Question

i’m working on applet that displays a moving banner horizontally and when this text banner
reaches the right boundary of the applet window it should be appear reversed from the start of the left boundary, i write the following class to do the work, the problem is that when the text banner reaches the right banner it crashes, the applet goes to infinite loop:

    import java.applet.*;
    import java.awt.*;
    import java.net.*;
    import java.io.*;
    import javax.swing.*;

    /*
    <applet code="banner" width=300 height=50>
    </applet>
    */
    public class TextBanner extends Applet implements Runnable 
    {
        String msg = "Islam Hamdy", temp="";
        Thread t = null;
        int state;
        boolean stopFlag;
        int x;
        String[] revMsg;
        int msgWidth;
        boolean isReached;

         public void init() 
        {

            setBackground(Color.BLACK);
            setForeground(Color.YELLOW);
        }

        // Start thread

        public void start() 
        {

            t = new Thread(this);
            stopFlag = false;
            t.start();
        }

        // Entry point for the thread that runs the banner.

        public void run() 
        {

            // Display banner
            while(true) 
            {
                try 
                {
                    repaint();
                    Thread.sleep(550);
                    if(stopFlag)
                        break;
                } catch(InterruptedException e) {}
            }
        }

        // Pause the banner.

        public void stop() 
        {
            stopFlag = true;
            t = null;
        }

        // Display the banner.

        public void paint(Graphics g) 
        {
        String temp2="";
        System.out.println("Temp-->"+temp);
        int result=x+msgWidth; 
        FontMetrics fm = g.getFontMetrics();
        msgWidth=fm.stringWidth(msg);
        g.setFont(new Font("ALGERIAN", Font.PLAIN, 30));        
        g.drawString(msg, x, 40);
        x+=10;
        if(x>bounds().width){
        x=0;
    }
        if(result+130>bounds().width){
         x=0;
        while((x<=bounds().width)){
        for(int i=msg.length()-1;i>0;i--){
        temp2=Character.toString(msg.charAt(i));
        temp=temp2+temp;
                    // it crashes here
        System.out.println("Before draw");
            g.drawString(temp, x, 40);
        System.out.println("After draw");
        repaint();
               }
            x++;
            }       // end while
         }          //end if

      }

     }

Answer 1

Let’s start with…

• Use Swing over AWT components (JApplet instead of Applet)
• Don’t override the paint methods of top level containers. There are lots of reasons, the major one that is going to effect you is top level containers are not double buffered.
• DO NOT, EVER, update the UI from any thread other the Event Dispatching Thread, in fact, for what you are trying to do, a Thread is simply over kill.
• DO NOT update animation states with the paint method. You’ve tried to perform ALL you animation within the paint method, this is not how paint works. Think of paint as a frame in film, it is up to (in your case) the thread to determine what frame where up to, and in fact, it should be preparing what should painted.
• You do not control the paint system. repaint is a “request” to the paint sub system to perform an update. The repaint manager will decide when the actual repaint will occur. This makes performing updates a little tricky…

Updated with Example

public class Reverse extends JApplet {

    // Set colors and initialize thread.
    public void init() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }
                setBackground(Color.BLACK);
                setForeground(Color.YELLOW);
                setLayout(new BorderLayout());
                add(new TextPane());
            }
        });
    }

    // Start thread
    public void start() {
    }

    // Pause the banner.
    public void stop() {
    }

    public class TextPane extends JPanel {

        int state;
        boolean stopFlag;
        char ch;
        int xPos;
        String masterMsg = "Islam Hamdy", temp = "";
        String msg = masterMsg;
        String revMsg;
        int msgWidth;

        private int direction = 10;

        public TextPane() {
            setOpaque(false);
            setBackground(Color.BLACK);
            setForeground(Color.YELLOW);

            setFont(new Font("ALGERIAN", Font.PLAIN, 30));

            // This only needs to be done one...
            StringBuilder sb = new StringBuilder(masterMsg.length());
            for (int index = 0; index < masterMsg.length(); index++) {
                sb.append(masterMsg.charAt((masterMsg.length() - index) - 1));
            }
            revMsg = sb.toString();

            // Main animation engine.  This is responsible for making
            // the decisions on where the animation is up to and how
            // to react to the edge cases...
            Timer timer = new Timer(100, new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    xPos += direction;
                    FontMetrics fm = getFontMetrics(getFont());
                    if (xPos > getWidth()) { // this condition fires when the text banner reaches the right banner
                        direction *= -1;
                        msg = revMsg;
                    } else if (xPos < -fm.stringWidth(masterMsg)) {
                        direction *= -1;
                        msg = masterMsg;
                    }
                    repaint();
                }
            });
            timer.setRepeats(true);
            timer.setCoalesce(true);
            timer.start();

        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            System.out.println(xPos);
            FontMetrics fm = g.getFontMetrics();
            msgWidth = fm.stringWidth(msg);
            g.drawString(msg, xPos, 40);
        }
    }
}

Updated with additional example

Now, if you want to be a little extra clever…you could take advantage of a negative scaling process, which will reverse the graphics for you…

Updated timer…

            Timer timer = new Timer(100, new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    xPos += direction;
                    FontMetrics fm = getFontMetrics(getFont());
                    System.out.println(xPos + "; " + scale);
                    if (scale > 0 && xPos > getWidth()) { // this condition fires when the text banner reaches the right banner
                        xPos = -(getWidth() + fm.stringWidth(msg));
                        scale = -1;
                    } else if (scale < 0 && xPos >= 0) {
                        xPos = -fm.stringWidth(msg);
                        scale = 1;
                    }
                    repaint();
                }
            });

And the updated paint method…

        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            g2d.scale(scale, 1);
            FontMetrics fm = g2d.getFontMetrics();
            msgWidth = fm.stringWidth(msg);
            g2d.drawString(msg, xPos, 40);
            g2d.dispose();
        }

Updated with “bouncing”…

This replaces the TextPane from the previous example

As the text moves beyond the right boundary, it will “reverse” direction and move back to the left, until it passes beyond that boundary, where it will “reverse” again…

public class TextPane    public class TextPane extends JPanel {

    int state;
    boolean stopFlag;
    char ch;
    int xPos;
    String msg = "Islam Hamdy";
    int msgWidth;

    private int direction = 10;

    public TextPane() {
        setBackground(Color.BLACK);
        setForeground(Color.YELLOW);

        setFont(new Font("ALGERIAN", Font.PLAIN, 30));

        Timer timer = new Timer(100, new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                xPos += direction;
                FontMetrics fm = getFontMetrics(getFont());
                if (xPos > getWidth()) {
                    direction *= -1;
                } else if (xPos < -fm.stringWidth(msg)) {
                    direction *= -1;
                }
                repaint();
            }
        });
        timer.setRepeats(true);
        timer.setCoalesce(true);
        timer.start();

    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g.create();
        FontMetrics fm = g2d.getFontMetrics();
        msgWidth = fm.stringWidth(msg);
        g2d.drawString(msg, xPos, 40);
        g2d.dispose();
    }
}