How to restrict the number of digits inside JPasswordField in Java?

A JPasswordField is a subclass of JTextField and each character entered in a JPasswordField can be replaced by an echo character. This allows confidential input for passwords. The important methods of JPasswordField are getPassword(), getText(), getAccessibleContext() and etc. By default, we can enter any number of digits inside JPasswordField. If we want to restrict the digits entered by a user by implementing a DocumentFilter class and need to override the replace() method.

Syntax

<strong>public void replace(DocumentFilter.FilterBypass fb, int offset, int length, String text, AttributeSet attrs) throws BadLocationException</strong>

Example

import java.awt.*;
import java.awt.*;
import javax.swing.*;
import javax.swing.text.*;
public class JPasswordFieldDigitLimitTest extends JFrame {
   private JPasswordField passwordField;
   private JPanel panel;
   public JPasswordFieldDigitLimitTest() {
      panel = new JPanel();
      ((FlowLayout) panel.getLayout()).setHgap(2);
      panel.add(new JLabel("Enter Pin: "));
      passwordField = new JPasswordField(4);
      PlainDocument document = (PlainDocument) passwordField.getDocument();
      document.setDocumentFilter(new <strong>DocumentFilter</strong>() {
         @Override
         public void <strong>replace</strong>(DocumentFilter.FilterBypass fb, int offset, int length, String text, AttributeSet attrs) throws BadLocationException {
            String string = fb.getDocument().getText(0, fb.getDocument().getLength()) + text;
               if (string.length() <= 4) {
                  super.replace(fb, offset, length, text, attrs);
               }
         }
      });
      panel.add(passwordField);
      add(panel);
      setSize(400, 300);
      setDefaultCloseOperation(EXIT_ON_CLOSE);
      setLocationRelativeTo(null);
      setVisible(true);
   }
   public static void main(String[] args) {
      new JPasswordFieldDigitLimitTest();
   }
}

Output