The KeyListener interface has three methods to be implemented: keyPressed, keyTyped and keyReleased. As each key is pressed you can do what you need to in this implemented listener. So for example, if the current key typed or the current contents of the text field is not to your approval, you can take the visual or logical action you need to take.
I don't like links generally, but here is a small tutorial on KeyListeners.
And here is an example to add a KeyListener to a JTextField:
usernameTextField.addKeyListener(new KeyAdapter() {
public void keyReleased(KeyEvent e) {
JTextField textField = (JTextField) e.getSource();
String text = textField.getText();
textField.setText(text.toUpperCase());
}
public void keyTyped(KeyEvent e) {
// TODO: Do something for the keyTyped event
}
public void keyPressed(KeyEvent e) {
// TODO: Do something for the keyPressed event
}
});