import javax.swing.ComboBoxModel;
import javax.swing.DefaultComboBoxModel;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import javax.swing.text.PlainDocument;

public class FilterJComboBox extends JComboBox{
    
    private FilterJComboBox self;
//    private UpperCaseDocument upperCaseDocument;
    private MyDocumentListener documentListener;
    private JTextField jtf;
    private String[] listItems ;
     
    public static void main(String...args){
        JFrame frame = new JFrame("FilterJComboBoxTest");
        FilterJComboBox filterComboBox = new FilterJComboBox();
        frame.getContentPane().add(filterComboBox);
        frame.setVisible(true);
        frame.pack();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
//        BasicComboBoxEditor b; 
    }
    
    public FilterJComboBox(){
        super();
        self = this;
        this.setEditable(true);
        this.setModel(new FilterModel());
        jtf = (JTextField)this.getEditor().getEditorComponent();
//        upperCaseDocument = new UpperCaseDocument(jtf);
//        upperCaseDocument.SetOnlyFirstLetterCapitalize(true);
        documentListener = new MyDocumentListener();
        
io

 protected synchronized final void writeLock() {
    try {
        while ((numReaders > 0) || (currWriter != null)) {
        if (Thread.currentThread() == currWriter) {
                    if (notifyingListeners) {
                        // Assuming one doesn't do something wrong in a
                        // subclass this should only happen if a
                        // DocumentListener tries to mutate the document.
                        throw new IllegalStateException(
                                      "Attempt to mutate in notification");
                    }
                    numWriters++;
                    return;
                }
        wait();
        }
        currWriter = Thread.currentThread();
            numWriters = 1;
    } catch (InterruptedException e) {
        throw new Error("Interrupted attempt to aquire write lock");
    }
    }