Logging Panel (help!)

Question

Hi everyone,
I want to add a logging panel to my frame. I want to use it in order to diplay text messages displayed by the program rather than printing it to the console.
I don't know how to direct the text to this panel. Would plz tell me how to do this? or give me a reference to read?
Many Thanks,
Somaya

Answer 1

I can't really understand why you want to redirect System.out/System.err ("printing to the console") if you are using a frame. Why print to the console instead of using GUI components only?
However odd it may seem, I thought it would be a fun experiment so I created a panel class you can use:
import javax.swing.*;
import java.awt.*;
import java.io.OutputStream;
import java.io.PrintStream;
import java.io.IOException;
* Panel class that can intercept and redirect System.out and/or System.err to internal scrollable JTextArea.
public class OutErrInterceptingPanel extends JPanel
    * @param interceptErrStream decides whether or not to intercept and redirect System.err to text area
    * @param interceptOutStream decides whether or not to intercept and redirect System.out to text area
   public OutErrInterceptingPanel(boolean interceptErrStream, boolean interceptOutStream)
      JTextArea logArea = new JTextArea();
      logArea.setEditable(false);
      setLayout(new BorderLayout());
      add(new JScrollPane(logArea), BorderLayout.CENTER);
      //Use fake/intercepting OutStream to connect PrintStream(-s) to JTextArea...
      OutputStream wrapper = null;
      if (interceptErrStream)
         wrapper = new StreamWrapper(logArea);
         System.setErr(new PrintStream(wrapper));
      if (interceptOutStream)
         if (wrapper == null)
            wrapper = new StreamWrapper(logArea);
         System.setOut(new PrintStream(wrapper));
    * Acts as an OutputStream but it intercepts all write calls and adds data to a JTextArea instead...
   private class StreamWrapper extends OutputStream
      private JTextArea outputSource;
      public StreamWrapper(JTextArea aOutputSource)
         outputSource = aOutputSource;
      public void write(int b) throws IOException
         //Convert and append to JTextArea (Note that append method is thread safe)
         if (b < 0)
            outputSource.append("?"); //Replace invalid char with a question mark
         else
            char[] chars = Character.toChars(b);
            for (int i = 0; i < chars.length; i++)
               outputSource.append(Character.toString(chars));
public void write(byte[] b) throws IOException
for (int i = 0; i < b.length; i++)
write(b[i]);
public void write(byte[] b, int off, int len) throws IOException
for (int i = off; i < off + len; i++)
write(b[i]);
And of course I also created a GUI application for testing the panel:
import javax.swing.*;
import java.awt.*;
public class TextAreaLogger extends JFrame
   public TextAreaLogger(JPanel logPanel)
      super("Sample Application on how to use OutErrInterceptingPanel");
      getContentPane().add(new JLabel("See OutErrInterceptingPanel in action below:"), BorderLayout.NORTH);
      getContentPane().add(logPanel, BorderLayout.CENTER);
   public static void main(String[] args)
      //Create GUI on the event dispatching thread
      try
         SwingUtilities.invokeAndWait(new Runnable()
            public void run()
               //Create intercepting panel (intercept System.out only) and use it in GUI
               JPanel logPanel = new OutErrInterceptingPanel(false, true);
               TextAreaLogger frame = new TextAreaLogger(logPanel);
               frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
               frame.setSize(450, 400);
               frame.setVisible(true);
      } catch (Exception e)
      //Print 25 lines to System.out 25 lines to System.err
      for (int i = 1; i <= 25; i++)
         System.out.println("This is a line "+ i +" written to System.out");
         System.err.println("This is a line "+ i +" written to System.err");
}My implementation may not be the best way of redirecting System.out and System.err to a text area, but it sure was fun to do! =)
Note. If I completely misinterpreted what you wanted help with - if all you really wanted was to know a litle bit about JTextArea - you may find this link useful (and you should probably just ignore the code above):
http://java.sun.com/docs/books/tutorial/uiswing/components/text.html