Java IO.pdf - Nguyen Dang Binh

public native synchronized void setDictionary(byte[] data,
int offset, int length)
Presetting a dictionary is never necessary and requires detailed
understanding of both the compression format used and the data to be
compressed. Putting the wrong data in your dictionary can actually
increase the file size. Unless you're a compression expert and you really
need every last byte of space you can save, I recommend letting the
deflater build the dictionary adaptively as the data is compressed.
I started with a highly compressible 44,392-byte text file (the output of
running FileDumper2.java on itself in decimal mode). Without
presetting the dictionary, it deflated to 3,859 bytes. My first attempt to
preset the dictionary to the ASCII digits, space, and \r\n actually
increased that size to 3,863 bytes. After carefully examining the data
and custom-designing a dictionary to fit it, I was able to deflate the data
to 3,852 bytes, saving a whopping 7 extra bytes or 0.18%. Of course,
the dictionary itself occupied 112 bytes, so it's truly arguable whether I
really saved anything.
Exact details are likely to vary from file to file. The only real possible
gain is for very short, very predictable files where zlib may not have
enough data to build a good dictionary before the end of stream is
reached. However, zlib uses a pretty good algorithm for building an
adaptive dictionary, and you're unlikely to do significantly better by
hand. I recommend you not worry about setting a dictionary, and simply
let the deflater build one for you.
Java I/O
If Inflater.inflate() decompresses the data later, the Inflater.getAdler() method will
return the Adler-32 checksum of the dictionary needed for decompression. However, you'll
need some other means to pass the dictionary itself between the deflater and the inflater. It is
not stored with the deflated file.
9.1.1.5 Set the input
Next you must set the input data to be deflated with one of the setInput() methods:
public void setInput(byte[] input)
public synchronized void setInput(byte[] input, int offset, int length)
The first method prepares the entire array to be deflated. The second method prepares the
specified subarray of data starting at offset and continuing for length bytes.
9.1.1.6 Deflate the data repeatedly until needsInput( ) returns true
Finally, you're ready to deflate the data. Once setInput() has filled the input buffer with
data, it is deflated through one of two deflate() methods:
public int deflate(byte[] output)
public native synchronized int deflate(byte[] output, int offset, int
length)
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