如何使用数据库制作Android单例和自定义数据类？

   private static DatabaseSingleton instance;

   private SQLiteDatabase database;

   private DatabaseSingleton(Context context) {

       // 初始化数据库

       database = new DatabaseHelper(context).getWritableDatabase();

   }

   public static synchronized DatabaseSingleton getInstance(Context context) {

       if (instance == null) {

           instance = new DatabaseSingleton(context);

       }

       return instance;

   }

   public SQLiteDatabase getDatabase() {

       return database;

   }

   // ...

   public void insertData(DataModel data) {

       ContentValues values = new ContentValues();

       values.put("column_name", data.getValue());

       database.insert("table_name", null, values);

   }

   public List<DataModel> getAllData() {

       List<DataModel> dataList = new ArrayList<>();

       Cursor cursor = database.query("table_name", null, null, null, null, null, null);

       if (cursor.moveToFirst()) {

           do {

               String value = cursor.getString(cursor.getColumnIndex("column_name"));

               DataModel data = new DataModel(value);

               dataList.add(data);

           } while (cursor.moveToNext());

       }

       cursor.close();

       return dataList;

   }

   // ...

   private DatabaseSingleton databaseSingleton;

   @Override

   protected void onCreate(Bundle savedInstanceState) {

       super.onCreate(savedInstanceState);

       setContentView(R.layout.activity_main);

       // 获取数据库实例

       databaseSingleton = DatabaseSingleton.getInstance(this);

       // 插入数据

       DataModel data = new DataModel("Some value");

       databaseSingleton.insertData(data);

       // 查询数据

       List<DataModel> dataList = databaseSingleton.getAllData();

       for (DataModel data : dataList) {

           Log.d("TAG", data.getValue());

       }

   }