Question

Hey I cant figure out what is wrong here.

Write to file:

byte[] dim = new byte[2];
dim[0] = (byte) deImgWidth; // Input 384
dim[1] = (byte) deImgHeight; // Input 216
out.write(dim);

Read from File

byte[] file = new byte[(int) f.length()];
FileInputStream fs = new FileInputStream(f);
fs.read(file);
deImgWidth = ((file[0]) & 0xFF); // output 128
deImgHeight = ((file[1]) & 0xFF); // output 216

How come I can retrieve the same deImgHeight value but not the same deImgWidth value?

Answer 1

384 doesn’t fit into an unsigned byte while 216 does. That’s why when casting the former you must lose information.

Narrowing conversions just preserve the lowest bits of a number, so if you do the extra & 0xFF when you read your value, you can later restore the sign (since Java uses two’s complement for negative numbers). 216 = 0b11011000 (fits into 8 bits) can be losslessly transformed, but 384 = 0b110000000 (that’s 9 bits) – when you take the lower 8 bits, you end up with 128.