import javax.swing.InputVerifier;
import javax.swing.JComponent;
import javax.swing.JTextField;
public class Main {
public static void main(String[] args) {
JTextField tf = new JTextField("");
tf.setInputVerifier(new MyInputVerifier());
}
}
class MyInputVerifier extends InputVerifier {
public boolean verify(JComponent input) {
JTextField tf = (JTextField) input;
String pass = tf.getText();
return pass.equals("A");
}
}