Access to SQLiteOpenHelper created in MainActivity

Question

I have created a SQLiteOpenHelper object in MainActivity:

    public class ExchangeActivity extends AppCompatActivity {


public CurrencyDBHelper db;
private Handler handler;
private int delay = 30000;
private DataHandler dataHandler;

@Override
protected void onCreate(Bundle savedInstanceState) {
    //create CurrencyDBHelper object
    db = new CurrencyDBHelper(this);
    Log.v("DBTag", "DB created");


    //Activity and UI
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_exchange);
    }}

My CurrencyDBHelper class code:

public class CurrencyDBHelper extends SQLiteOpenHelper {

public static final int DATABASE_VERSION = 1;
public static final String DATABASE_NAME = "LastCurrency.db";

//constructor
public CurrencyDBHelper(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
}

@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL("CREATE TABLE CURRENCY ("
            + "_id INTEGER PRIMARY KEY AUTOINCREMENT, "
            + "NAME TEXT, "
            + "RATE REAL); ");
}

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

}

I need to call the db from the other class. How can I do this from the other class?

I tryed to create SQLiteOpenHelper not in MainActivity, but it does not work for me. It seems like I have to use Context, but I dont undestand hot do this.

Please help. Thanks!

@MatPag i've edited question (added CurrencyDBHelper class code) — Anton Prokopov, Jul 21 '17 at 18:14
@MatPag There is no errors. I just can't uderstand how to call variable of MainActivity (in my case it's ExchangeActivity) from the other class. In case of ordinary java classes we usually use className.objectName.varName.But it does not work in case of Activities — Anton Prokopov, Jul 21 '17 at 18:28
Follow this guide: https://github.com/codepath/android_guides/wiki/Local-Databases-with-SQLiteOpenHelper — MatPag, Jul 21 '17 at 20:32

score 2 · Accepted Answer · answered Jul 21 '17 at 21:45

1) Move the DBHelper class to be a class in it's own right i.e CurrencyDBHelper.java should be :-

public class CurrencyDBHelper extends SQLiteOpenHelper {

    public static final int DATABASE_VERSION = 1;
    public static final String DATABASE_NAME = "LastCurrency.db";

    //constructor
    public CurrencyDBHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE CURRENCY ("
                + "_id INTEGER PRIMARY KEY AUTOINCREMENT, "
                + "NAME TEXT, "
                + "RATE REAL); ");
    }

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

    }
}

2) In the activity in which you want to use the database use:-

db = new CurrencyDBHelper(this);

Note! this will NOT create the database. The database will only be created when an attempt is made to open it e.g. the getWRiteableDatabase or the getReadableDatabase is called.

so 3)

SQLiteDatabase mydb = db.getWritableDatabase();

will result in the database being created (i.e the DBHelper's onCreate method will be called).

Note! onCreate will only be called once for the lifetime of the database file. As such if you want to amend the structure of the database, when developing it's probably easiest to clear the Apps Data or uninstall and then re-install the App.

At this stage there will be no data in the table.

You could however query the empty table e.g :-

        Cursor csr = mydb.query("CURRENCY",null,null,null,null,null,null,null);
        Log.v("MYDB","Table CURRENCY has " +
                Integer.toString(csr.getCount()) +
                " rows"
        );
        for (int i=0; i < csr.getColumnCount(); i++) {
            Log.v("MYDB","Table CURRENCY has a column named " +
                    csr.getColumnName(i)
            );
        }
        csr.close();

The using activity could then be :-

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        db = new CurrencyDBHelper(this);
        SQLiteDatabase mydb = db.getWritableDatabase();
        Cursor csr = mydb.query("CURRENCY",null,null,null,null,null,null,null);
        Log.v("MYDB","Table CURRENCY has " +
                Integer.toString(csr.getCount()) +
                " rows"
        );
        for (int i=0; i < csr.getColumnCount(); i++) {
            Log.v("MYDB","Table CURRENCY has a column named " +
                    csr.getColumnName(i)
            );
        }
        csr.close();
    }

The output to the log would be along the lines of :-

07-21 17:38:07.290 1886-1886/mjt.cardoniser V/MYDB: Table CURRENCY has 0 rows
07-21 17:38:07.290 1886-1886/mjt.cardoniser V/MYDB: Table CURRENCY has a column named _id
07-21 17:38:07.290 1886-1886/mjt.cardoniser V/MYDB: Table CURRENCY has a column named NAME
07-21 17:38:07.290 1886-1886/mjt.cardoniser V/MYDB: Table CURRENCY has a column named RATE

Thank you for your answer! It is a great tutorial for work with QSLite in Android. — Anton Prokopov, Jul 22 '17 at 18:54

Access to SQLiteOpenHelper created in MainActivity

1 Answers1