Why repaint is automatically called

Question

in the slot machine program below, you try to run it, play once , after that you leave the bet textfield to be 0, and click play again, when you click down the JOptionMessage dialog, three slot is atomatically set back to lemon, but it shoudn't be that, I didn't call repaint in my program, why
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
public class Slot extends JFrame implements ActionListener,Runnable
     ImageIcon img;
     JButton play;
     JPanel leftCanvas;
     JPanel centerCanvas;
     JPanel rightCanvas;
     JTextField amount;
     JTextField bet;
     SpinThread t1;
     SpinThread t2;
     SpinThread t3;
     Thread t;
     int credit,betAmount;
     AudioClip audioClip=Applet.newAudioClip(this.getClass().getResource("win.au"));
     public Slot(String s) {
     /** Constructor */
      super( s );
      setSize(300,200);
      img = new ImageIcon("images/lemon.gif") ;
      Container c = getContentPane();
      c.setLayout( new BorderLayout() );
      /** Panel with Label showing title */
     JPanel title = new JPanel ();
      title.setFont(new Font("TimesRoman", Font.BOLD, 16));
      title.add(new Label("Slot Machine Simulation"));
      c.add(title, "North");
      /** Panel with 3 Canvases showing pictures */
      JPanel slotPanel = new JPanel(new GridLayout(1, 3));
      leftCanvas = new JPanel();
      centerCanvas = new JPanel();
      rightCanvas = new JPanel() ;
      slotPanel.add(leftCanvas);
      slotPanel.add(centerCanvas);
      slotPanel.add(rightCanvas) ;
      c.add(slotPanel,"Center") ;
      /** the Bottom panel showing the credit, betting amount and play button */
      JPanel messagePanel = new JPanel(new GridLayout(1,3));
      /** credit panel */
      JPanel creditPanel = new JPanel();
      creditPanel.setLayout(new FlowLayout()) ;
      JLabel label1  = new JLabel("Credit") ;
      creditPanel.add(label1) ;
      amount = new JTextField("1000") ;
      amount.setHorizontalAlignment(JTextField.RIGHT) ;
      amount.setEditable(false) ;
      creditPanel.add(amount) ;
      messagePanel.add(creditPanel) ;     
      /** betting panel*/
      JPanel bettingPanel = new JPanel();
      bettingPanel.setLayout(new FlowLayout()) ;
      JLabel label2  = new JLabel("Bet") ;
      bettingPanel.add(label2) ;
      bet = new JTextField(3) ;
      bet.setText("50") ;
      bet.setHorizontalAlignment(JTextField.RIGHT) ;
      bettingPanel.add(bet) ;
      messagePanel.add(bettingPanel) ;
     /** button panel */
      JPanel buttonPanel = new JPanel();
      buttonPanel.setLayout(new FlowLayout()) ;
      play = new JButton("PLAY") ;
      play.addActionListener(this) ;
      buttonPanel.add(play) ;
      messagePanel.add(buttonPanel) ;
     /** add the last panel to the south end */ 
      c.add(messagePanel,"South") ;
      credit=Integer.parseInt(amount.getText());
      betAmount=Integer.parseInt(bet.getText());
   public void paint(Graphics g)
   {  super.paint(g);
          img.paintIcon(leftCanvas,leftCanvas.getGraphics(),0,0);
      img.paintIcon(centerCanvas,centerCanvas.getGraphics(),0,0);
      img.paintIcon(rightCanvas,rightCanvas.getGraphics(),0,0);
   public void check()
   {credit=Integer.parseInt(amount.getText());
    betAmount=Integer.parseInt(bet.getText());
        if(t1.index==t2.index&&t2.index==t3.index&&t3.index==6)
             credit=credit+1000000;
             audioClip.play();
             //Splash win=new Splash();
             //win.setSize(200,200);
             //win.setVisible(true);
    else if(t1.index==t2.index&&t2.index==t3.index)
            credit=credit+betAmount*100;
        else if(t1.index==t2.index||t2.index==t3.index||t3.index==t1.index)
             credit=credit+betAmount*10;
        else
             credit=credit-betAmount;
        amount.setText(String.valueOf(credit));
        bet.setText("0");
   public void actionPerformed(ActionEvent e)
        if(e.getSource()==play)
             credit=Integer.parseInt(amount.getText());
        betAmount=Integer.parseInt(bet.getText());
             if(betAmount<=0)
             JOptionPane.showMessageDialog(this,"Bet must be positive!");
       else  if(betAmount>100||betAmount>credit)
             JOptionPane.showMessageDialog(this,"Bet exceed the upper limit(100) or not enough credit!");
       else if(betAmount<credit&&betAmount<=100&&betAmount>0)
            t1=new SpinThread(leftCanvas,20);
        t2=new SpinThread(centerCanvas,40);
        t3=new SpinThread(rightCanvas,60);
        t=new Thread(this);
         t.setPriority(1);
            t1.start();
             t2.start();
             t3.start();
             t.start();
   public void run()
            boolean done=false;
            while(!done)
                if(!(t1.isAlive()||t2.isAlive()||t3.isAlive()))
                     done=true;
            check();
   public static void main(String[] args)
         Slot mySlot=new Slot("Slot Machine Simulation");
         mySlot.show();
         mySlot.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
class SpinThread extends Thread
     JPanel p;
     int delay;
     ImageIcon[] img={new ImageIcon("images/ball.gif"),
     new ImageIcon("images/bar.gif"),new ImageIcon("images/bell.gif"),
     new ImageIcon("images/fruit.gif"),new ImageIcon("images/lemeon.gif"),
     new ImageIcon("images/seven.gif")};
     int index;
     public SpinThread(JPanel p,int delay)
          super();
          this.p=p;
          this.delay=delay;
     public void run()
          int i=0;
          while(i<delay)
               index=Math.round((float)Math.random()*5);
               display(p.getGraphics());
               try{
                    sleep(delay);
               }catch(InterruptedException e){;}
               i++;
     public void display(Graphics g)
       img[index].paintIcon(p,g,0,0);
}

Answer 1

That's the way repaint works
Anytime a java component which lies beneath some other window is uncovered, the UI thread will do a repaint