Полезная информация

Java Fundamental Classes Reference

Previous Chapter 18 Next
 

18. The java.util.zip Package

Contents:
CheckedInputStream
CheckedOutputStream
Checksum
CRC32
DataFormatException
Deflater
DeflaterOutputStream
GZIPInputStream
GZIPOutputStream
Inflater
InflaterInputStream
ZipEntry
ZipException
ZipFile
ZipInputStream
ZipOutputStream

The package java.util.zip is new as of Java 1.1. It contains classes that provide support for general-purpose data compression and decompression using the ZLIB compression algorithms. The important classes in java.util.zip are those that provide the means to read and write data that is compatible with the popular GZIP and ZIP formats: GZIPInputStream, GZIPOutputStream, ZipInputStream, and ZipOutputStream. Figure 18.1 shows the class hierarchy for the java.util.zip package.

Figure 18.1: The java.text package

[Graphic: Figure 18-1]

It is easy to use the GZIP and ZIP classes because they subclass java.io.FilterInputStream and java.io.FilterOutputStream. For example, to decompress GZIP data, you simply create a GZIPInputStream around the input stream that represents the compressed data. As with any InputStream, you could be reading from a file, a socket, or some other data source. You can then read decompressed data by calling the read() methods of the GZIPInputStream. The following code fragment creates a GZIPInputStream that reads data from the file sample.gz :

FileInputStream inFile;
try { 
    inFile = new FileInputStream("sample.gz"); 
} catch (IOException e) {
    System.out.println("Couldn't open file.");
    return;
}
GZIPInputStream in = new GZIPInputStream(inFile);
// Now use in.read() to get decompressed data.

Similarly, you can compress data using the GZIP format by creating a GZIPOutputStream around an output stream and using the write() methods of GZIPOutputStream. The following code fragment creates a GZIPOutputStream that writes data to the file sample.gz :

FileOutputStream outFile;
try { 
    outFile = new FileOutputStream("sample.gz"); 
} catch (IOException e) {
    System.out.println("Couldn't open file.");
    return;
}
GZIPOutputStream out = new GZIPOutputStream(outFile);
// Now use out.write() to write compressed data.

A ZIP file, or archive, is not quite as easy to use because it may contain more than one compressed file. A ZipEntry object represents each compressed file in the archive. When you are reading from a ZipInputStream, you must first call getNextEntry() to access an entry, and then you can read decompressed data from the stream, just like with a GZIPInputStream. When you are writing data to a ZipOutputStream, use putNextEntry() before you start writing each entry in the archive. The ZipFile class is provided as a convenience for reading an archive; it allows nonsequential access to the entries in a ZIP file.

The remainder of the classes in java.util.zip exist to support the GZIP and ZIP classes. The generic Deflater and Inflater classes implement the ZLIB algorithms; they are used by DeflaterOutputStream and InflaterInputStream to decompress and compress data. The Checksum interface and the classes that implement it, Adler32 and CRC32, define algorithms that generate checksums from stream data. These checksums are used by the CheckedInputStream and CheckedOutputStream classes.

Adler32

Name

Adler32

Synopsis

Class Name:

java.util.zip.Adler32

Superclass:

java.lang.Object

Immediate Subclasses:

None

Interfaces Implemented:

java.util.zip.Checksum

Availability:

New as of JDK 1.1

Description

The Adler32 class implements the Checksum interface using the Adler-32 algorithm. This algorithm is significantly faster than CRC-32 and almost as reliable.

Class Summary

public class java.util.zip.Adler32 extends java.lang.Object
             implements java.util.zip.Checksum {
  // Constructors
  public Adler32();
  
  // Instance Methods
  public long getValue();
  public void reset();
  public void update(int b);
  public void update(byte[] b);
  public native void update(byte[] b, int off, int len);
}

Constructors

Adler32

public Adler32()

Description

This constructor creates an Adler32 object.

Instance Methods

getValue

public long getValue()

Returns

The current checksum value.

Implements

Checksum.getValue()

Description

This method returns the current value of this checksum.

reset

public void reset()

Implements

Checksum.reset()

Description

This method resets the checksum to its initial value, making it appear as though it has not been updated by any data.

update

public void update(int b)

Parameters

b

The value to be added to the data stream for the checksum calculation.

Implements

Checksum.update(int)

Description

This method adds the specified value to the data stream and updates the checksum value. The method uses only the lowest eight bits of the given int.

public void update(byte[] b)

Parameters

b

An array of bytes to be added to the data stream for the checksum calculation.

Description

This method adds the bytes from the specified array to the data stream and updates the checksum value.

public native void update(byte[] b, int off, int len)

Parameters

b

An array of bytes to be added to the data stream for the checksum calculation.

off

An offset into the byte array.

len

The number of bytes to use.

Implements

Checksum.update(byte[], int, int)

Description

This method adds len bytes from the specified array, starting at off, to the data stream and updates the checksum value.

Inherited Methods

Method

Inherited From

Method

Inherited From

clone()

Object

equals(Object)

Object

finalize()

Object

getClass()

Object

hashCode()

Object

notify()

Object

notifyAll()

Object

toString()

Object

wait()

Object

wait(long)

Object

wait(long, int)

Object

   

See Also

Checksum, CRC32


Previous Home Next
Vector Book Index CheckedInputStream

Java in a Nutshell Java Language Reference Java AWT Java Fundamental Classes Exploring Java