Question

I am learning how to use a sql database in an Android activity. I am learning from this tutorial. I have an activity in which I use the database handler, from the tutorial.
I make a declaration of the variable inside of activity class.

DatabaseHandler db;

In the oncreate method I create a new DatabaseHandler object:

DatabaseHandler db = new DatabaseHandler(this);

I want to use the database after a button has been clicked. So I use the connection in the onClick method, but I get a null pointer exception unless I create a new object in the onclick. Why is that, with different type of objects I can declare variables like I did in this example.

The constructor of the DatabaseHandler is:

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

The super being SQLiteOpenHelper.

Answer 1

You’re getting the null exception because you’re instantiating incorrectly. you declared your db a field, but then instantiated a different object DatabaseHandler db = new DatabaseHandler(this); <– as a local variable. Thus your onClick only has access to the uninstantiated field. to fix you need to replace what you have with this in your oncreate.

db = new DatabaseHandler(this);

or if the onClick is in onCreate, by applying the final modifier. (and removing the field object)