[Chapter 18] Checksum

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| Chapter 18 The java.util.zip Package |

# Checksum

## Name

Checksum

## Synopsis

Interface Name:

  `java.util.zip.Checksum`

Super-interface:

  None

Immediate Sub-interfaces:

  None

Implemented By:

  `java.util.zip.Adler32`, `java.util.zip.CRC32`

Availability:

  New as of JDK 1.1

## Description

The `Checksum` interface defines the methods that are needed to compute a checksum value for a stream of data. The checksum value can be used for error checking purposes. Note, however, that the checksum value must fit into a `long` value, so this interface is not suitable for cryptographic checksum algorithms.

The `Adler32` and `CRC32` classes implement the `Checksum` interface, using the Adler-32 and CRC-32 algorithms, respectively. The `CheckedInputStream` and `CheckedOutputStream` classes provide a higher-level mechanism for computing checksums on data streams.

## Class Summary

    public abstract interface java.util.zip.Checksum {
      // Methods
      public abstract long getValue();
      public abstract void reset();
      public abstract void update(int b);
      public abstract void update(byte[] b, int off, int len);
    }

## Methods

### getValue

#### public abstract long getValue()

Returns

  The current checksum value.

Description

  This method returns the current value of this checksum.

### reset

#### public abstract void 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 abstract void update(int b)

Parameters

  `b`

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

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 abstract 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.

Description

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

## See Also

`Adler32`, `CheckedInputStream`, `CheckedOutputStream`, `CRC32`

CheckedOutputStream |  | CRC32