Java Reference
In-Depth Information
Adler32 and CRC32 checksums for HELLO
Adler32: 72089973
CRC32: 3242484790
Adler32
is faster than
CRC32
. However,
CRC32
gives a more robust checksum. Checksum is frequently used to
check for data corruption.
CheckedInputStream
and
CheckedOutputStream
are two concrete decorator classes in the
InputStream/OutputStream
class family. They are in the
java.util.zip
package. They work with a
Checksum
object.
Note that
Checksum
is an interface, and the
Adler32
and
CRC32
classes implement the interface.
CheckedInputStream
computes a checksum as you read data from a stream and
CheckedOutputStream
computes the checksum as you
write data to a stream. The
ZipEntry
class lets you compute the
CRC32
checksum for an entry in a ZIP file using its
getCrc()
method.
Compressing Byte Arrays
You can use the
Deflater
and
Inflater
classes in the
java.util.zip
package to compress and decompress data
in a byte array, respectively. These classes are the basic building blocks for compression and decompression in
Java. You may not use them directly very often. You have other high-level, easy-to-use classes in Java to deal with
data compression. Those classes are
DeflaterInputStream
,
DeflaterOutputStream
,
GZIPInputStream
,
ZipFile
,
GZIPOutputStream
,
ZipInputStream
, and
ZipOutputStream
. I will discuss these classes in detail in subsequent sections.
Using the
Deflater
and
Inflater
classes is not straightforward. You need to use the following steps to compress
data in a byte array.
1.
Create a
Deflater
object.
2.
Set the input data to be compressed using the
setInput()
method.
3.
Call the
finish()
method indicating that you have supplied all input data.
4.
Call the
deflate()
method to compress the input data.
5.
Call the
end()
method to end the compression process.
You can create an object of the
Deflater
class using one of its constructors.
// Uses the no-args constructor
Deflater compressor = new Deflater();
Other constructors of the
Deflater
class let you specify the level of compression. You can specify the
compression level using one of the constants in the
Deflater
class. Those constant are
BEST_COMPRESSION
,
BEST_
SPEED
,
DEFAULT_COMPRESSION
, and
NO_COMPRESSION
. There is a trade-off in choosing between the best compression
and the best speed. The best speed means lower compression ratio and the best compression means slower
compression speed.
// Uses the best compression
Deflater compressor = new Deflater(Deflater.BEST_COMPRESSION);
By default, the compressed data using the
Deflater
object will be in the ZLIB format. If you want the compressed
data to be in GZIP or PKZIP format, you need to specify that by using the
boolean
flag as
true
in the constructor.
// Uses the best speed compression and GZIP format
Deflater compressor = new Deflater(Deflater.BEST_SPEED, true);
You can supply the input data to the
Deflater
object in a byte array.
