SwingのDocumentEventの追加について

内部クラスを使ってDocumentEventを使いたいのですが、addDocumentListenerの使い方がいまいちわかりません。どこを修正すべきでしょうか？ よろしくお願いします。

import java.awt.*;
import java.awt.event.*;
import javax.lang.model.element.Element;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextArea;
import javax.swing.JTextField;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import javax.swing.text.Caret;
import javax.swing.text.Document;

class CaesarFrame{
    JFrame frame;
    JTextField textField1, textField2;
    JButton button;
    JPanel panel1, panel2;
    JComboBox<Object> comboBox1;
    Object[] ob = {'A','B','C','D','E','F','G','H','I','J','K','L'
            ,'M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
    Container contentPane;
    JLabel label;

    // Constructor
    CaesarFrame(){  
        frame = new JFrame("SwingLab");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 110);
        frame.setLayout(new BorderLayout());
        frame.setLocationRelativeTo(null);

        contentPane = frame.getContentPane();
        textField1 = new JTextField(20);
        textField2 = new JTextField(28);
        textField2.setEditable(false);
        button = new JButton("Code!");
        comboBox1 = new JComboBox<Object>(ob);
        label = new JLabel("Output:");
        panel1 = new JPanel();
        panel2 = new JPanel();

        panel1.add(comboBox1);
        panel1.add(textField1);
        panel1.add(button);
        panel2.add(label);
        panel2.add(textField2);

        contentPane.add(panel1, BorderLayout.NORTH);
        contentPane.add(panel2,BorderLayout.WEST);

        frame.setVisible(true);

        ActionListener buttonCode = new OkButtonActionListener();
        button.addActionListener(buttonCode);

        KeyAdapter key = new InputFieldKeyListener();
        textField1.addKeyListener(key);
        //この箇所がよくわかりません。
        KeyAdapter key2 = new InputFieldKeyListener();
        Document doc = ((DocumentEvent) key2).getDocument();
        doc.addDocumentListener((DocumentListener) key);

    }

    class InputFieldKeyListener extends KeyAdapter implements DocumentListener{
        public void keyTyped(KeyEvent e) {
            char temp = e.getKeyChar();
            String ch =String.valueOf(temp);
            char offset = (char) comboBox1.getSelectedItem();
            textField2.setText(Caesar.caesarCode(ch,offset));
        }

        @Override
        public void changedUpdate(DocumentEvent e) {
            System.out.println("Changed");
            }

        @Override
        public void insertUpdate(DocumentEvent e) {
            // TODO Auto-generated method stub
            System.out.println("Inserted");
            }

        @Override
        public void removeUpdate(DocumentEvent e) {
            // TODO Auto-generated method stub
            System.out.println("Removed");
            }


    }



    private class OkButtonActionListener implements ActionListener{
        @Override
        public void actionPerformed(ActionEvent e) {
            String message = textField1.getText();
            char offset = (char) comboBox1.getSelectedItem();
            textField2.setText(Caesar.caesarCode(message,offset));
        }
    }
}