A byte in Java ranges from -128 to +127. So a Java byte is not unsigned but signed. To get values above 127, you'll have to cast the byte to a type which can hold larger numbers, such as a short. However just casting it will make it stay negative, so you'll have to make some "addition", like this:
byte b = (byte)203; // turns into -53
// convert to short
short s = b<0?(short)(256 - (-b)):b; // s now contains 203
edit. Should probably say that I'm not the most experienced programmer in Java, so there might be a nicer/better way to do this.