Bitmask in big endian

Question

This isn't a question as much as it's a sanity check!

If you needed to read 4 bytes into Java as a bitmask in Big endian and those bytes were:

0x00, 0x01, 0xB6, 0x02.

Making that into an int would be: 112130

The binary would be: 00000000000000011010011000000010

The endian of a series of bytes wouldn't affect the bit position, would it?

Thanks

Tony

Answer 1

Endian-ness reflects the ordering of bytes, but not the ordering of the bits within those bytes.

Let's say I want to represent the (two-byte) word 0x9001. If I just type this out in binary, that would be 1001000000000001.

If I dump the bytes (from lower address to higher) on a big-endian machine, I would see 10010000 00000001.

If I dump the bytes (from lower address to higher) on a little-endian machine, I would see 00000001 10010000.

Answer 2

In general, if the thing you're reading from is giving you whole bytes, then you don't need to worry about the order of bits making up those bytes: it is just the order of the bytes that matters, as you correctly suppose.

The time you might have to worry about the "endianness" of individual bits is where you're actually reading/writing a stream of bits rather than whole bytes (e.g. if you were writing a compression algorithm that operated at the bit level, you'd have to make a decision about what order to write the bits in).

Answer 3

The only thing you have to pay attention is how exactly you "read 4 bytes into Java" - that's where endianness matters and you can mess it up (DataInputStream assumes big endian). Once the value you've read has become the int 112130, you're set.