Wrtie unsigned char and unsigned int into a binary file

Question

Hi, I need write some data into some specific file format (.ov2) for other applications to read.
Here is the requirement of the data:
First byte: An unsigned char value, always "2" means simple POI entry
Next 4 byte: An unsigned int value,
Next 4 byte: A signed integer value.
Next 4 byte: A signed integer value
then next : A zero-terminated Ascii string
Here is my code:
                   String name = "name";
                   byte type = 2;
                 int size = name.length() + 13 +1; //1+4+4+4 and there is a 0 at last
                 int a = 1;
                 int b =2;
                 ds.writeByte(type); //1 byte, need save it as unsigned char
                     ds.writeInt(size); //4 //need to be an unsignged int
                     ds.writeInt(ilont); //4 //signed int
                     ds.writeInt(ilatt); //4 //signed int
                     //write zero-terminated ascii string
                     for(int n=0; n<name.length(); n++){
                          ds.writeByte(name.charAt(n));
                     ds.writeByte(0);                    This code could not get the correct result and I think I must do some sign to unsign conversion, but I don't know how to do it. Any help?
Thanks for you attention.

Answer 1

You don't have to do anything special at all. What's in a int or a byte is what you, as a programmer, say it is. Java treats int's as signed but, in fact, with 2's complement arithmatic most operations are exactly the same. Load and store, add and subtract - it makes no difference whether a variable is signed or unsigned. It's only when you convert the number, for example to a wider type or to a string representation that you need to know the difference.
When you read an unsigned byte and you want to widen it to an integer you have to clip the top bytes of the integer e.g.
int len = input.get() & 0xff;This is because, since java sees a byte as signed it will extend the sign bit into the top bytes of the int. "& 0xff" forces them to zero.
Writing it, you don't have to do anything special at all - it will autmatically get clipped. The above is probably the only concesion you'll ever need to make to unsigned binary. (You'd need to do the same thing from unsigned int to long, but this almost never happens).
When I read ov2 files I used a mapped byte buffer, because it allows you to set either big-endian or little-endian and I wasn't sure which ov2 files used. (Sorry, I've forgotten).