Question

I guess it’s some basic mistake since I’m still a rookie.

package pl.cashflowmanager;

//all necessary imports are here

    public class SummaryOverMonths extends ExpandableListActivity {


        private CfmDbAdapter db;
        private final Cursor monthsCountQuery = db.SQLDb.rawQuery("select * from expenses", null); //crashes here with: 12-10 12:51:15.101: E/AndroidRuntime(348): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{pl.cashflowmanager/pl.cashflowmanager.SummaryOverMonths}: java.lang.NullPointerException


    //rest code goes there

I commented the problem. The query is fine, the database is working. Should the cursor go inside the method or what?

Answer 1

You’re getting a NullPointerException because you haven’t instantiated the CfmDbAdapter yet. You have created a field for it, yes, but you haven’t put anything in it before trying to call your query on it.
So you’re asking a non-existing item to perform an action, causing the NullPointer.

Instantiate it first like so:

private CfmDbAdapter db = new CfmDbAdapter(); // Or whatever constructor it expects.
private final Cursor monthsCountQuery = db.SQLDb.rawQuery("select * from expenses", null); 

That should do the trick.