[JAVA] Swing PinPad

package pinpad;

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class Pinpad extends JFrame implements ActionListener{
    JPanel jplControlPanel = new JPanel();
    JPanel jplLabelPanel = new JPanel();
    JPanel jplButtonPanel = new JPanel();
    JLabel jlbPasscode = new JLabel("Wpisz PIN:");
    JPasswordField jpwPasscode = new JPasswordField(4);
    JButton jbtNumber;
    StringBuilder inputPin = new StringBuilder("");

    public Pinpad() {
        jplLabelPanel.setLayout(new BorderLayout());
        jplButtonPanel.setLayout(new GridLayout(4,3));

        jplLabelPanel.add(jlbPasscode, BorderLayout.CENTER);
        jplLabelPanel.add(jpwPasscode, BorderLayout.SOUTH);

        for (int i = 1; i< 10; i++){
            jbtNumber = new JButton(i+"");
            jbtNumber.addActionListener(this);
            jplButtonPanel.add(jbtNumber);
        }

        jbtNumber = new JButton(" ");
        jbtNumber.addActionListener(this);
        jplButtonPanel.add(jbtNumber);
        jbtNumber = new JButton(0+"");
        jbtNumber.addActionListener(this);
        jplButtonPanel.add(jbtNumber);
        jbtNumber = new JButton("C");
        jbtNumber.addActionListener(this);
        jplButtonPanel.add(jbtNumber);

        jplControlPanel.setLayout(new BorderLayout());
        jplControlPanel.add(jplLabelPanel, BorderLayout.CENTER);
        jplControlPanel.add(jplButtonPanel, BorderLayout.SOUTH);

        add(jplControlPanel);

       /* Toolkit tk = Toolkit.getDefaultToolkit();
        int xSize = ((int) tk.getScreenSize().getWidth()/4);
        int ySize = ((int) tk.getScreenSize().getHeight()/4);
*/
        setPreferredSize(new Dimension(200, 200));
        setVisible(true);
        pack();
    }

    @Override public void actionPerformed(ActionEvent e){
        if(e.getActionCommand().equals("C")) {
            jpwPasscode.setText("");
            inputPin.delete(0, inputPin.length());
            System.out.println(inputPin);
        }

        for (int i = 1; i< 10; ++i){
            if(e.getActionCommand().equals(i+"")){
                inputPin.append(i);
                jpwPasscode.setText(inputPin.toString());
                System.out.println(inputPin);
            }
        }
    }

    public static void main(String[] args) {
        new Pinpad();
    }

}