java.nio.charset

Class CharsetDecoder

java.lang.Object

java.nio.charset.CharsetDecoder

public abstract class CharsetDecoder
extends Object

An engine that can transform a sequence of bytes in a specific charset into a sequence of sixteen-bit Unicode characters.

The input byte sequence is provided in a byte buffer or a series of such buffers. The output character sequence is written to a character buffer or a series of such buffers. A decoder should always be used by making the following sequence of method invocations, hereinafter referred to as a decoding operation:

1. Reset the decoder via the reset method, unless it has not been used before;

2. Invoke the decode method zero or more times, as long as additional input may be available, passing false for the endOfInput argument and filling the input buffer and flushing the output buffer between invocations;

3. Invoke the decode method one final time, passing true for the endOfInput argument; and then

4. Invoke the flush method so that the decoder can flush any internal state to the output buffer.

Each invocation of the decode method will decode as many bytes as possible from the input buffer, writing the resulting characters to the output buffer. The decode method returns when more input is required, when there is not enough room in the output buffer, or when a decoding error has occurred. In each case a CoderResult object is returned to describe the reason for termination. An invoker can examine this object and fill the input buffer, flush the output buffer, or attempt to recover from a decoding error, as appropriate, and try again.

There are two general types of decoding errors. If the input byte sequence is not legal for this charset then the input is considered malformed. If the input byte sequence is legal but cannot be mapped to a valid Unicode character then an unmappable character has been encountered.

How a decoding error is handled depends upon the action requested for that type of error, which is described by an instance of the CodingErrorAction class. The possible error actions are to ignore the erroneous input, report the error to the invoker via the returned CoderResult object, or replace the erroneous input with the current value of the replacement string. The replacement has the initial value "\uFFFD"; its value may be changed via the replaceWith method.

The default action for malformed-input and unmappable-character errors is to report them. The malformed-input error action may be changed via the onMalformedInput method; the unmappable-character action may be changed via the onUnmappableCharacter method.

This class is designed to handle many of the details of the decoding process, including the implementation of error actions. A decoder for a specific charset, which is a concrete subclass of this class, need only implement the abstract decodeLoop method, which encapsulates the basic decoding loop. A subclass that maintains internal state should, additionally, override the implFlush and implReset methods.

Instances of this class are not safe for use by multiple concurrent threads.

Since:

1.4

Author:

Mark Reinhold, JSR-51 Expert Group

See Also:

ByteBuffer, CharBuffer, Charset, CharsetEncoder

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	CharsetDecoder(Charset cs, float averageCharsPerByte, float maxCharsPerByte) Initializes a new decoder.

Method Summary

Methods

Modifier and Type	Method and Description
float	averageCharsPerByte() Returns the average number of characters that will be produced for each byte of input.
Charset	detectedCharset() Retrieves the charset that was detected by this decoder (optional operation).
Charset	charset() Returns the charset that created this decoder.
protected void	implReplaceWith(String newReplacement) Reports a change to this decoder's replacement value.
protected void	implReset() Resets this decoder, clearing any charset-specific internal state.
boolean	isAutoDetecting() Tells whether or not this decoder implements an auto-detecting charset.
boolean	isCharsetDetected() Tells whether or not this decoder has yet detected a charset (optional operation).
float	maxCharsPerByte() Returns the maximum number of characters that will be produced for each byte of input.
String	replacement() Returns this decoder's replacement value.
CharsetDecoder	replaceWith(String newReplacement) Changes this decoder's replacement value.
CharsetDecoder	reset() Resets this decoder, clearing any internal state.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

CharsetDecoder

protected CharsetDecoder(Charset cs,
              float averageCharsPerByte,
              float maxCharsPerByte)

Initializes a new decoder. The new decoder will have the given chars-per-byte values and its replacement will be the string "\uFFFD".

Parameters:

averageCharsPerByte - A positive float value indicating the expected number of characters that will be produced for each input byte

maxCharsPerByte - A positive float value indicating the maximum number of characters that will be produced for each input byte

Throws:

IllegalArgumentException - If the preconditions on the parameters do not hold

Method Detail

charset

public final Charset charset()

Returns the charset that created this decoder.

Returns:

This decoder's charset

replacement

public final String replacement()

Returns this decoder's replacement value.

Returns:

This decoder's current replacement, which is never null and is never empty

replaceWith

public final CharsetDecoder replaceWith(String newReplacement)

Changes this decoder's replacement value.

This method invokes the implReplaceWith method, passing the new replacement, after checking that the new replacement is acceptable.

Parameters:

newReplacement - The new replacement; must not be null and must have non-zero length

Returns:

This decoder

Throws:

IllegalArgumentException - If the preconditions on the parameter do not hold

implReplaceWith

protected void implReplaceWith(String newReplacement)

Reports a change to this decoder's replacement value.

The default implementation of this method does nothing. This method should be overridden by decoders that require notification of changes to the replacement.

Parameters:

newReplacement -

averageCharsPerByte

public final float averageCharsPerByte()

Returns the average number of characters that will be produced for each byte of input. This heuristic value may be used to estimate the size of the output buffer required for a given input sequence.

Returns:

The average number of characters produced per byte of input

maxCharsPerByte

public final float maxCharsPerByte()

Returns the maximum number of characters that will be produced for each byte of input. This value may be used to compute the worst-case size of the output buffer required for a given input sequence.

Returns:

The maximum number of characters that will be produced per byte of input

reset

public final CharsetDecoder reset()

Resets this decoder, clearing any internal state.

This method resets charset-independent state and also invokes the implReset method in order to perform any charset-specific reset actions.

Returns:

This decoder

implReset

protected void implReset()

Resets this decoder, clearing any charset-specific internal state.

The default implementation of this method does nothing. This method should be overridden by decoders that maintain internal state.

isAutoDetecting

public boolean isAutoDetecting()

Tells whether or not this decoder implements an auto-detecting charset.

The default implementation of this method always returns false; it should be overridden by auto-detecting decoders to return true.

Returns:

true if, and only if, this decoder implements an auto-detecting charset

isCharsetDetected

public boolean isCharsetDetected()

Tells whether or not this decoder has yet detected a charset (optional operation).

If this decoder implements an auto-detecting charset then at a single point during a decoding operation this method may start returning true to indicate that a specific charset has been detected in the input byte sequence. Once this occurs, the detectedCharset method may be invoked to retrieve the detected charset.

That this method returns false does not imply that no bytes have yet been decoded. Some auto-detecting decoders are capable of decoding some, or even all, of an input byte sequence without fixing on a particular charset.

The default implementation of this method always throws an UnsupportedOperationException; it should be overridden by auto-detecting decoders to return true once the input charset has been determined.

Returns:

true if, and only if, this decoder has detected a specific charset

Throws:

UnsupportedOperationException - If this decoder does not implement an auto-detecting charset

detectedCharset

public Charset detectedCharset()

Retrieves the charset that was detected by this decoder (optional operation).

If this decoder implements an auto-detecting charset then this method returns the actual charset once it has been detected. After that point, this method returns the same value for the duration of the current decoding operation. If not enough input bytes have yet been read to determine the actual charset then this method throws an IllegalStateException.

The default implementation of this method always throws an UnsupportedOperationException; it should be overridden by auto-detecting decoders to return the appropriate value.

Returns:

The charset detected by this auto-detecting decoder, or null if the charset has not yet been determined

Throws:

IllegalStateException - If insufficient bytes have been read to determine a charset

UnsupportedOperationException - If this decoder does not implement an auto-detecting charset