Well, I don’t know if this is the best solution, but we can test the file against various CharsetDecoders and see if any of them reports no errors. Here is a class implementing this behaviour (note: the code below will open and read the file and test it against the decoder until EOF is reached – if an error occurs it proceeds to the next decoder etc. – so if you specify a great number of charsets to be tested, or test large files, it will be slow ) :

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import java.io.BufferedInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStreamReader;
import java.nio.ByteBuffer;
import java.nio.charset.CharacterCodingException;
import java.nio.charset.Charset;
import java.nio.charset.CharsetDecoder;
 
/**
  *
  * @author Georgios Migdos
  */
public class CharsetDetector {
 
     public Charset detectCharset(File f, String[] charsets) {
 
         Charset charset = null ;
 
         for (String charsetName : charsets) {
             charset = detectCharset(f, Charset.forName(charsetName));
             if (charset != null ) {
                 break ;
             }
         }
 
         return charset;
     }
 
     private Charset detectCharset(File f, Charset charset) {
         try {
             BufferedInputStream input = new BufferedInputStream( new FileInputStream(f));
 
             CharsetDecoder decoder = charset.newDecoder();
             decoder.reset();
 
             byte [] buffer = new byte [ 512 ];
             boolean identified = false ;
             while ((input.read(buffer) != - 1 ) && (!identified)) {
                 identified = identify(buffer, decoder);
             }
 
             input.close();
 
             if (identified) {
                 return charset;
             } else {
                 return null ;
             }
 
         } catch (Exception e) {
             return null ;
         }
     }
 
     private boolean identify( byte [] bytes, CharsetDecoder decoder) {
         try {
             decoder.decode(ByteBuffer.wrap(bytes));
         } catch (CharacterCodingException e) {
             return false ;
         }
         return true ;
     }
 
     public static void main(String[] args) {
         File f = new File( "example.txt" );
 
         String[] charsetsToBeTested = { "UTF-8" , "windows-1253" , "ISO-8859-7","GBK" };
 
         CharsetDetector cd = new CharsetDetector();
         Charset charset = cd.detectCharset(f, charsetsToBeTested);
 
         if (charset != null ) {
             try {
                 InputStreamReader reader = new InputStreamReader( new FileInputStream(f), charset);
                 int c = 0 ;
                 while ((c = reader.read()) != - 1 ) {
                     System.out.print(( char )c);
                 }
                 reader.close();
             } catch (FileNotFoundException fnfe) {
                 fnfe.printStackTrace();
             } catch (IOException ioe){
                 ioe.printStackTrace();
             }
 
         } else {
             System.out.println( "Unrecognized charset." );
         }
     }