import javax.swing.*;    
import java.awt.*;    

public class MyPanel extends JPanel {
    @Override
    public void paintComponent(Graphics g){    
        // clear the previous painting
        super.paintComponent(g);    
        Graphics2D g2 = (Graphics2D)g;    
        g2.setColor(Color.blue);    
        g2.drawOval(0, 0, 20,20);  
        g2.fillOval(50,50,20,20);    
    }
}

g2.drawOval(int x,int y,int height, int width);

This method will draw an oval at specified x and y position with given height and width. 

g2.fillOval(int x,int y,int height, int width); This method will fill an oval at specified x and y position with given height and width.