android - How can I split a long, single SQLiteOpenHelper into several classes, one for each table

Question

I know this has been asked a couple of times before, but in all those questions neither the OP's nor the people who answered, provided clear examples.

So what I'm trying to ask here is if having a class like this

public class MyDatabaseDB {

    // database constants
    public static final String DB_NAME = "mydatabase.db";
    public static final int    DB_VERSION = 1;

    // list table constants
    public static final String LIST_TABLE = "list";

    public static final String LIST_ID = "_id";
    public static final int    LIST_ID_COL = 0;

    public static final String LIST_NAME = "list_name";
    public static final int    LIST_NAME_COL = 1;

    // task table constants
    public static final String TASK_TABLE = "task";

    public static final String TASK_ID = "_id";
    public static final int    TASK_ID_COL = 0;

    public static final String TASK_LIST_ID = "list_id";
    public static final int    TASK_LIST_ID_COL = 1;

    public static final String TASK_NAME = "task_name";
    public static final int    TASK_NAME_COL = 2; 

    // CREATE and DROP TABLE statements
    public static final String CREATE_LIST_TABLE = 
            "CREATE TABLE " + LIST_TABLE + " (" + 
            LIST_ID   + " INTEGER PRIMARY KEY AUTOINCREMENT, " + 
            LIST_NAME + " TEXT    UNIQUE)";

    public static final String CREATE_TASK_TABLE = 
            "CREATE TABLE " + TASK_TABLE + " (" + 
            TASK_ID         + " INTEGER PRIMARY KEY AUTOINCREMENT, " + 
            TASK_LIST_ID    + " INTEGER, " + 
            TASK_NAME       + " TEXT " + 
           )";

    public static final String DROP_LIST_TABLE = 
            "DROP TABLE IF EXISTS " + LIST_TABLE;

    public static final String DROP_TASK_TABLE = 
            "DROP TABLE IF EXISTS " + TASK_TABLE;

    private static class DBHelper extends SQLiteOpenHelper {

        public DBHelper(Context context, String name, 
                CursorFactory factory, int version) {
            super(context, name, factory, version);
        }

        @Override
        public void onCreate(SQLiteDatabase db) {
            // create tables
            db.execSQL(CREATE_LIST_TABLE);
            db.execSQL(CREATE_TASK_TABLE);

            // insert lists
            db.execSQL("INSERT INTO list VALUES (1, 'Hobbies')");
            db.execSQL("INSERT INTO list VALUES (2, 'Sports')");

            // insert sample tasks
            db.execSQL("INSERT INTO task VALUES (1, 1, 'Play the guitar')");
            db.execSQL("INSERT INTO task VALUES (2, 1, 'Play video games')");
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, 
                int oldVersion, int newVersion) {

            Log.d("Task list", "Upgrading db from version " 
                    + oldVersion + " to " + newVersion);


            db.execSQL(MyDatabaseDB.DROP_LIST_TABLE);
            db.execSQL(MyDatabaseDB.DROP_TASK_TABLE);
            onCreate(db);
        }
    }

    // database object and database helper object
    private SQLiteDatabase db;
    private DBHelper dbHelper;

    // constructor
    public MyDatabaseDB(Context context) {
        dbHelper = new DBHelper(context, DB_NAME, null, DB_VERSION);
    }

    // private methods
    private void openReadableDB() {
        db = dbHelper.getReadableDatabase();
    }

    private void openWriteableDB() {
        db = dbHelper.getWritableDatabase();
    }

    private void closeDB() {
        if (db != null)
            db.close();
    }

    // public methods   

    public long insertTask(Task task) {
        ContentValues cv = new ContentValues();
        cv.put(TASK_LIST_ID, task.getListId());
        cv.put(TASK_NAME, task.getName());        

        this.openWriteableDB();
        long rowID = db.insert(TASK_TABLE, null, cv);
        this.closeDB();

        return rowID;
    }    

    public int updateTask(Task task) {
        ContentValues cv = new ContentValues();
        cv.put(TASK_LIST_ID, task.getListId());
        cv.put(TASK_NAME, task.getName());        

        String where = TASK_ID + "= ?";
        String[] whereArgs = { String.valueOf(task.getId()) };

        this.openWriteableDB();
        int rowCount = db.update(TASK_TABLE, cv, where, whereArgs);
        this.closeDB();

        return rowCount;
    }    

    public int deleteTask(long id) {
        String where = TASK_ID + "= ?";
        String[] whereArgs = { String.valueOf(id) };

        this.openWriteableDB();
        int rowCount = db.delete(TASK_TABLE, where, whereArgs);
        this.closeDB();

        return rowCount;
    }
}

This is a very reduced version of my class , built using some code I found on-line. In this example I'm only showing the code for two of my tables : List and Task, and just some of the sql methods for the Task table : insertTask,updateTask, and deleteTask.

Even though the code shown above works, I don't think it would be nice to have all the code for let's say ten tables all in the same class. So I tried to split all these code into several classes , one for each table. Something like this:

public class MyDatabaseDB {

    // database constants
    public static final String DB_NAME = "mydatabase.db";
    public static final int    DB_VERSION = 1;


    private static class DBHelper extends SQLiteOpenHelper {

        public DBHelper(Context context, String name, 
                CursorFactory factory, int version) {
            super(context, name, factory, version);
        }

        @Override
        public void onCreate(SQLiteDatabase db) {
            // create tables
            db.execSQL(ListDAL.CREATE_LIST_TABLE);
            db.execSQL(TaskDAL.CREATE_TASK_TABLE);

            // insert lists
            db.execSQL("INSERT INTO list VALUES (1, 'Hobbies')");
            db.execSQL("INSERT INTO list VALUES (2, 'Sports')");

            // insert sample tasks
            db.execSQL("INSERT INTO task VALUES (1, 1, 'Play the guitar')");
            db.execSQL("INSERT INTO task VALUES (2, 1, 'Play video games')");
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, 
                int oldVersion, int newVersion) {

            Log.d("Task list", "Upgrading db from version " 
                    + oldVersion + " to " + newVersion);


            db.execSQL(ListDAL.DROP_LIST_TABLE);
            db.execSQL(TaskDAL.DROP_TASK_TABLE);
            onCreate(db);
        }
    }

    // database object and database helper object
    private SQLiteDatabase db;
    private DBHelper dbHelper;

    // constructor
    public MyDatabaseDB(Context context) {
        dbHelper = new DBHelper(context, DB_NAME, null, DB_VERSION);
    }

    // private methods
    private void openReadableDB() {
        db = dbHelper.getReadableDatabase();
    }

    private void openWriteableDB() {
        db = dbHelper.getWritableDatabase();
    }

    private void closeDB() {
        if (db != null)
            db.close();
    }   

}

These are the two new classes, the I created to put the code related to a specific table :

The ListDAL doesn't have much code

public class ListDAL {

   // list table constants
    public static final String LIST_TABLE = "list";

    public static final String LIST_ID = "_id";
    public static final int    LIST_ID_COL = 0;

    public static final String LIST_NAME = "list_name";
    public static final int    LIST_NAME_COL = 1;

    // CREATE and DROP TABLE statements
    public static final String CREATE_LIST_TABLE = 
            "CREATE TABLE " + LIST_TABLE + " (" + 
            LIST_ID   + " INTEGER PRIMARY KEY AUTOINCREMENT, " + 
            LIST_NAME + " TEXT    UNIQUE)";

    public static final String DROP_LIST_TABLE = 
            "DROP TABLE IF EXISTS " + LIST_TABLE;


}

The TaskDAL class is the one that contains most of the code, and it is in this class that I have problems, specifically in the insertTask,updateTask and deleteTask with calls like this.openWriteableDB(),this.openWriteableDB() or calls like db.insert(TASK_TABLE, null, cv).

Since these methods are no longer inside TaskDAL, I can't get access to them. I tried passing some references to these methods to be used in place of this or db, but it didn't work

public class TaskDAL {
    // task table constants
    public static final String TASK_TABLE = "task";

    public static final String TASK_ID = "_id";
    public static final int    TASK_ID_COL = 0;

    public static final String TASK_LIST_ID = "list_id";
    public static final int    TASK_LIST_ID_COL = 1;

    public static final String TASK_NAME = "task_name";
    public static final int    TASK_NAME_COL = 2; 

    // CREATE and DROP TABLE statements
    public static final String CREATE_TASK_TABLE = 
            "CREATE TABLE " + TASK_TABLE + " (" + 
            TASK_ID         + " INTEGER PRIMARY KEY AUTOINCREMENT, " + 
            TASK_LIST_ID    + " INTEGER, " + 
            TASK_NAME       + " TEXT " + 
           )";
    public static final String DROP_TASK_TABLE = 
            "DROP TABLE IF EXISTS " + TASK_TABLE;       

     // public methods   

    public long insertTask(Task task) {
        ContentValues cv = new ContentValues();
        cv.put(TASK_LIST_ID, task.getListId());
        cv.put(TASK_NAME, task.getName());        

        this.openWriteableDB();
        long rowID = db.insert(TASK_TABLE, null, cv);
        this.closeDB();

        return rowID;
    }    

    public int updateTask(Task task) {
        ContentValues cv = new ContentValues();
        cv.put(TASK_LIST_ID, task.getListId());
        cv.put(TASK_NAME, task.getName());        

        String where = TASK_ID + "= ?";
        String[] whereArgs = { String.valueOf(task.getId()) };

        this.openWriteableDB();
        int rowCount = db.update(TASK_TABLE, cv, where, whereArgs);
        this.closeDB();

        return rowCount;
    }    

    public int deleteTask(long id) {
        String where = TASK_ID + "= ?";
        String[] whereArgs = { String.valueOf(id) };

        this.openWriteableDB();
        int rowCount = db.delete(TASK_TABLE, where, whereArgs);
        this.closeDB();

        return rowCount;
    }
}

So, have any of you ever tried to so something similar?? If I managed to correctly separate the DB code into several classes, would I be still able to JOIN tables??

P.S. Please do not close this question, if you think I did something wrong please tell me and I'll try to correct it

Answer 1

do you think you could give some hints on how to do it

This has nothing much to do with Android, and even not all that much to do with Java. Decomposing long programming structures (e.g., classes in Java) into smaller structures has standard techniques, called design patterns, with language-specific implementations.

For example, you could go with the composite pattern:

• Define an interface -- I'll call it TableHelper here -- that has onCreate() and onUpdate() methods that match those on SQLiteOpenHelper

• Define N classes, one per table, that implement the TableHelper interface and provide the create and upgrade logic for that table (along with whatever other business logic you want to have on those classes)

• Have your SQLiteOpenHelper define a TableHelper[] containing instances of your TableHelper classes, and have it delegate onCreate() and onUpgrade() to those TableHelper instances by iterating over the array