Enable or disable a JButton based on JTextField content in Java

import java.awt.FlowLayout;
/*w w w .  j  a  v  a2  s  .  c  o m*/
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import javax.swing.text.Document;

public class Main {
  public static void main(String[] args) {
    JFrame f = new JFrame("Text Field Elements");

    f.setLayout(new FlowLayout());

    JButton button = new JButton("foo");
    JTextField textfield = new JTextField(10);
    Document document;
    document = textfield.getDocument();
    document.addDocumentListener(new JButtonStateController(button));

    f.add(button);
    f.add(textfield);
    f.pack();
    f.setVisible(true);

  }


}
class JButtonStateController implements DocumentListener {
  JButton button;
  JButtonStateController(JButton b) {
    button = b;
  }

  public void changedUpdate(DocumentEvent e) {
    disableIfEmpty(e);
  }

  public void insertUpdate(DocumentEvent e) {
    disableIfEmpty(e);
  }

  public void removeUpdate(DocumentEvent e) {
    disableIfEmpty(e);
  }

  public void disableIfEmpty(DocumentEvent e) {
    button.setEnabled(e.getDocument().getLength() > 0);
  }
}