com.ibm.misc
Class CharacterDecoder

java.lang.Object
  |
  +--com.ibm.misc.CharacterDecoder
Direct Known Subclasses:
BASE64Decoder

public abstract class CharacterDecoder
extends Object

This class defines the decoding half of character encoders. A character decoder is an algorithim for transforming 8 bit binary data that has been encoded into text by a character encoder, back into original binary form. The character encoders, in general, have been structured around a central theme that binary data can be encoded into text that has the form:

	[Buffer Prefix]
	[Line Prefix][encoded data atoms][Line Suffix]
	[Buffer Suffix]
 
Of course in the simplest encoding schemes, the buffer has no distinct prefix of suffix, however all have some fixed relationship between the text in an 'atom' and the binary data itself. In the CharacterEncoder and CharacterDecoder classes, one complete chunk of data is referred to as a buffer. Encoded buffers are all text, and decoded buffers (sometimes just referred to as buffers) are binary octets. To create a custom decoder, you must, at a minimum, overide three abstract methods in this class.
bytesPerAtom which tells the decoder how many bytes to expect from decodeAtom
decodeAtom which decodes the bytes sent to it as text.
bytesPerLine which tells the encoder the maximum number of bytes per line.
In general, the character decoders return error in the form of a CEFormatException. The syntax of the detail string is
	DecoderClassName: Error message.
 
Several useful decoders have already been written and are referenced in the See Also list below.

See Also:
CEFormatException, CharacterEncoder, BASE64Decoder

Constructor Summary
CharacterDecoder()
           
 
Method Summary
protected abstract  int bytesPerAtom()
          Return the number of bytes per atom of decoding
protected abstract  int bytesPerLine()
          Return the maximum number of bytes that can be encoded per line
protected  void decodeAtom(InputStream aStream, OutputStream bStream, int l)
          This method does an actual decode.
 byte[] decodeBuffer(InputStream in)
          Decode the contents of the inputstream into a buffer.
 void decodeBuffer(InputStream aStream, OutputStream bStream)
          Decode the text from the InputStream and write the decoded octets to the OutputStream.
 byte[] decodeBuffer(String inputString)
          Alternate decode interface that takes a String containing the encoded buffer and returns a byte array containing the data.
protected  void decodeBufferPrefix(InputStream aStream, OutputStream bStream)
          decode the beginning of the buffer, by default this is a NOP.
protected  void decodeBufferSuffix(InputStream aStream, OutputStream bStream)
          decode the buffer suffix, again by default it is a NOP.
protected  int decodeLinePrefix(InputStream aStream, OutputStream bStream)
          This method should return, if it knows, the number of bytes that will be decoded.
protected  void decodeLineSuffix(InputStream aStream, OutputStream bStream)
          This method post processes the line, if there are error detection or correction codes in a line, they are generally processed by this method.
protected  int readFully(InputStream in, byte[] buffer, int offset, int len)
          This method works around the bizarre semantics of BufferedInputStream's read method.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

CharacterDecoder

public CharacterDecoder()
Method Detail

bytesPerAtom

protected abstract int bytesPerAtom()
Return the number of bytes per atom of decoding

bytesPerLine

protected abstract int bytesPerLine()
Return the maximum number of bytes that can be encoded per line

decodeBufferPrefix

protected void decodeBufferPrefix(InputStream aStream,
                                  OutputStream bStream)
                           throws IOException
decode the beginning of the buffer, by default this is a NOP.

decodeBufferSuffix

protected void decodeBufferSuffix(InputStream aStream,
                                  OutputStream bStream)
                           throws IOException
decode the buffer suffix, again by default it is a NOP.

decodeLinePrefix

protected int decodeLinePrefix(InputStream aStream,
                               OutputStream bStream)
                        throws IOException
This method should return, if it knows, the number of bytes that will be decoded. Many formats such as uuencoding provide this information. By default we return the maximum bytes that could have been encoded on the line.

decodeLineSuffix

protected void decodeLineSuffix(InputStream aStream,
                                OutputStream bStream)
                         throws IOException
This method post processes the line, if there are error detection or correction codes in a line, they are generally processed by this method. The simplest version of this method looks for the (newline) character.

decodeAtom

protected void decodeAtom(InputStream aStream,
                          OutputStream bStream,
                          int l)
                   throws IOException
This method does an actual decode. It takes the decoded bytes and writes them to the OuputStream. The integer l tells the method how many bytes are required. This is always <= bytesPerAtom().

readFully

protected int readFully(InputStream in,
                        byte[] buffer,
                        int offset,
                        int len)
                 throws IOException
This method works around the bizarre semantics of BufferedInputStream's read method.

decodeBuffer

public void decodeBuffer(InputStream aStream,
                         OutputStream bStream)
                  throws IOException
Decode the text from the InputStream and write the decoded octets to the OutputStream. This method runs until the stream is exhausted.
Throws:
CEFormatException - An error has occured while decoding
CEStreamExhausted - The input stream is unexpectedly out of data

decodeBuffer

public byte[] decodeBuffer(String inputString)
                    throws IOException
Alternate decode interface that takes a String containing the encoded buffer and returns a byte array containing the data.
Throws:
CEFormatException - An error has occured while decoding

decodeBuffer

public byte[] decodeBuffer(InputStream in)
                    throws IOException
Decode the contents of the inputstream into a buffer.