Can't draw on JFrame or its ContentPane.

[James Brooks]

Hi all,

I'm sorry, I'm having an awful day and a really hard time figuring out why I can't paint the Car objects onto a JFrame OR its ContentPane. The JFrame appears, but no cars appear on it. Anything that can point me in the right direction? This is homework, so I don't want a spoon-fed solution, but maybe if you see something that I'm overlooking, it will help. As usual, thanks in advance!

  
package project5;
import javax.swing.JFrame;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.Timer;
import javax.swing.JOptionPane;
import java.awt.Graphics2D;
import java.awt.Container;
public class CarMover {
    static Car c1;
    static Car c2;
    static Car c3;
    static CarComponent cc;
    static TimerListener tl;
    static Timer t;
    static JFrame jf;
    static Graphics2D g2d;
    static Container cont;
  public static class TimerListener implements ActionListener{
    public void actionPerformed(ActionEvent event){
        c1.translate(1, 0);
        c2.translate(1, 0);
        c3.translate(1, 0);
        jf.repaint();
    }
  }

    public static void main(String[] args) {
        jf = new JFrame();
        jf.setSize(300, 400);
        jf.setVisible(true);
        cont = jf.getContentPane();
        jf.setContentPane(cont);
        cc = new CarComponent();
        c1 = new Car(30, 30);
        c2 = new Car(100, 50);
        c3 = new Car(170, 70);

        cc.add(c1);
        c1.draw(((java.awt.Graphics2D)(cont.getGraphics())));
        cc.add(c2);
        c2.draw(((java.awt.Graphics2D)(jf.getGraphics())));
        cc.add(c3);
        c3.draw(((java.awt.Graphics2D)(jf.getGraphics())));
        jf.repaint();
        tl = new TimerListener();
        t = new Timer(50, tl);
        t.start();
        

    }
}

  

Car class:

package project5;

import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.geom.Ellipse2D;
import java.awt.geom.Line2D;
import java.awt.geom.Point2D;

/**
   A car shape that can be positioned anywhere on the screen.
*/
public class Car
{
   /**
      Constructs a car with a given top left corner
      @param x the x coordinate of the top left corner
      @param y the y coordinate of the top left corner
   */
   public Car(int x, int y)
   {
      xLeft = x;
      yTop = y;
   }

   /**
      Draws the car.
      @param g2 the graphics context
   */
   public void draw(Graphics2D g2)
   {
      Rectangle body 
            = new Rectangle(xLeft, yTop + 10, 60, 10);      
      Ellipse2D.Double frontTire 
            = new Ellipse2D.Double(xLeft + 10, yTop + 20, 10, 10);
      Ellipse2D.Double rearTire
            = new Ellipse2D.Double(xLeft + 40, yTop + 20, 10, 10);

      // The bottom of the front windshield
      Point2D.Double r1 
            = new Point2D.Double(xLeft + 10, yTop + 10);
      // The front of the roof
      Point2D.Double r2 
            = new Point2D.Double(xLeft + 20, yTop);
      // The rear of the roof
      Point2D.Double r3 
            = new Point2D.Double(xLeft + 40, yTop);
      // The bottom of the rear windshield
      Point2D.Double r4 
            = new Point2D.Double(xLeft + 50, yTop + 10);

      Line2D.Double frontWindshield 
            = new Line2D.Double(r1, r2);
      Line2D.Double roofTop 
            = new Line2D.Double(r2, r3);
      Line2D.Double rearWindshield
            = new Line2D.Double(r3, r4);
   
      g2.draw(body);
      g2.draw(frontTire);
      g2.draw(rearTire);
      g2.draw(frontWindshield);      
      g2.draw(roofTop);      
      g2.draw(rearWindshield);      
   }
   public void translate(int x, int y){
       this.xLeft += x;
       this.yTop += y;
   }

   private int xLeft;
   private int yTop;
}
    

[James Brooks]

OK, sorry, I see now (finally!!!) that paint() was wiping the slate clean, so maybe I can take it from here. Thanks!

[Rob Camick]

that paint() was wiping the slate clean, so maybe I can take it from here.

And the problem is that you can't control when Swing decides the frame needs to be repainted.

Read the section from the Swing tutorial on Custom Painting. Basically, custom painting is done by override the paintComponent() method of a JPanel or JComponent. That way when Swing decides the frame needs to be repainted, your custom component will repaint itself correctly.

You should never use the getGraphics() method to get a Graphics object. The Graphics object will be passed to you in the paintComponent() method.

Also, get rid of all the static methods and variables. That is the sign of a poorly designed program. Again the Swing tuturial has proper working examples.