在Java中將無符號的32位整數轉換為字節數組，長度為4字節，反之亦然

Question

一個無符號的32位整數，min為0，max為2的32平方減1。我想將其限制為長度為4個字節的字節數組，反之亦然。 當我運行主方法打擊時，一切都是-1嗎？我很困惑。 如何從字節數組中獲取最大值並將其轉換為字節數組？

public static void main(String[] args) {
    long l = (long) Math.pow(2, 32);
    l--;
    byte[] bs = toBytes(l);
    for(byte b:bs){
        System.out.println(b);
    }
    System.out.println("------");
    byte[] arr = { (byte) 0xff, (byte) 0xff,(byte) 0xff,(byte) 0xff };
    System.out.println(fromByteArray(arr));

}

static byte[] toBytes(long i)
{
  byte[] result = new byte[4];

  result[0] = (byte) (i >> 24);
  result[1] = (byte) (i >> 16);
  result[2] = (byte) (i >> 8);
  result[3] = (byte) (i /*>> 0*/);

  return result;
}

 static long fromByteArray(byte[] bytes) {
     return (bytes[0]& 0xFF) << 24 | (bytes[1] & 0xFF) << 16 | (bytes[2] & 0xFF) << 8 | (bytes[3] & 0xFF);
}

Answer 1

您確實需要研究二進制補碼的工作原理，以了解計算機中的帶符號數字。

但是，要展示有符號和無符號值之間的差異，請參見此處：

long maxAsLong = 4294967295L; // Unsigned int max value
System.out.println("maxAsLong = " + maxAsLong);

int max = (int) maxAsLong;
System.out.println("max (unsigned) = " + Integer.toUnsignedString(max) +
                                 " = " + Integer.toUnsignedString(max, 16));
System.out.println("max (signed) = " + Integer.toString(max) +
                               " = " + Integer.toString(max, 16));

byte[] maxBytes = ByteBuffer.allocate(4).putInt(max).array();
System.out.print("maxBytes (unsigned): ");
for (byte b : maxBytes)
    System.out.print(Byte.toUnsignedInt(b) + " ");
System.out.println();
System.out.print("maxBytes (signed): ");
for (byte b : maxBytes)
    System.out.print(b + " ");
System.out.println();

int max2 = ByteBuffer.allocate(4).put(maxBytes).rewind().getInt();
System.out.println("max2 (unsigned) = " + Integer.toUnsignedString(max2) +
                                  " = " + Integer.toUnsignedString(max2, 16));

輸出量

maxAsLong = 4294967295
max (unsigned) = 4294967295 = ffffffff
max (signed) = -1 = -1
maxBytes (unsigned): 255 255 255 255 
maxBytes (signed): -1 -1 -1 -1 
max2 (unsigned) = 4294967295 = ffffffff