How do you reference the same instance of a class when its passed in as a parameter in another class?

Question

I have extended SQLiteOpenHelper in this one class and have used the following method to update my database.

public void UpDateDataBase(ItemClass itemClass){

    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues contentValues = new ContentValues();
    contentValues.put(KEY_ItemClass_NAME, itemClass.getTaskName());
    contentValues.put(KEY_STATUS, itemClass.getStatus());
    db.update(TABLE_NAME, contentValues, KEY_ID + " = ?", new String[]{String.valueOf(itemClass.getId())});
}

public static DataBaseHelper getInstance(){
    return  dbHelper;
}

}

And when I try to call the UpdateDataBase() method in my other class using

DataBaseHelper.getInstance().UpDateDataBase(new ItemClass());

It tells me that I'm referencing a null object. How do I reference the same instance of the class?

Thank you.

Attempt to invoke virtual method 'void demoapp.app.android.sqlitedatabaseapplication.DataBaseHelper.UpDateDataBase(demoapp.app.android.sqlitedatabaseapplication.ItemClass)' on a null object reference

Answer 1

You need a make your DatabaseHelper class a singleton. A singleton is a design pattern that restricts the instantiation of a class to a single object. Alex Lockwood wrote a really simple and well explained article about this regarding Databases: http://www.androiddesignpatterns.com/2012/05/correctly-managing-your-sqlite-database.html

Also, in Java it is a convention to for your class method names to start with a lowercase letter: updateDatabase instead of UpdateDatabase Camelcase consists in putting a upercase letter at the beginning of a new word in a string without spaces: updateDatabase instead of UpDateDataBase

Answer 2

From Activity

DatabaseHelper databaseHelper=new DatabaseHelper(Your Activity Context)
databaseHelper.methodName();

in DatabaseHelper create a Constructor

public DatabaseHelper(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
}

i hope this will help you