Java Swing - Dibuja N Circulos

import javax.swing.*;
import java.awt.*;
import java.awt.Color;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;

public class PS3{
    static JFrame frame;
    static JButton button;
    static JTextField label;
    static CenterPanel panel;
    static int N;
    static boolean buttonPressed = false;
    public static void main (String args[]) {
        PS3 gui = new PS3();
        gui.go();
    }
    public void go(){
        frame = new JFrame();
        button = new JButton("Ingresar numero");
        label = new JTextField();
        panel = new CenterPanel();

        frame.setSize(600,600);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new BorderLayout());
        frame.setVisible(true);

        frame.add(label, BorderLayout.PAGE_START);
        frame.add(panel, BorderLayout.CENTER);
        frame.add(button, BorderLayout.PAGE_END);

        button.addActionListener(new buttonAction());
        panel.addMouseListener(new panelAction());
    }
    class buttonAction implements ActionListener{
        public void actionPerformed(ActionEvent a){
            buttonPressed = false;
            String answer = label.getText();
            try{
                N = Integer.parseInt(answer);
                if(N<1){
                    int E = Integer.parseInt("error.");
                }else{
                    frame.setTitle("Dibujo "+N+" circulos");
                    buttonPressed = true;
                }
            }catch(NumberFormatException ex){
                JOptionPane.showMessageDialog(null,"El numero ingresado no es valido","ERROR!",JOptionPane.INFORMATION_MESSAGE);
            }
        }
    }class panelAction implements MouseListener{
        public void mouseClicked(MouseEvent m){
            if(!buttonPressed){
                JOptionPane.showMessageDialog(null,"No se ha ingresado un numero.","ERROR!",JOptionPane.INFORMATION_MESSAGE);
            }for(int i = 1; i<=N ; i++){
                panel.draw(m,i);
            }
        }public void mouseEntered(MouseEvent m){

        }public void mouseExited(MouseEvent m){

        }public void mousePressed(MouseEvent m){

        }public void mouseReleased(MouseEvent m){

        }
    }public class CenterPanel extends JPanel{
        public void paintComponent(Graphics g){
            super.paintComponent(g);
            this.setBackground(Color.WHITE);
        }public void draw(MouseEvent m,int i){
            super.getGraphics().setColor(Color.BLACK);
            super.getGraphics().drawOval(m.getX(),m.getY(),i*50,i*50);
        }
    }
}