import javax.swing.*;
import javax.swing.text.*;

public class S04FirstAutoCompletion extends PlainDocument {
    JComboBox comboBox;
    ComboBoxModel model;
    // flag to indicate if setSelectedItem has been called
    // subsequent calls to remove/insertString should be ignored
    boolean selecting=false;
    
    public S04FirstAutoCompletion(final JComboBox comboBox) {
        this.comboBox = comboBox;
        model = comboBox.getModel();
    }
    
    public void remove(int offs, int len) throws BadLocationException {
        // return immediately when selecting an item
        if (selecting) return;
        super.remove(offs, len);
    }
    
    public void insertString(int offs, String str, AttributeSet a) throws BadLocationException {
        // return immediately when selecting an item
        if (selecting) return;
        // insert the string into the document
        super.insertString(offs, str, a);
        // lookup and select a matching item
        Object item = lookupItem(getText(0, getLength()));
        setSelectedItem(item);
        // remove all text and insert the completed string
        super.remove(0, getLength());
        super.insertString(0, item.toString(), a);
        // select the completed part
        JTextComponent editor = (JTextComponent) comboBox.getEditor().getEditorComponent();
        editor.setSelectionStart(offs+str.length());
        editor.setSelectionEnd(getLength());
    }
    
    private void setSelectedItem(Object item) {
        selecting = true;
        model.setSelectedItem(item);
        selecting = false;
    }
    
    private Object lookupItem(String pattern) {
        // 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 (currentItem.toString().startsWith(pattern)) {
                return currentItem;
            }
        }
        // no item starts with the pattern => return null
        return null;
    }
    
    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 S04FirstAutoCompletion(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();
            }
        });
    }
}