swing - java for-loop in GUI TextArea

Question

the question is really simple....I have a for loop to print a list of titles and append the results to the TextArea of GUI. for example the list contains titles A, B, C. Every time the TextArea should display the titles one by one. But, in my case, the print-out is always shows the whole list of titles. My TextArea method is called inside the for loop. it should be appended the print-out for each loop but not entire list. how to solve it please help...

sorry for the confusing...I am using JTextArea to show the results. here is the code:

    private static JTextArea textArea1;
 ....
 ....
    textArea1 = new JTextArea(26, 38);
    textArea1.setLineWrap(true);
    textArea1.setEditable(false);
    sbrText = new JScrollPane(textArea1);
    sbrText.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
....
....

public static void getStringText() {

    String ptime = passdata.getTime();
    String ppid = passdata.getPageid();
    String ptitle = passdata.getTitle();
    String previd = passdata.getrevid();
    String pparentid = passdata.getParentId();
    String puser = passdata.getUser();
    String pcomments = passdata.getComments();

    textArea1.append("Timestamp: " + ptime + "
" + "Pageid: " + ppid
            + "
" + "Title:" + ptitle + "
" + "Revid:" + previd + "
"
            + "Parentid:" + pparentid + "
" + "User:" + puser + "
"
            + "Comments:" + pcomments + "
" + newline + newline);
    textArea1.setCaretPosition(textArea1.getDocument().getLength());
    // System.out.println(passToText.getPageid());
}

the getStringText() method is called inside a for loop.

here is the GUI i built. it shows the title with its information, cannot be print one by one means, in the for loop, it should be printed like A, and then B. the animation on the GUI should be shown A and then shown B. But in my case, it shows A,B, C at the same time...Sorry It hard to explain what I am seeing....

Answer 1

With out evidence to the contrie, it sounds like you're looping within the context of the Event Dispatching Thread.

This means that until you exit the loop (and the method), the contents of the JTextArea won't be updated to the screen

Swing is a single threaded environment. The Event Dispatching Thread is responsible for, amongst other things, processing paint requests. Anything that blocks this thread will prevent it from processing painting updates.

It is also required that all updates and interactions you make to the UI are done from within the context of the EDT...

You have a couple of options...

You could use a javax.swing.Timer, which will allow to trigger an ActionEvent on a regular bases, which is triggered within the context of the EDT. This means that while the timer is "waiting", it won't block the EDT...

You could use a SwingWorker, which allows you perform processing in a background thread, but provides you with the ability publish results back to the EDT and process these updates within the EDT...

Check out Concurrency in Swing for more details

Updated with example

javax.swing.Timer acts as kind of loop (all be it an uncontrolled loop). Every period, it will trigger. You need to treat as if you have just looped another iteration and update the state of the UI as required...

import java.awt.BorderLayout;
import java.awt.EventQueue;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class TextTimer {

    public static void main(String[] args) {
        new TextTimer();
    }

    public TextTimer() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException ex) {
                } catch (InstantiationException ex) {
                } catch (IllegalAccessException ex) {
                } catch (UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public static final String[] TEXT = new String[]{
        "Why, you wanna tell me how to live my life?",
        "Who, are you to tell me if it's black or white?",
        "Mama, can you hear me? Try to understand.",
        "Is innocence the difference between a boy and a man?",
        "My daddy lived the lie, that's just the price that he paid",
        "Sacrificed his life, just slavin' away.",
        "",
        "Ohhh, if there's one thing I hang onto,",
        "That gets me through the night.",
        "I ain't gonna do what I don't want to,",
        "I'm gonna live my life.",
        "Shining like a diamond, rolling with the dice,",
        "Standing on the ledge, I show the wind how to fly.",
        "When the world gets in my face,",
        "I say, Have A Nice Day.",
        "Have A Nice Day",
        "",
        "Take a look around you; nothing's what it seems",
        "We're living in the broken home of hopes and dreams,",
        "Let me be the first to shake a helping hand.",
        "Anybody brave enough to take a stand,",
        "I've knocked on every door, on every dead end street,",
        "Looking for forgiveness,",
        "what's left to believe?",
        "",
        "Ohhh, if there's one thing I hang onto,",
        "That gets me through the night.",
        "I ain't gonna do what I don't want to,",
        "I'm gonna live my life.",
        "Shining like a diamond, rolling with the dice,",
        "Standing on the ledge, I show the wind how to fly.",
        "When the world gets in my face,",
        "I say, Have A Nice Day.",
        "Have A Nice Day.",
        "",
        "[Guitar Solo]",
        "",
        "Ohhh, if there's one thing I hang onto,",
        "That gets me through the night.",
        "I ain't gonna do what I don't want to,",
        "I'm gonna live my life.",
        "Shining like a diamond, rolling with the dice,",
        "Standing on the ledge, I show the wind how to fly.",
        "When the world gets in my face,",
        "I say, Have A Nice Day.",
        "Have A Nice Day.",
        "Have A Nice Day.",
        "Have A Nice Day.",
        "Have A Nice Day.",
        "",
        "When The world keeps trying, to drag me down,",
        "I gotta raise my hands, I'm gonna stand my ground.",
        "Well I say, Have A Nice Day.",
        "Have A Nice Day",
        "Have A Nice Day"
    };

    public class TestPane extends JPanel {

        private JTextArea ta;
        private int currentLine = 0;

        public TestPane() {

            setLayout(new BorderLayout());
            ta = new JTextArea(20, 40);
            add(new JScrollPane(ta));

            Timer timer = new Timer(500, new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    String text = TEXT[currentLine];
                    ta.append(text + "
");
                    ta.setCaretPosition(ta.getText().length());
                    currentLine++;
                    if (currentLine >= TEXT.length) {
                        ((Timer)e.getSource()).stop();
                    }
                }
            });
            timer.start();            
        }        
    }
}