NOTE
NIO Buffer
ByteBuffer position, limit, capacity, flip/clear/compact, and heap versus direct buffers.
This is a historical learning note and may contain outdated or incomplete understanding.
NIO buffers track capacity, position, and limit. Writes advance the position; flip() changes the buffer from writing into it to reading from the written region.
clear() prepares the whole buffer for new writes without zeroing its bytes. compact() preserves unread bytes while making remaining capacity available for more input.
Heap buffers are backed by managed Java memory. Direct buffers can reduce copying in some native I/O paths but use native memory and have different allocation/lifetime costs. Measure before assuming direct is always faster.