如何将光标移到JScrollPane内JTextArea的TOP

Question

For a long time I've been living with the cursor being at the bottom of my JTextArea named txaOutput after being populated in a JScrollPane even though I WANT it positioned at the TOP.

Tthe only help I've found has referred to setting the caret position to 0 (eg, txaOutput.setCaretPosition(0); ), which I'd never figured out how to make work properly.

Today I just went through every conceivable method for JTextArea and finally found that, BEFORE populating it, this line seems to do what I need:

txaOutput.insert(" ", 1 );

SURELY this isn't the best or only way.

Here's the class for the text area:

package masterwords;
import gbl.GBConstraints;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.GridBagLayout;
import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.ScrollPaneConstants;
import javax.swing.WindowConstants;

public class HelpOutput extends JFrame {

  private JScrollPane scrPnl;
  private JTextArea txaOutput;  

  public     HelpOutput() /* constructor */ {

    scrPnl = new JScrollPane();

    txaOutput    = new JTextArea();

    setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);

    scrPnl.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);
    scrPnl.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
    scrPnl.setViewportBorder(BorderFactory.createEtchedBorder());
    scrPnl.setAutoscrolls(false);
    scrPnl.setPreferredSize(new Dimension(500, 555));

    txaOutput.setFont(new Font("Courier New", 0, 14)); 
    scrPnl.setViewportView(txaOutput);

    setLayout(new GridBagLayout());

    add(scrPnl, new GBConstraints(0,0).ipad(200, 300).spanX(100).spanY(90));

    txaOutput.insert(" ", 1 );  // ********** WITHOUT THIS CURSOR IS AT BOTTOM
    setVisible(true);
    pack();
  }

  public void appendHelp(String s){
    txaOutput.append(s);
  }
}

Here's how I've always called it but it NEVER worked until adding line with ************ above:

  private void btnHelpActionPerformed (ActionEvent evt) {                                        

    HelpOutput helpOutput = new HelpOutput();

    Scanner sc = openHelp();
    while(sc.hasNext())
      helpOutput.appendHelp(sc.next());

   // txaOutput.setCaretPosition(0); // THIS DOES NOTHING so commented out!!!
  }   

Prior to adding the line with all the **************s, NOTHING ELSE I tried put the cursor at the TOP of the text area--always the bottom.

WHAT SHOULD I BE DOING? What I AM doing seems a kludge.

* EDIT, THANKS TO JAVANATOR *

Rename variable txaOutput to txaHelpOutput ; problem solved. New key lines:

  private void btnHelpActionPerformed (ActionEvent evt) {                                        

    HelpOutput helpOutput = new HelpOutput();

    Scanner sc = openHelp();
    while(sc.hasNext())
      helpOutput.appendHelp(sc.next());

   txaHelpOutput.setCaretPosition(0);
   // ^^^^
  }   


public class HelpOutput extends JFrame {

  private JTextArea txaHelpOutput;  
  //                   ^^^^

  public     HelpOutput() /* constructor */ {

    txaHelpOutput    = new JTextArea();
    // ^^^^

    scrPnl.setViewportView(txaHelpOutput);
    //                        ^^^^

    // LOSE THIS LINE!! txaHelpOutput.insert(" ", 1 ); 
  }

Answer 1

First, txaOutput.insert(" ", 1 ); inserts a space into your help text, which is probably not what you want.

Second, you create a HelpOutput object, append text to it, but then you call setCaretPosition on some other object referenced by txaOutput . You need to call setCaretPosition on the HelpOutput object's JTextArea . This is easily done by creating a method in HelpOutput which calls setCaretPosition(0) .

The following code will produce a text area with the carrot at the top, using setCaretPosition(0) .

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.event.ActionEvent;

import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.ScrollPaneConstants;
import javax.swing.SwingUtilities;
import javax.swing.WindowConstants;

public class HelpOutput extends JFrame
{
    private static final long   serialVersionUID    = -1323914827861467580L;
    private JScrollPane         scrPnl;
    private JTextArea           txaOutput;

    public static void main(String[] args)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            @Override
            public void run()
            {
                btnHelpActionPerformed(null);
            }
        });
    }

    public HelpOutput()
    {

        scrPnl = new JScrollPane();
        txaOutput = new JTextArea();

        setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);

        scrPnl.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);
        scrPnl.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
        scrPnl.setViewportBorder(BorderFactory.createEtchedBorder());
        scrPnl.setAutoscrolls(false);
        scrPnl.setPreferredSize(new Dimension(500, 555));

        txaOutput.setFont(new Font("Courier New", 0, 14));
        scrPnl.setViewportView(txaOutput);

        setLayout(new BorderLayout());

        add(scrPnl, BorderLayout.CENTER);

        setVisible(true);
        pack();
    }

    public void appendHelp(String s)
    {
        txaOutput.append(s);
    }

    public void putCarrotAtTop()
    {
        txaOutput.setCaretPosition(0);
    }

    private static void btnHelpActionPerformed(ActionEvent evt)
    {
        HelpOutput helpOutput = new HelpOutput();

        helpOutput
                .appendHelp("Lots of help\nLots of help\nLots of help\nLots of help\nLots of help\n");
        helpOutput.putCarrotAtTop();
    }
}