import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;

public class Foo {

    private JTextField feld, feld2, feld3;
    private DocumentListener listener;

    public Foo(){
        JFrame frame = new JFrame("DocumentListenerDemo");
        JPanel panel = new JPanel();

        initListener();

        feld = new JTextField(10);
        feld.getDocument().addDocumentListener(listener);
        feld2 = new JTextField(10);
        feld2.getDocument().addDocumentListener(listener);
        feld3 = new JTextField(12);

        panel.add(feld);
        panel.add(feld2);
        panel.add(feld3);
        frame.add(panel);
        frame.pack();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    private void initListener() {
        listener = new DocumentListener() {
            @Override
            public void removeUpdate(DocumentEvent e) {
                checkEquality();
            }

            @Override
            public void insertUpdate(DocumentEvent e) {
                checkEquality();
            }

            @Override
            public void changedUpdate(DocumentEvent e) {
            }
        };
    }


    public void checkEquality() {
        feld3.setText("Beide sind "+(feld.getText().equals(feld2.getText())?"gleich":"nicht gleich"));
    }

    public static void main(String[] args) throws Exception {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new Foo();
            }
        });
    }
}