Swing paintComponent not working properly

Question

I don't understand why i have to set Component.setPreferredSize() to draw and why my ovals are not placed in one place. Also i got some other questions, which are described below.

public class PaintPanel extends JPanel implements ActionListener
{
    private void initStructure()
    {
        for (int i : new Range(MAX_AGENTS)) {
            Agent agent = new Agent();
            agents.add(agent);
            add(agent);

        }
    }

//i want to override parent class 'add', to easy call 'actionPerformed' on children elements.  
    public Component add(Element comp)
    {
        elements.add(comp);
        return super.add(comp);
    }


    @Override
    public void actionPerformed(ActionEvent e)
    {
        for(Element element: elements){
            element.actionPerformed(e);
        }

        repaint();
    }


  //ELEMENT is abstract class: 'public class Element extends JComponent implements ActionListener'

  public class Agent extends Element
  {
    public Agent()
    {
        super();
 //     setPreferredSize(new Dimension(120,120)); !!!! a-a, i don't know the future size of the oval or triangle, i don't want and i can't set this :(
        setVisible(true);
    }


    @Override
    protected void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        System.out.println("draw"); //called

        Point p1 = new Point(1, 1);
        Point p2 = new Point(101, 101);
        Point p3 = new Point(10, 10);

        int[] xs = { p1.x, p2.x, p3.x };
        int[] ys = { p1.y, p2.y, p3.y };

        Polygon triangle = new Polygon(xs, ys, xs.length);
        g.setColor(new Color(255,255,255)); // !! never painted
        g.fillPolygon(triangle); //!! never painted

        g.drawOval(10,10,10,10); // !!!! painted only when i set preferredSize
    }

    @Override
    public void actionPerformed(ActionEvent e)
    {
        System.out.println("i am alive"); //working
    }
}

In case, if i set preferred size i got the picture below.

So.

Question

1) Is there any patterns to not set preferred size and draw component inside Component calling paint() on contentPanel?

2) Why g.fillPolygon not working?

3) Why my ovals are not placed in one point?

Answer 1

Why my ovals are not placed in one point?

I would guess you are adding your Agent components to a JPanel. By default a JPanel uses a FlowLayout. So each component is placed 120 pixels apart and flow to a new line when the row is filled.

i don't know the future size of the oval or triangle, i don't want and i can't set this

Don't use the drawOval(...) method. Instead use a Shape object that represents an oval. Then you can get the size of the Shape and use this value in the getPreferredSize() method mentioned by @hovercraft.

Check out Playing With Shapes for more info on this concept. Of course if you use this concept you would need to define the Shapes as instance variable so the Shape can be referenced by both the paintComponent() and getpreferredSize() methods.

Answer 2

The issue is if the JPanel doesn't have a preferred size, and if it is being added to a container that uses a layout manager that doesn't fill the container (such as FlowLayout), then how will the GUI know what size the drawing JPanel should be? I've heard that better than calling setPreferredSize(...) on your JPanel is to override its Dimension getPreferredSize() method (ask kleopatra, a Swing expert on this site).

Regarding:

Why g.fillShape not working?

Check the API -- does Graphics have a fillShape method? Nope. But Graphics2D has a fill(Shape s) method, and that's what you want.

Why my ovals are not placed in one point?

Please clarify this and provide details. What do you mean by "placed in one point"? What behavior exactly are you expecting and why?

---

Edit: your triangle is not being drawn because all the points are co-linear!

---

For example:

import java.awt.*;

import javax.swing.*;

public class MyDrawingPanel extends JPanel {
   private static final int PREF_W = 100;
   private static final int PREF_H = PREF_W;
   // private Point p1 = new Point(1, 1);
   private Point p1 = new Point(30, 1);
   private Point p2 = new Point(100, 101);
   // private Point p3 = new Point(10, 10);
   private Point p3 = new Point(50, 10);
   private int[] xs = { p1.x, p2.x, p3.x };
   private int[] ys = { p1.y, p2.y, p3.y };
   private Polygon triangle = new Polygon(xs, ys, xs.length);


   public MyDrawingPanel() {
   }

   @Override
   public Dimension getPreferredSize() {
      if (isPreferredSizeSet()) {
         return super.getPreferredSize();
      }
      return new Dimension(PREF_W, PREF_H);
   }

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      g.setColor(new Color(255, 255, 255)); // !! never painted
      g.fillPolygon(triangle); // !! never painted

      g.drawOval(10, 10, 10, 10); // !!!! painted only when i set preferredSize
   }

   private static void createAndShowGui() {
      int rows = 4;
      int cols = 4;
      JPanel gridPanel = new JPanel(new GridLayout(rows, cols));
      for (int i = 0; i < rows; i++) {
         for (int j = 0; j < cols; j++) {
            gridPanel.add(new MyDrawingPanel());
         }
      }

      JFrame frame = new JFrame("PaintPanel");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(gridPanel);
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }

}