Create a sorted List Model for JList in Java

import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.SortedSet;
import java.util.TreeSet;
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import javax.swing.AbstractListModel;
import javax.swing.JFrame;
import javax.swing.JList;
import javax.swing.JScrollPane;

public class Main {
  public static void main(String args[]) {
    JFrame frame = new JFrame("Custom Tip Demo");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    JList list = new JList();
    SortedListModel model = new SortedListModel();
    list.setModel(model);
    model.add("a");
    model.add("c");
    model.add("b");

    JScrollPane scrollPane = new JScrollPane(list);
    frame.getContentPane().add(scrollPane);
    frame.setSize(300, 300);
    frame.setVisible(true);
  }
}
class SortedListModel extends AbstractListModel {

  SortedSet model;

  public SortedListModel() {
    model = new TreeSet();
  }

  public int getSize() {
    return model.size();
  }

  public Object getElementAt(int index) {
    return model.toArray()[index];
  }

  public void add(Object element) {
    if (model.add(element)) {
      fireContentsChanged(this, 0, getSize());
    }
  }

  public void addAll(Object elements[]) {
    Collection c = Arrays.asList(elements);
    model.addAll(c);
    fireContentsChanged(this, 0, getSize());
  }

  public void clear() {
    model.clear();
    fireContentsChanged(this, 0, getSize());
  }

  public boolean contains(Object element) {
    return model.contains(element);
  }

  public Object firstElement() {
    return model.first();
  }

  public Iterator iterator() {
    return model.iterator();
  }

  public Object lastElement() {
    return model.last();
  }

  public boolean removeElement(Object element) {
    boolean removed = model.remove(element);
    if (removed) {
      fireContentsChanged(this, 0, getSize());
    }
    return removed;
  }
}