java - 问一个比较基础的问题，为什么0x8FFFFFF 等于 128 MB ？

Question

怎么换算的啊？进制数之间可以转换，为什么两个不同计量单位之间也可以转换啊？
代码如下：

import java.nio.*;
import java.nio.channels.*;
import java.io.*;
import static net.mindview.util.Print.*;

public class LargeMappedFiles {
  static int length = 0x8FFFFFF; // 128 MB
  public static void main(String[] args) throws Exception {
    MappedByteBuffer out =
      new RandomAccessFile("test.dat", "rw").getChannel()
      .map(FileChannel.MapMode.READ_WRITE, 0, length);
    for(int i = 0; i < length; i++)
      out.put((byte)'x');
    print("Finished writing");
    for(int i = length/2; i < length/2 + 6; i++)
      printnb((char)out.get(i));
  }
}

Answer 1

It should be a clerical error. You can use a calculator to calculate that 128MB is 0x8000000, or you can test it yourself and the generated test.data will be 143MB

RandomAccessFile randomAccessFile = new RandomAccessFile("test.data", "rw");
FileChannel channel = randomAccessFile.getChannel();
ByteBuffer buffer = channel.map(FileChannel.MapMode.READ_WRITE, 0, 0x8FFFFFF);

Answer 2

Convert to binary. . Count to 0

Answer 3

Even if the unit of 0x8FFFFFF is B, it is about 144MB after conversion. 0x8000000 B is equal to 128MB

Answer 4

It should be that the code and comments are out of sync. For example, it was originally 0x8000000, which is 128MB. Later, it was changed to 0x8FFFFFFF for some reason, but I forgot to change the comment. Or simply a clerical error. This feels like a typical "useless" comment.

Answer 5

0x8FFFFFF is just a number, without unit, how can it be equal to 128M

Answer 6

This seems to be a problem about conversion between complement code, inverse code and source code.