Então, estou tentando classificar um listview de arrayadapter simples. Aqui está o que eu tentei:

    ListView lv;
String[] months = {
        "January",
        "February",
        "March",
        "April",
        "May",
        "June",
        "July",
        "August",
        "September",
        "October",
        "November",
        "December"
};

ArrayAdapter<String> adapter;
ArrayList<String> list;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    lv = (ListView)findViewById (R.id.listView1);
    lv.setFastScrollEnabled(true);
    adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, months);
    lv.setAdapter(adapter);

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // TODO Auto-generated method stub

    switch (item.getItemId()) {
    case R.id.sortasc:
        list = new ArrayList<String>(Arrays.asList(months));
        Collections.sort(list);
        for(int i=0; i < list.size(); i++){
            System.out.println(list.get(i));
        }
        adapter.clear();
        adapter = new ArrayAdapter<String>(this,
                android.R.layout.simple_list_item_1, list);
        adapter.notifyDataSetChanged();
        return true;
    case R.id.sortdesc:

        return true;
    default:
        return super.onOptionsItemSelected(item);
    }
}

Mas estou recebendo UnsupportedOperationException, o que há de errado com isso? Eu realmente preciso de ajuda. Desde já, obrigado!