Check input with InputVerifier for JTextField in Java

import javax.swing.InputVerifier;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JTextField;
/*  w  w w .  j av a  2 s  . c  om*/
public class Main {
  public static void main(String[] args) {
    JTextField tf = new JTextField();
    tf.setInputVerifier(new MyInputVerifier());

    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setLayout(new java.awt.GridLayout(0, 1));

    frame.add(tf);
    frame.setSize(150, 150);
    frame.setVisible(true);
    tf.requestFocus();
  }
}
class MyInputVerifier extends InputVerifier {
  public boolean verify(JComponent input) {
    JTextField tf = (JTextField) input;
    String pass = tf.getText();
    return pass.equals("A");
  }
}