Hex.java

public class Hex {

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

  private static final char[] FIRST_CHAR = new char[256];
  private static final char[] SECOND_CHAR = new char[256];

  static {
    for (int i = 0; i < 256; i++) {
      FIRST_CHAR[i] = HEX_DIGITS[(i >> 4) & 0xF];
      SECOND_CHAR[i] = HEX_DIGITS[i & 0xF];
    }
  }

  private static final byte[] DIGITS = new byte['f' + 1];

  static {
    for (int i = 0; i <= 'f'; i++) {
      DIGITS[i] = -1;
    }
    for (byte i = 0; i < 10; i++) {
      DIGITS['0' + i] = i;
    }
    for (byte i = 0; i < 6; i++) {
      DIGITS['A' + i] = (byte) (10 + i);
      DIGITS['a' + i] = (byte) (10 + i);
    }
  }

  /**
  * Convert an int [0-255] to a hexadecimal string representation.
  *
  * @param value int value.
  */
  public static String byte2Hex(int value) {
    if (value > 255 || value < 0) {
      throw new IllegalArgumentException("The int converting to hex should be in range 0~255");
    }
    return String.valueOf(FIRST_CHAR[value]) + String.valueOf(SECOND_CHAR[value]);
  }

  /**
  * Quickly converts a byte array to a hexadecimal string representation.
  *
  * @param array byte array, possibly zero-terminated.
  */
  public static String encodeHex(byte[] array, boolean zeroTerminated) {
    char[] cArray = new char[array.length * 2];

    int j = 0;
    for (int i = 0; i < array.length; i++) {
      int index = array[i] & 0xFF;
      if (index == 0 && zeroTerminated) {
        break;
      }

      cArray[j++] = FIRST_CHAR[index];
      cArray[j++] = SECOND_CHAR[index];
    }

    return new String(cArray, 0, j);
  }

  /**
  * Quickly converts a hexadecimal string to a byte array.
  */
  public static byte[] decodeHex(String hexString) {
    int length = hexString.length();

    if ((length & 0x01) != 0) {
      throw new IllegalArgumentException("Odd number of characters.");
    }

    boolean badHex = false;
    byte[] out = new byte[length >> 1];
    for (int i = 0, j = 0; j < length; i++) {
      int c1 = hexString.charAt(j++);
      if (c1 > 'f') {
        badHex = true;
        break;
      }

      final byte d1 = DIGITS[c1];
      if (d1 == -1) {
        badHex = true;
        break;
      }

      int c2 = hexString.charAt(j++);
      if (c2 > 'f') {
        badHex = true;
        break;
      }

      final byte d2 = DIGITS[c2];
      if (d2 == -1) {
        badHex = true;
        break;
      }

      out[i] = (byte) (d1 << 4 | d2);
    }

    if (badHex) {
      throw new IllegalArgumentException("Invalid hexadecimal digit: " + hexString);
    }

    return out;
  }
}