Set MaxLength to Password field

Question

Hi All,
I have developed small login window kind of application. I want to set maximam length for the password field. Narratively, I want to show the message "Password should contain upto 8 characters" if user types more than 8 letters. How to do that?
You can view my code below
import javax.swing.JPasswordField;
import javax.swing.JLabel;
import javax.swing.JTextField;
import javax.swing.event.CaretListener;
import javax.swing.event.CaretEvent;
import javax.swing.text.*;
public class PassWord
    private JFrame fr;
    private JPanel panel;
    private JLabel userIDLabel;
    private JLabel passwordLabel;
    private JTextField userID;
    private JPasswordField password;
    int i=0;
    public void createFrame()
        panel = new JPanel();
        panel.setLayout(new GridLayout(2,2));
        userIDLabel = new JLabel("Userid");
        passwordLabel = new JLabel("Password");
        panel.add(userIDLabel);
        userID = new JTextField("Your UserID");
        password = new JPasswordField();
        password.setEchoChar('*');
        password.setColumns(8);
        /*password.addCaretListener(new CaretListener()
            public void caretUpdate(CaretEvent e)
                i++;
                if(i>=8)
                   password.exit(0);
        panel.add(userID);
        panel.add(passwordLabel);
        panel.add(password);
        fr = new JFrame("Authentication");
        fr.getContentPane().add(panel);
        fr.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        fr.setSize(200,200);
        fr.setLocation(300,200);
        fr.setVisible(true);
        fr.pack();
    public static void main(String args[])
        new PassWord().createFrame();
}

Answer 1

Sir,
Remember that behind a swing control lives a Model.
The most appropriate way to provide constraints (such as max length) is to build the constraint in to the Model itself.
Text controls provide a getDocument() method which returns the Document (model) the control is associated with.
You will notice that Document provides a method to add a DocumentListener to it.
A DocumentListener added to a Document is notified of any changes made to the document.
So, suppose your document has a max length of 8.
When you receive notification of a change, you can check to see if this maximum length has been exceeded.
If it has, you can, for example remove the extra characters from the model.
There are probably other ways to do this - but remember that the Swing control is just a View of the underlying Model. The best way to enforce constraints is in the model.