Why the repaint() method not working here?

Question

public class Ova extends JPanel implements ActionListener{

int x=0;
Timer timer=new Timer(100,this);
int y=0,x1=5,y1=5;

public static void main(String[] ds)
{
    Ova ss=new Ova();
    ss.nn();    
}

private void nn() {     
    JFrame frame=new JFrame("fram");
    frame.setSize(1000,600);
    JPanel as=new JPanel();
    frame.add(as);  
    frame.add(new Ova());
    frame.setVisible(true);

    timer.start();
}

public void paintComponent(Graphics g)
{
    super.paintComponent(g);    
    g.drawOval(x, y, 50, 50);
}

public void actionPerformed(ActionEvent e) {
    if (x<0 || x>950){
        x1=-x1; 
    }
    if (y<0 || y>530)
    {
        y1=-y1;
    }

    x=x+x1;
    y=y+y1;

    repaint();
}
}

Whenever I put timer.start() inside paintComponent() the repaint() method works but if I start the timer outside the paint component method the repaint method does not work.Please explain the reasons. Thank you.

Answer 1

Problem

You have mis-matched reference issue

Start with this snippet...

public class Ova extends JPanel implements ActionListener {

    int x = 0;
    Timer timer = new Timer(100, this);
    int y = 0, x1 = 5, y1 = 5;

    public static void main(String[] ds) {
        Ova ss = new Ova();
        ss.nn();
    }

    private void nn() {
        JFrame frame = new JFrame("fram");
        frame.setSize(1000, 600);

        JPanel as = new JPanel();
        frame.add(as);
        frame.add(new Ova());
        frame.setVisible(true);
        timer.start();

    }

First, main creates an instance of Ova , then the method nn creates a new instance of Ova and then you start the timer ...

Now the question you need to ask is, which Ova s timer did you start...the one on the screen or the one main created...

I can tell, it was the one that main created, which is not visible on the screen

(A possible) Solution

Start by never creating a JFrame inside a JPanel (or other component). The only time I would do something like that is via a static support method...

Remove the nn method and replace it with a begin (or something like that) method that starts the timer .

In the main method, create an instance of Ova , add it to an instance of JFrame and call Ova 's begin method...

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class Ova extends JPanel implements ActionListener {

    int x = 0;
    Timer timer = new Timer(100, this);
    int y = 0, x1 = 5, y1 = 5;

    public static void main(String[] ds) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                Ova ova = new Ova();

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(ova);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);

                ova.begin();
            }
        });
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(600, 600);
    }

    private void begin() {
        timer.start();
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);

        g.drawOval(x, y, 50, 50);

    }

    @Override
    public void actionPerformed(ActionEvent e) {
        if (x < 0 || x > 950) {
            x1 = -x1;
        }
        if (y < 0 || y > 530) {
            y1 = -y1;

        }

        x = x + x1;
        y = y + y1;

        repaint();

    }
}

Avoid calling setSize on anything, instead, override the getPreferredSize method of the component and return a preferred size and then use pack on the JFrame to wrap the frame around this. You will get far better and reliable results.