Java IO.pdf - Nguyen Dang Binh

Java I/O
legally unencumbered—that is, not covered by any patents—lossless data-compression library
for use on virtually any computer hardware and operating system."
Without going into excessive detail, zip, gzip, and zlib all compress data in more or less the
same way. Repeated bit sequences in the input data are replaced with pointers back to the first
occurrence of that bit sequence. Other tricks are used, but this is basically how these
compression schemes work and has certain implications for compression and decompression
code. First, you can't randomly access data in a compressed file. To decompress the nth byte
of data, you must first decompress bytes 1 through n-1 of the data. Second, a single twiddled
bit doesn't just change the meaning of the byte it's part of. It also changes the meaning of
bytes that come after it in the data, since subsequent bytes may be stored as copies of the
previous bytes. Therefore, compressed files are much more susceptible to corruption than
uncompressed files. For more general information about compression and archiving
algorithms and formats, the comp.compression FAQ is a good place to start. See
http://www.faqs.org/faqs/compression-faq/part1/preamble.html.
9.1.1 Deflating Data
The Deflater class contains methods to compress blocks of data. You can choose the
compression format, the level of compression, and the compression strategy. There are nine
steps to deflating data with the Deflater class:
1. Construct a Deflater object.
2. Choose the strategy (optional).
3. Set the compression level (optional).
4. Preset the dictionary (optional).
5. Set the input.
6. Deflate the data repeatedly until needsInput() returns true.
7. If more input is available, go back to step 5 to provide additional input data.
Otherwise, go to step 8.
8. Finish the data.
9. If there are more streams to be deflated, reset the deflater.
More often than not, you don't use this class directly. Instead, you use a Deflater object
indirectly through one of the compressing stream classes like DeflaterInputStream or
DeflaterOutputStream. These classes provide more convenient programmer interfaces for
stream-oriented compression than the raw Deflater methods.
9.1.1.1 Constructing deflaters
There are three Deflater() constructors:
public Deflater(int level, boolean useGzip)
public Deflater(int level)
public Deflater()
The most general constructor allows you to set the level of compression and the format used.
Compression level is specified as an int between and 9. is no compression; 9 is maximum
compression. Generally, the higher the compression level, the smaller the output will be and
the longer the compression will take. Four mnemonic constants are available to select
particular levels of compression. These are:
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