Java Swing How to - Move a circle automatically with Timer

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Question

We would like to know how to move a circle automatically with Timer.

Answer

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
//from ww  w.j a  v  a  2s. c  o  m
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;

public class Main {
  public static void main(String[] args) {
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.add(new TestPane());
    frame.pack();
    frame.setVisible(true);
  }
}

class TestPane extends JPanel {
  int x = 0,y = 100,radius = 20,xDelta = 2;

  public TestPane() {
    Timer timer = new Timer(40, new ActionListener() {
      @Override
      public void actionPerformed(ActionEvent e) {
        x += xDelta;
        if (x + (radius * 2) > getWidth()) {
          x = getWidth() - (radius * 2);
          xDelta *= -1;
        } else if (x < 0) {
          x = 0;
          xDelta *= -1;
        }
        repaint();
      }
    });
    timer.start();
  }

  @Override
  public Dimension getPreferredSize() {
    return new Dimension(200, 200);
  }

  @Override
  protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.setColor(Color.RED);
    g.fillOval(x, y - radius, radius * 2, radius * 2);
  }
}

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