2's complement hex number to decimal in java

This seems to trick java into converting the number without forcing a positive result:

Integer.valueOf("FFFF",16).shortValue(); // evaluates to -1 (short)

Of course this sort of thing only works for 8, 16, 32, and 64-bit 2's complement:

Short.valueOf("FF",16).byteValue(); // -1 (byte)
Integer.valueOf("FFFF",16).shortValue(); // -1 (short)
Long.valueOf("FFFFFFFF",16).intValue(); // -1 (int)
new BigInteger("FFFFFFFFFFFFFFFF",16).longValue(); // -1 (long)

Example here.

Just write a utility method:

 public static Integer twosComp(String str) throws java.lang.Exception {
       Integer num = Integer.valueOf(str, 16);
       return (num > 32767) ? num - 65536 : num;
 }

Tests:

 twosComp("7FFF") -> 32767
 twosComp("8000") -> -32768
 twosComp("FFFF") -> -1

This seems to work reasonably well. It can be fooled by passing it non-standard length strings: "FFF" maps to -1. Zero padding will correct the fault.

You are not clear on what type return you want, so I have returned Number, at whatever size is appropriate.

public Number hexToDec(String hex)  {
   if (hex == null) {
      throw new NullPointerException("hexToDec: hex String is null.");
   }

   // You may want to do something different with the empty string.
   if (hex.equals("")) { return Byte.valueOf("0"); }

   // If you want to pad "FFF" to "0FFF" do it here.

   hex = hex.toUpperCase();

   // Check if high bit is set.
   boolean isNegative =
      hex.startsWith("8") || hex.startsWith("9") ||
      hex.startsWith("A") || hex.startsWith("B") ||
      hex.startsWith("C") || hex.startsWith("D") ||
      hex.startsWith("E") || hex.startsWith("F");

   BigInteger temp;

   if (isNegative) {
      // Negative number
      temp = new BigInteger(hex, 16);
      BigInteger subtrahend = BigInteger.ONE.shiftLeft(hex.length() * 4);
      temp = temp.subtract(subtrahend);
   } else {
      // Positive number
      temp = new BigInteger(hex, 16);
   }

   // Cut BigInteger down to size.
   if (hex.length() <= 2) { return (Byte)temp.byteValue(); }
   if (hex.length() <= 4) { return (Short)temp.shortValue(); }
   if (hex.length() <= 8) { return (Integer)temp.intValue(); }
   if (hex.length() <= 16) { return (Long)temp.longValue(); }
   return temp;
}

Sample output:

"33" -> 51
"FB" -> -5
"3333" -> 13107
"FFFC" -> -4
"33333333" -> 53687091
"FFFFFFFD" -> -3
"3333333333333333" -> 3689348814741910323
"FFFFFFFFFFFFFFFE" -> -2
"33333333333333333333" -> 241785163922925834941235
"FFFFFFFFFFFFFFFFFFFF" -> -1