Creo que el código siguiente te dará algunas ideas.

/**
<p>
@author Carlos Hoces
*/
public class NewJFrame extends javax.swing.JFrame {

private double xCoord, yCoord, diameter;

/** Creates new form NewJFrame */
public NewJFrame() {
diameter = 50;
initComponents();
Executors.newScheduledThreadPool(1).scheduleAtFixedRate(new Runnable() {

@Override
public void run() {
xCoord++;
yCoord++;
diameter++;
jPanel1.repaint();
}
}, 1, 1, TimeUnit.SECONDS);
}

/** This method is called from within the constructor to
initialize the form.
WARNING: Do NOT modify this code. The content of this method is
always regenerated by the Form Editor.
*/
@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">
private void initComponents() {

jPanel1 = new Circulo();

setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
jPanel1.setLayout(jPanel1Layout);
jPanel1Layout.setHorizontalGroup(
jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGap(0, 400, Short.MAX_VALUE)
);
jPanel1Layout.setVerticalGroup(
jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGap(0, 300, Short.MAX_VALUE)
);

getContentPane().add(jPanel1, java.awt.BorderLayout.CENTER);

pack();
}// </editor-fold>

/**
@param args the command line arguments
*/
public static void main(String args[]) {
/* Create and display the form */
java.awt.EventQueue.invokeLater(new Runnable() {
@Override
public void run() {
new NewJFrame().setVisible(true);
}
});
}

private class Circulo extends JPanel {

@Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
((Graphics2D) g).fill(new Ellipse2D.Double(xCoord, yCoord, diameter, diameter));
}
}

// Variables declaration - do not modify
private javax.swing.JPanel jPanel1;
// End of variables declaration
}