How to control the behavior of setCaretPosition( getDocument().getLength() ) in keylistener method?

Hi Experts,

   This is a strange behavior of setCaretPosition( getDocument().getLength() ). This line locates in keylistener method "public void keyTyped( KeyEvent event )" of file AgentTextPane.java.

   The codes attached below are to demo my problem. In order to get a feel quickly what the problem is. Follow the steps please:

1. Compile AgentTextPane.java and TextPaneDisplay.java and run TextPaneDisplay.
2. Type "teh" once and type some thing else after it, you will see the words after "teh"(which has been changed to "the") are in "red". (It should be in default style, gray in color)
3.  Type "teh" again, then the words after second "teh" are back to default style(which is agent style, gray in color).

    What I need is after a word comes out from spell-checking, its style should be changed(in this case, red in color). The rest of words typed  after it stay with the default style( agent style, which is gray in color).

    Currently, the problem is that the default style was not changed back when the spell checker was used for the first time. The default style was changed back when the checkSpelling was called the second time. It follows the pattern. The default style is only changed back on the even number calls.

   If I comment out the line in keyTyped method, the problem is gone. But I do need this line of code to stay there.
   Could you explain me why it behaves like this? What is the better way  to replace the same feature? (which to keep caret at the end of document all the time). Thanks!

[1] AgentTextPane.java

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.text.*;


public class AgentTextPane extends JTextPane implements KeyListener{

      /** The stopping point of the last word break */
      private int stop = -1;

      /** The style for agent */
      private Style agentStyle = getTextStyle("AgentStyle", Color.gray); // default style

      /** The styled document agent uses */
    private StyledDocument agentDoc;

    public AgentTextPane( StyledDocument doc ){
        super(doc);

            //Get the document from text pane
            agentDoc = (StyledDocument)getDocument();

            //Sets the logical style for agent to use from the beginning position
            agentDoc.setLogicalStyle(0, agentStyle);
        addKeyListener( this );

    }

    // ----------------------------------------------------------------------
    // METHODS FOR THE KEYLISTENER INTERFACE
    // ----------------------------------------------------------------------
    public void keyPressed( KeyEvent event ){
        switch( event.getKeyCode() )
        {

            case KeyEvent.VK_END:
                setCaretPosition( getDocument().getLength() );
                break;

            case KeyEvent.VK_HOME:
                setCaretPosition( 0 );
                break;

            default:
                // Update stop point after hitting whitespace...
                if( Character.isWhitespace(event.getKeyChar()) )
                    checkSpelling();
                break;
        }

        return;
    }

    public void keyTyped( KeyEvent event ){
            if( event.getKeyChar() == '\b'  &&  (getCaretPosition() - stop) <= 1 )
                  event.consume();

            // If the caret isn't already at the end of the document, I need to
            // put it there, because I do not want users to insert text into the
            // middle of my document...
 setCaretPosition( getDocument().getLength() ); //<----------this is the line causing the problem, take it  out problem gone
            return;
    }
    public void keyReleased( KeyEvent event ){
        // EMPTY
    }

    /**
     * Checks the spelling of the last inserted word.
     *
     */

    private void checkSpelling()
    {
       StyledDocument doc = (StyledDocument)getDocument();

               // See if my stop value is out of sync with the document...
               if( stop < 0  ||  stop >= doc.getLength() )
                   stop = 0;

        // Retrieve the text to check and send it to the spell checker...
        try{
                   String text     = doc.getText(stop, doc.getLength() - stop).trim();
                   String replace  = checkWord( text.trim() );

            Style theStyle = getTextStyle("MyStyle", Color.red);

                     if(!replace.equals(text.trim())){
                       if( stop > 0 )
                           ++stop;

                // Remove the misspelled word and add in the replacement...
                 doc.remove( stop, doc.getLength() - stop );
                       doc.insertString( stop, replace, null );

                //  Change the color of the replacement text so it is
                //  clear that the word came from the spell-checker...
                       doc.setCharacterAttributes(stop, replace.length(), theStyle, true);

            }
        }
        catch( Exception ignored ){
        }
         stop = doc.getLength();
        return;
    }

  // Set the style for the styled document in a JTextArea
  private Style getTextStyle(String styleName, Color foregroundColor){
        StyledDocument styleDoc = (StyledDocument)getDocument();
        Style style;

        // Set the style attributes for the doc
        style = styleDoc.addStyle("AgentStyle", null);
        // Bold
        StyleConstants.setBold(style, true);
        // Font family
        StyleConstants.setFontFamily(style, "SansSerif");
        // Font size
        StyleConstants.setFontSize(style, 30);
        // Foreground color
        StyleConstants.setForeground(style, foregroundColor);
            return style;
  }

  // Fake one for simulation
    public String checkWord ( String word) {
          String replacedWord;
          String wrongStr1 = "teh";
          String wrongStr2 = "experter";
          String wrongStr3 = "exp";

          if (word.equals(wrongStr1)){
             replacedWord = "the";
            }
            else {
                       if (word.equals(wrongStr2))
                      replacedWord = "EXPERT";
                              else {
                                       if(word.equals(wrongStr3))
                                          replacedWord = "EXPERT";
                                          else
                                          replacedWord = word;
                                  }
                     }
          return replacedWord;
 }
}

[2] TextPaneDisplay.java
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.text.*;

public class TextPaneDisplay extends JFrame {

      StyledDocument doc = new DefaultStyledDocument();


    AgentTextPane agentArea = new AgentTextPane(doc);

    public TextPaneDisplay() {

        JScrollPane scrollingArea = new JScrollPane(agentArea);
        scrollingArea.setBorder(BorderFactory.createEmptyBorder(10,5,10,5));

        // Get the content pane, set layout, add to center
        Container content = this.getContentPane();
        content.setLayout(new BorderLayout());
        content.add(scrollingArea, BorderLayout.CENTER);
        this.pack();
    }

    public static void main(String[] args) {
        JFrame win = new TextPaneDisplay();
        win.setTitle("AgentTextPane");
        win.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        win.setSize( 500, 400 );
        win.setVisible(true);
    }
}