Understanding Java bytes

Question

So at work yesterday, I had to write an application to count the pages in an AFP file. So I dusted off my MO:DCA spec PDF and found the structured field BPG (Begin Pag

Answer 1

In order to obtain the binary representation of a negative number you calculate two's complement:

• Get the binary representation of the positive number
• Invert all the bits
• Add one

Let's do -72 as an example:

0100 1000    72
1011 0111    All bits inverted
1011 1000    Add one

So the binary (8-bit) representation of -72 is 10111000.

What is actually happening to you is the following: You file has a byte with value 10111000. When interpreted as an unsigned byte (which is probably what you want), this is 88.

In Java, when this byte is used as an int (for example because read() returns an int, or because of implicit promotion), it will be interpreted as a signed byte, and sign-extended to 11111111 11111111 11111111 10111000. This is an integer with value -72.

By ANDing with 0xff you retain only the lowest 8 bits, so your integer is now 00000000 00000000 00000000 10111000, which is 88.