

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?

推荐答案

请参阅 http：// en .wikipedia.org / wiki / Arithmetic_overflow

要修复java，请尝试使用 long 代替。

To fix in java, try using a long instead.

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

或相反的操作顺序

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

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