java - Calling function from another activity

Question

I have an activity that contains all the functions for controlling my database, and I want all other activities to use to these functions when interacting with the database. Below is an example of my problem.

Clock script:

public class Clock extends Activity
{
    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_clock);

        Data.createTable(); //<<<
    }
        //...
}

Data script:

public class Data extends Activity
{
    SQLiteDatabase mydb;
    private static String DBNAME = "SHIFTS.db";
    private static String TABLE = "MY_SHIFTS"; 

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_data);
    }   

    public void createTable() //<<<
    {
        try
        {
            mydb = openOrCreateDatabase(DBNAME, Context.MODE_PRIVATE,null);
            mydb.execSQL("CREATE TABLE IF  NOT EXISTS "+ TABLE +" (ID INTEGER PRIMARY KEY, STARTDATE TEXT, ENDDATE TEXT, LENGTH INTEGER, TYPE INTEGER);");
            mydb.close();
        }
        catch(Exception e)
        {
            Toast.makeText(getApplicationContext(), "Error in creating table", Toast.LENGTH_LONG).show();
        }
    }
    // ... lots more functions
}

Error message:

Cannot make a static reference to the non-static method createTable() from the type Data.

And when I trying to make method static, it just causes more problems.

Also if I try Data data = new Data(); data.createTable(); I get a NullPointerException.

What is the problem here?

Answer 1

Add the static keyword to your shared methods

public static void createTable()

Then use:

Data.createTable();

somewhere in another Class / Fragment / Activity.

I do so and it works.

Well, my "Data" is a normal Class, not an Activity, but I also have an Activity with shared methods in the very same way.

This is my dbTableCreate method:

// Create the database table if it doesn't exist
protected final static void dbTableCreate(final Context ctx)
{
    SQLiteDatabase db = null;
    try
    {
        db = ctx.openOrCreateDatabase(DB_NAME, Context.MODE_PRIVATE, null);
        db.execSQL
        (
            "CREATE TABLE IF NOT EXISTS " + DB_TABLE +
            " (date DATETIME PRIMARY KEY NOT NULL DEFAULT (CURRENT_DATE), " +
            "score INTEGER DEFAULT (null));"
        );
    }
    catch (final SQLiteException se)
    {
        System.out.println
        (
            ctx.getClass().getSimpleName() + "
" +
                "Could not create the database"
        );
        se.printStackTrace();
    }
    finally
    {
        db.close();
    }
}

And I call it like:

Context ctx = getApplicationContext();
CLS_DB.dbTableCreate(ctx);

The main difference between my class and yours is that mine is declared as

public final class CLS_DB
{
    /* ----------------------------- Constants ------------------------------ */

    private final static String DB_NAME = "pat.db";
    private final static String DB_TABLE = "tests";

    /* ------------------------------ Methods ------------------------------- */

And not as an Activity.
I'm not initializing the Class, and it has no constructor.
It's just a bin of shared (and not, for doing operations not seen anywhere else in my code) methods.