10进制是人类最熟悉的数字计算
2进制是机器最基本的单位
16进制呢?是为了方便表示最基本的数据传输单位字节,因为一个字节8位,16进制2位就能轻松表示1个字节。
比如 15
16进制:print 0x0F
2进制:print 0b1111
255
16进制:print 0xFF
2进制:print 0b11111111
如何把一个数字转换成字节数组?
以10001为例:
i = 10001
bin(i):
'0b10011100010001'
int两个字节表示:
- 高位:100111
- 低位:00010001
获取高位的值:big = i>>8 //位移8位
获取低位的值:little = (byte)i17 //强制转换成byte会丢弃超过byte范围的高位值
decode:
result = big<<8 | little
print result
这里解释一下:
全部代码:
public class BruteForceCoding { private final static int BSIZE = Byte.SIZE / Byte.SIZE; // 1 private final static int SSIZE = Short.SIZE / Byte.SIZE; // 2 private final static int ISIZE = Integer.SIZE / Byte.SIZE; // 4 private final static int LSIZE = Long.SIZE / Byte.SIZE; // 8 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(); } // Warning: Untested preconditions (e.g., 0 <= size <= 8) 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; } // Warning: Untested preconditions (e.g., 0 <= size <= 8) public static long decodeIntBigEndian(byte[] val, int offset, int size) { long rtn = 0; for (int i = 0; i < size; i++) { rtn = (rtn << Byte.SIZE) | ((long) val[offset + i] & BYTEMASK); } return rtn; } public static void main(String[] args) { // System.out.println(Long.toBinaryString(10001)); // System.out.println(Long.toBinaryString(17)); // System.out.println((byte) 10001); byte byteVal = 101; // one hundred and one short shortVal = 10001; // ten thousand and one int intVal = 100000001; // one hundred million and one long longVal = 1000000000001L; // one trillion and one 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)); // Decode several fields long value = decodeIntBigEndian(message, BSIZE, SSIZE); System.out.println("Decoded short = " + value); value = decodeIntBigEndian(message, BSIZE + SSIZE + ISIZE, LSIZE); System.out.println("Decoded long = " + value); // Demonstrate dangers of conversion offset = 4; value = decodeIntBigEndian(message, offset, BSIZE); System.out.println("Decoded value (offset " + offset + ", size " + BSIZE + ") = " + value); byte bVal = (byte) decodeIntBigEndian(message, offset, BSIZE); System.out.println("Same value as byte = " + bVal); //10001 } }
简单解释一下:
byteArrayToDecimalString():
该方法把给定数组中的每个字节作为一个无符号十进制数打印出来。BYTEMASK 的作用是防止在字节数值转换成 int 类型时,发生符号扩展(sign-extended),即转换成无符号整型。
encodeIntBigEndian():
赋值语句的右边,首先将数值向右移动,以使我们需要的字节处于该数值的低 8 位中。然后,将移位后的数转换成 byte 型,并存入字节数组的适当位置。在转换过程中,除了低 8位以外,其他位都将丢弃。这个过程将根据给定数值所占字节数迭代进行。该方法还将返回存入数值后字节数组中新的偏移位置,因此我们不必做额外的工作来跟踪偏移量。
decodeIntBigEndian():
根据给定数组的字节大小进行迭代,通过每次迭代的左移操作,将所取得字节的值累积到一个 long 型整数中。