CharsetDecoder Class in Java

For encoding and decoding tasks, many methods are offered in Charset Encoder and Charset Decoder classes in Java. The Charset Decoder class is used for text handling to convert bytes to characters. The Charset decoder accepts a sequence of bytes as its input and displays Unicode characters as output. The input provided to the charset decoder must belong to the UTF-8 character set. In Software Development, charset is used for Text handling and decoding control processes.

What is CharsetDecoder?

The Charset Decoder class is imported from the "java.nio.charset" package with functionalities of the decoder class complementing with the encoder class. Input buffers are used to process the input byte sequence as distinct buffers. The output of each buffer is written to a character buffer which upon concatenation forms meaningful strings.

This decoder is used by making the following sequence of method invocations:

1. The reset method resets the value of the decoder unless previously used.
2. Decode method with the endOfInput argument as "false" fills the input buffer and flushes the output buffer between invocations. The false value of the argument conveys that the input set of bytes may not be complete. So, decoder will process as many bytes as possible in the input buffer.
3. The decode method with the endOfInput argument as "true" is passed one final time and then the flush method is used so that the decoder can flush any internal state to the output buffer.

The above sequence of operations together comprise the functions of the decode method. Each invocation of this decoding method will decode as many bytes as possible from the input buffer and write the resulting characters to the output buffer.

Syntax of CharsetDecoder

public abstract class CharsetDecoder extends Object

Constructor of CharsetDecoder

Methods in CharsetDecoder

Above table describing modifier, method and it's description has been mentioned according to the documentation in Java SDK 21. The java SE APIs 21 defines core Java platform for general purpose computing.

Error Handling during decoding

There are 2 cases of error handling in CharsetDecoder class namely, malformed byte sequence or unmappable character. The error can be dealt with using ignore, report or replace with actions. Malformed inputs can be reported using onMalformedInput method.

// Java Program to Implement
// CharsetDecoder Class

import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.charset.Charset;
import java.nio.charset.CharsetDecoder;//decoding operations library
import java.nio.charset.CoderResult;// result promtomg!
import java.nio.charset.CodingErrorAction; // Error message handling

class GFG {
    public static void main (String[] args) {
        byte[] bytes = { (byte) 0x40, (byte) 0x40};// '@' is being passed twice in UTF - 8
          CharsetDecoder decoder = Charset.forName("UTF-8").newDecoder();
      
          //Error handling actions for malformed and unmappable characters
          decoder.onMalformedInput(CodingErrorAction.REPLACE);
        decoder.onUnmappableCharacter(CodingErrorAction.REPORT);
      
          //I/O buffer creation
          CharBuffer charStore = CharBuffer.allocate(bytes.length);
        ByteBuffer utfStore = ByteBuffer.wrap(bytes);
      
          //Output string instance to concatenate each decoded byte
          StringBuilder decodedText = new StringBuilder();
        CoderResult result;
      
          do {
            result = decoder.decode(utfStore, charStore, false);
            charStore.flip();
            decodedText.append(charStore);
            charStore.clear(); 

            if (result.isError()) {
                // Error handling logic
                if (result.isMalformed()) {
                    System.err.println("Encountered malformed byte sequence!");//Malformed error
                } else if (result.isUnmappable()) {
                    System.err.println("Encountered unmappable character!");//Unmappable error
                }
            }
          
        } while (!result.isUnderflow());
      
        System.out.println("Decoded Text: " + decodedText); // Decoded text is shown as output!
    }
}

Decoded Text: @@

Explanation of the above Program:

The above code depicts how '@' is being decoded using CharsetDecoder library with correct error handling procedure. Following steps are involved in above code:

• bytes is an input list of type byte containing '@' passed as UTF-8 encoded values.
• Error handling is performed such that malformed inputs are replaced and unmappable inputs are reported.
• Two buffers CharBuffer and byteBuffer are created to store output characters and input byte sequence respectively.
• decodedText is used to concate each output character and display the output to users.
• The do-while performs the decoding task for each byte in the input byte sequence using previously created buffers under underflow condition.
• After completing the loop output of the decosing program is prointed to the terminal as '@@'.