JcomboBox con suggerimento testo

[music_ale]

Voglio realizzare un campo di testo (JComboBox) in cui, mentre si scrive si apre la tendina con i suggerimenti. Ho cercato di farlo da solo ma ho difficoltà.
Ho trovato questo codice in rete, ma analizzandolo passo passo mi perdo e non riesco a capire come funziona.
C'è qualcuno che sa aiutarmi ?

import java.awt.event.*;
import javax.swing.*;
import javax.swing.text.*;

public class S09ShowPopup extends PlainDocument {
    JComboBox comboBox;
    ComboBoxModel model;
    JTextComponent editor;
    // flag to indicate if setSelectedItem has been called
    // subsequent calls to remove/insertString should be ignored
    boolean selecting = false;

    public S09ShowPopup(final JComboBox comboBox) {
        this.comboBox = comboBox;
        model = comboBox.getModel();
        editor = (JTextComponent) comboBox.getEditor().getEditorComponent();

        comboBox.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                System.out.println("actionListener");
                if (!selecting) highlightCompletedText(0);
            }
        });

        editor.addKeyListener(new KeyAdapter() {
            public void keyPressed(KeyEvent e) {
                // if  (e.getKeyCode() == KeyEvent.VK_S)
                System.out.println("addKeyListener");
                if (comboBox.isDisplayable()) comboBox.setPopupVisible(true);
            }
        });
    }

    public void remove(int offs, int len) throws BadLocationException {
        // return immediately when selecting an item
        System.out.println("remove");
        if (selecting) return;
        super.remove(offs, len);
    }

    public void insertString(int offs, String str, AttributeSet a) throws BadLocationException {
        // return immediately when selecting an item
        System.out.println("insertString");
        if (selecting) return;

        // insert the string into the document
        super.insertString(offs, str, a);
        System.out.println("super.insertString");

        // lookup and select a matching item
        System.out.println("\n"+getText(0, getLength()));
        Object item = lookupItem(getText(0, getLength()));

        if (item != null) {

            System.out.println("dentro if di insertString "+item);
            setSelectedItem(item);

        } else {
            
            System.out.println("dentro else di insertString "+item);

            // keep old item selected if there is no match
            item = comboBox.getSelectedItem();

            // imitate no insert (later on offs will be incremented by str.length(): selection won't move forward)
            offs = offs - str.length();

            // provide feedback to the user that his input has been received but can not be accepted
            comboBox.getToolkit().beep(); // when available use: UIManager.getLookAndFeel().provideErrorFeedback(comboBox);
        }
        // setText(item.toString());
        // select the completed part
        highlightCompletedText(offs + str.length());
    }

    private void setText(String text) throws BadLocationException {
        // remove all text and insert the completed string
        System.out.println("setText");
        super.remove(0, getLength());
        super.insertString(0, text, null);
    }

    private void highlightCompletedText(int start) {
        System.out.println("highlightCompletedText");
        editor.setSelectionStart(start);
        editor.setSelectionEnd(getLength());
    }

    private void setSelectedItem(Object item) {
        System.out.println("setSelectedItem");
        selecting = true;
        model.setSelectedItem(item);
        selecting = false;
    }

    private Object lookupItem(String pattern) {
        
        System.out.println("\nlookupItem");
        
        System.out.println("\nprima della getSelectedItem");
        Object selectedItem = model.getSelectedItem();
        System.out.println("\ndopo della getSelectedItem");

        // System.out.println("\n"+selectedItem);
        System.out.println(model.getSize());
        comboBox.addItem("prova");
        // qui viene chiamata la remove e poi la insertString


        System.out.println(model.getSize());

        // only search for a different item if the currently selected does not match
        if (selectedItem != null && startsWithIgnoreCase(selectedItem.toString(), pattern)) {
            return selectedItem;
        } else {
            // iterate over all items
            for (int i=0, n=model.getSize(); i < n; i++) {
                Object currentItem = model.getElementAt(i);
                // current item starts with the pattern?
                if (startsWithIgnoreCase(currentItem.toString(), pattern)) {
                    return currentItem;
                }
            }
        }
        
        System.out.println("No match");
        // no item starts with the pattern => return null
        return null;
    }

    // checks if str1 starts with str2 - ignores case
    private boolean startsWithIgnoreCase(String str1, String str2) {
        // System.out.println("startsWithIgnoreCase");
        System.out.println(str1.toUpperCase()+" -- "+str2.toUpperCase());
        System.out.println(str1.toUpperCase().startsWith(str2.toUpperCase()));
        return str1.toUpperCase().startsWith(str2.toUpperCase());
    }

    private static void createAndShowGUI() {
        // the combo box (add/modify items if you like to)
        JComboBox comboBox = new JComboBox(new Object[] {"Ester", "Jordi", "Jordina", "Jorge", "Sergi"});
        // has to be editable
        comboBox.setEditable(true);
        // get the combo boxes editor component
        JTextComponent editor = (JTextComponent) comboBox.getEditor().getEditorComponent();
        // change the editor's document
        editor.setDocument(new S09ShowPopup(comboBox));

        // create and show a window containing the combo box
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(3);
        frame.getContentPane().add(comboBox);
        frame.pack(); frame.setVisible(true);
    }


    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}