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java.util.zip

Class Adler32

Implemented Interfaces:
Checksum
public class Adler32
extends Object
implements Checksum
Computes Adler32 checksum for a stream of data. An Adler32 checksum is not as reliable as a CRC32 checksum, but a lot faster to compute.

The specification for Adler32 may be found in RFC 1950. (ZLIB Compressed Data Format Specification version 3.3)

From that document:

"ADLER32 (Adler-32 checksum) This contains a checksum value of the uncompressed data (excluding any dictionary data) computed according to Adler-32 algorithm. This algorithm is a 32-bit extension and improvement of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073 standard.

Adler-32 is composed of two sums accumulated per byte: s1 is the sum of all bytes, s2 is the sum of all s1 values. Both sums are done modulo 65521. s1 is initialized to 1, s2 to zero. The Adler-32 checksum is stored as s2*65536 + s1 in most- significant-byte first (network) order."

"8.2. The Adler-32 algorithm

The Adler-32 algorithm is much faster than the CRC32 algorithm yet still provides an extremely low probability of undetected errors.

The modulo on unsigned long accumulators can be delayed for 5552 bytes, so the modulo operation time is negligible. If the bytes are a, b, c, the second sum is 3a + 2b + c + 3, and so is position and order sensitive, unlike the first sum, which is just a checksum. That 65521 is prime is important to avoid a possible large class of two-byte errors that leave the check unchanged. (The Fletcher checksum uses 255, which is not prime and which also makes the Fletcher check insensitive to single byte changes 0 <-> 255.)

The sum s1 is initialized to 1 instead of zero to make the length of the sequence part of s2, so that the length does not have to be checked separately. (Any sequence of zeroes has a Fletcher checksum of zero.)"

Since:
JDK 1.1
See Also:
InflaterInputStream, DeflaterOutputStream

Constructor Summary

Adler32()
Creates a new instance of the Adler32 class.

Method Summary

long
getValue()
Returns the Adler32 data checksum computed so far.
void
reset()
Resets the Adler32 checksum to the initial value.
void
update(byte[] buffer)
Updates the checksum with the bytes taken from the array.
void
update(byte[] buf, int off, int len)
Updates the checksum with the bytes taken from the array.
void
update(int bval)
Updates the checksum with the byte b.

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

Adler32

public Adler32()
Creates a new instance of the Adler32 class. The checksum starts off with a value of 1.

Method Details

getValue

public long getValue()
Returns the Adler32 data checksum computed so far.
Specified by:
getValue in interface Checksum

reset

public void reset()
Resets the Adler32 checksum to the initial value.
Specified by:
reset in interface Checksum

update

public void update(byte[] buffer)
Updates the checksum with the bytes taken from the array.
Parameters:
buffer - an array of bytes

update

public void update(byte[] buf,
                   int off,
                   int len)
Updates the checksum with the bytes taken from the array.
Specified by:
update in interface Checksum
Parameters:
buf - an array of bytes
off - the start of the data used for this update
len - the number of bytes to use for this update

update

public void update(int bval)
Updates the checksum with the byte b.
Specified by:
update in interface Checksum
Parameters:
bval - the data value to add. The high byte of the int is ignored.
Adler32.java - Computes Adler32 data checksum of a data stream Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc. This file is part of GNU Classpath. GNU Classpath is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version. GNU Classpath is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with GNU Classpath; see the file COPYING. If not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. Linking this library statically or dynamically with other modules is making a combined work based on this library. Thus, the terms and conditions of the GNU General Public License cover the whole combination. As a special exception, the copyright holders of this library give you permission to link this library with independent modules to produce an executable, regardless of the license terms of these independent modules, and to copy and distribute the resulting executable under terms of your choice, provided that you also meet, for each linked independent module, the terms and conditions of the license of that module. An independent module is a module which is not derived from or based on this library. If you modify this library, you may extend this exception to your version of the library, but you are not obligated to do so. If you do not wish to do so, delete this exception statement from your version.

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