What is the purpose of ByteBuffer's flip method? (And why is it called "flip"?)

java flip bytebuffer
Suzan Cioc picture Suzan Cioc · Feb 10, 2013 · Viewed 55k times · Source

Why does ByteBuffer's flip() method called "flip"? What is "flipped" here? According to apidoc, two successive flips won't restore original state, and multiple flips will probably tend limit() to become zero.

Can I "unflip" somehow to reuse bytes went out of a limit?

Can I concatenate tail to be flipped with some other data?

Answer

nneonneo picture nneonneo · Feb 10, 2013

One fairly common use case for the ByteBuffer is to construct some data structure piece-by-piece and then write that whole structure to disk. flip is used to flip the ByteBuffer from "reading from I/O" (putting) to "writing to I/O" (getting): after a sequence of puts is used to fill the ByteBuffer, flip will set the limit of the buffer to the current position and reset the position to zero. This has the effect of making a future get or write from the buffer write all of what was put into the buffer and no more.

After finishing the put, you might want to reuse the ByteBuffer to construct another data structure. To "unflip" it, call clear. This resets the limit to the capacity (making all of the buffer usable), and the position to 0.

So, a typical usage scenario:

ByteBuffer b = new ByteBuffer(1024);
for(int i=0; i<N; i++) {
    b.clear();
    b.put(header[i]);
    b.put(data[i]);
    b.flip();
    out.write(b);
}

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