Thread not wanting to work

Question

If so someone could explain how to get this to paint and why it isn't painting that would be awesome!!!

public class Main extends JPanel implements Runnable {

    public void run() {

        System.out.println("g");
        repaint();
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawRect(50, 50, 200, 200);
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame("Physics!");
        frame.setSize(500, 500);
        frame.setBackground(Color.BLUE);
        frame.setVisible(true);

        Main physics = new Main();
        Thread t = new Thread(physics);
        t.start();
    }

}

Answer 1

You never add physics to the JFrame

Main physics = new Main();
JFrame frame = new JFrame("Physics!");
frame.add(physics);

---

Side Note

• When painting on JPanel override getPreferredSize() so the panel has a preferred size, then you can just pack() the frame, as you should be doing, instead of setting it's size

  @Override
  public Dimension getPreferredSize() {
      return new Dimension(500, 500);
  }
  ...
  frame.pack();
  // frame.setSize(500, 500);
  

• Also, paintComponent should be protected not public

• Also see Initial Threads for running Swing apps on the Event Dispatch Thread

Answer 2

You have to add the JPanel to the JFrame:

    JFrame frame = new JFrame("Physics!");
    Main physics = new Main();
    Thread t = new Thread(physics);
    t.start();
    frame.setContentPane(physics); // Add it like this
    frame.setSize(500, 500);
    frame.setBackground(Color.BLUE);
    frame.setVisible(true);

Answer 3

Try putting t.join() in the main method after t.start() .. Your main method should wait for the thread to finish.