常见的MD5等摘要运算的结果都是字节数组，通常我们见到的是转换后的十六进制字符串，今天就分享一下转换的代码。

package com.jianggujin.codec;

public class JHex {

   private static final char[] DIGITS_UPPER = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D',
         'E', 'F' };

   public static byte[] decode(char[] data) {
      int len = data.length;
      if ((len & 0x01) != 0) {
         throw new IllegalArgumentException("Odd number of characters.");
      }
      byte[] out = new byte[len >> 1];
      // two characters form the hex value.
      for (int i = 0, j = 0; j < len; i++) {
         int f = toDigit(data[j], j) << 4;
         j++;
         f = f | toDigit(data[j], j);
         j++;
         out[i] = (byte) (f & 0xFF);
      }
      return out;
   }

   public static char[] encode(byte[] data) {
      return encode(data, DIGITS_UPPER);
   }

   private static char[] encode(byte[] data, char[] toDigits) {
      int l = data.length;
      char[] out = new char[l << 1];
      // two characters form the hex value.
      for (int i = 0, j = 0; i < l; i++) {
         out[j++] = toDigits[(0xF0 & data[i]) >>> 4];
         out[j++] = toDigits[0x0F & data[i]];
      }
      return out;
   }

   public static String encodeString(byte[] data) {
      return new String(encode(data));
   }

   private static int toDigit(char ch, int index) {
      final int digit = Character.digit(ch, 16);
      if (digit == -1) {
         throw new IllegalArgumentException("Illegal hexadecimal character " + ch + " at index " + index);
      }
      return digit;
   }
}