Java – Working with Unsigned Values

bit-manipulationjava

I'm parsing unsigned bits from a DatagramSocket. I have a total of 24bits (or 3 bytes) coming in – they are: 1 unsigned 8bit integer followed by a 16bit signed integer. But java never stores anything more than a signed byte into a byte/byte array? When java takes in these values, do you lose that last 8th bit? 

DatagramSocket serverSocket = new DatagramSocket(666);
        byte[] receiveData = new byte[3]; <--Now at this moment I lost my 8th bit

        System.out.println("Binary Server Listing on Port: "+port);

        while (true)
        {
            DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
            serverSocket.receive(receivePacket);
            byte[] bArray = receivePacket.getData();
            byte b = bArray[0];

        }

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Did I now lose this 8th bit since I turned it into a byte? Was it wrong I initialized a byte array of 3 bytes?

Best Answer

When java takes in these values, do you lose that last 8th bit?

No. You just end up with a negative value when it's set.

So to get a value between 0 and 255, it's simplest to use something like this:

int b = bArray[0] & 0xff;

First the byte is promoted to an int, which will sign extend it, leading to 25 leading 1 bits if the high bit is 1 in the original value. The & 0xff then gets rid of the first 24 bits again :)

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