Swing中Java如何限定JTextFied的输入长度?

package er;

import java.awt.BorderLayout;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
import javax.swing.WindowConstants;

public class TextFieldTest extends javax.swing.JFrame {
    private JTextField field;

    /** * Auto-generated main method to display this JFrame */
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                TextFieldTest inst = new TextFieldTest();
                inst.setLocationRelativeTo(null);
                inst.setVisible(true);
            }
        });
    }

    public TextFieldTest() {
        super();
        initGUI();
    }

    private void initGUI() {
        try {
            setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
            {
                field = new LimitedTextField();
                getContentPane().add(field, BorderLayout.CENTER);
            }
            pack();
            this.setSize(400, 70);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    class LimitedTextField extends JTextField {
        private int maxLength = 5;

        public void setMaxLength(int maxLength) {
            this.maxLength = maxLength;
        }

        public int getMaxLength() {
            return this.maxLength;
        }

        public LimitedTextField() {
            this.addKeyListener(new KeyAdapter() {
                public void keyTyped(KeyEvent e) {
                    int code = e.getKeyChar();
                    if (code == KeyEvent.VK_BACK_SPACE) {
                        e.isActionKey();
                    } else if (getDocument().getLength() >= maxLength) {
                        e.consume();
                    }
                }
            });
        }
    }
}