// 菜单项控制登出
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // 不分组,id为first,第一个出现在前面,文字为登出
        menu.add(Menu.NONE, Menu.FIRST, 1, "登出").setIcon(getWallpaper());
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
        case Menu.FIRST:
            Editor editor = sharedPreferences.edit();
            editor.remove("username");
            editor.commit();
            Intent intent = new Intent(HomeworkListActivity.this,
                    LoginActivity.class);
            startActivity(intent);
            break;
        }
        return false;
    }