java - 位摆弄 : Encoding Unsigned Primitives

Question

下面是一个类，它将无符号基元类型编码为字节数组，并将编码的字节数组返回为十进制字符串。我从概念上理解 encodeIntBigEndian 和 byteArrayToDecimalString 的工作原理。不过，我希望能澄清以下问题:

1. Why/how shifting the val by ((size - i - 1) * Byte.SIZE) produces an unsigned java byte value.
2. Also, why does applying a byte mask of 0xff convert the byte to a decimal string value.

public class BruteForceCoding {
 private static byte byteVal = 101; // one hundred and one
 private static short shortVal = 10001; // ten thousand and one
 private static int intVal = 100000001; // one hundred million and one
 private static long longVal = 1000000000001L;// one trillion and one

 private final static int BSIZE = Byte.SIZE / Byte.SIZE;
 private final static int SSIZE = Short.SIZE / Byte.SIZE;
 private final static int ISIZE = Integer.SIZE / Byte.SIZE;
 private final static int LSIZE = Long.SIZE / Byte.SIZE;

 private final static int BYTEMASK = 0xFF; // 8 bits
 public static String byteArrayToDecimalString(byte[] bArray) {
  StringBuilder rtn = new StringBuilder();
  for (byte b : bArray) {
   rtn.append(b & BYTEMASK).append(" ");
  }
  return rtn.toString();
 }

 public static int encodeIntBigEndian(byte[] dst, long val, int offset, int size) {
  for (int i = 0; i < size; i++) {
   dst[offset++] = (byte) (val >> ((size - i - 1) * Byte.SIZE));
  }
  return offset;
 }

 public static void main(String[] args) {
  byte[] message = new byte[BSIZE + SSIZE + ISIZE + LSIZE];
  // Encode the fields in the target byte array
  int offset = encodeIntBigEndian(message, byteVal, 0, BSIZE);
  offset = encodeIntBigEndian(message, shortVal, offset, SSIZE);
  offset = encodeIntBigEndian(message, intVal, offset, ISIZE);
  encodeIntBigEndian(message, longVal, offset, LSIZE);
  System.out.println("Encoded message: " + byteArrayToDecimalString(message));
 }
}

Answer 1

1)它本身并没有。它的作用是将值向下移动一个字节为单位。但是，当与丢弃高位的 (byte) 转换结合使用时，这相当于从值中提取各个字节的移位和掩码操作。

2)事实并非如此。它屏蔽掉高位，保留值的低八位(一个字节)——与前一种情况中执行的转换为字节的操作相同。但是，默认情况下，将字节呈现为字符串会生成一个包含表示其二进制值(从 0 到 255)的十进制数的字符串，并且在调用 .append() 时会隐式发生这种情况。