实现下拉框显示所有曾经登陆的用户名的小功能

点击注册键，会把用户名存储到数据库中；点击下拉图标，显示出所有存储在数据库中的用户名。
public class MainActivity extends Activity implements OnClickListener{
private PopupWindow pop;
private EditText username_et;
private ImageView drop_down_iv;
private Button register_btn;
private ListView listView;
private List list = new ArrayList();
private UserDao userDao;

protected void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    setContentView(R.layout.activity_main);    initView();    userDao = new UserDao(this);}private void initView() {    username_et = (EditText) findViewById(R.id.username_et);    drop_down_iv = (ImageView) findViewById(R.id.username_drop_down_iv);    register_btn = (Button) findViewById(R.id.register_btn);    username_et.setOnClickListener(this);    drop_down_iv.setOnClickListener(this);    register_btn.setOnClickListener(this);}@Overridepublic void onClick(View v) {    switch (v.getId()) {    case R.id.register_btn:        String name = username_et.getText().toString();        userDao.insert(name, null);        username_et.setText("");        Toast.makeText(this, "登陆成功", Toast.LENGTH_SHORT).show();        break;    case R.id.username_drop_down_iv:        getPopWindow();        pop.showAsDropDown(v,100,0);        break;    }}private void getPopWindow() {    if (pop != null){        pop.dismiss();        return;    }else {        initPopWindow();    }}private void initPopWindow() {    View contentView = View.inflate(this,R.layout.pop_contentview,null);    listView = (ListView) contentView.findViewById(R.id.username_lv);    list = userDao.query();    ArrayAdapter<User> adapter = new ArrayAdapter<User>(this, android.R.layout.simple_list_item_1, list);    listView.setAdapter(adapter);    listView.setOnItemClickListener(new OnItemClickListener() {        @Override        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {            User user = list.get(position);            String name = user.getName();            username_et.setText(name);        }    });    pop = new PopupWindow(contentView,LayoutParams.MATCH_PARENT,LayoutParams.WRAP_CONTENT,true);    //触摸事件,对显示出来的东西进行监听    contentView.setOnTouchListener(new OnTouchListener() {        public boolean onTouch(View v, MotionEvent event) {            if (pop != null && pop.isShowing()) {                pop.dismiss();                pop = null;            }            return false;        }    });}

}

用于处理数据库的Dao层
public class UserDao {
private MyOpendhelper myOpendhelper;
private SQLiteDatabase db;
private static final String TABLE_NAME = “user”;
private Context context;

public UserDao(Context context){    myOpendhelper = new MyOpendhelper(context);    this.context = context;}public void insert(String userName,String password){    db = myOpendhelper.getWritableDatabase();    ContentValues values = new ContentValues();    values.put("userName", userName);    values.put("password", password);    db.insert(TABLE_NAME, null, values);    Toast.makeText(context,"插入成功",Toast.LENGTH_SHORT).show();    db.close();}public List<User> query(){    List<User> list = new ArrayList<User>();    db = myOpendhelper.getWritableDatabase();    Cursor cursor = db.query(TABLE_NAME, null, null, null, null, null, null,null);    while (cursor.moveToNext()) {        int id = cursor.getInt(0);        String name = cursor.getString(1);        String pwd = cursor.getString(2);        User user = new User();        user.setName(name);        user.setPassword(pwd);        user.setId(id);        list.add(user);    }    return list;}   

}

继承SQLiteOpenHelper的工具类
public class MyOpendhelper extends SQLiteOpenHelper{

public MyOpendhelper(Context context) {    super(context,"info.db",null,1);}@Overridepublic void onCreate(SQLiteDatabase db) {    String createsql = "create table user(_id integer primary key autoincrement,username,password)";    db.execSQL(createsql);}@Overridepublic void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {}

}