Sum query in sqlite in android

Question

I'm new to java,android and sqlite and i'm stuck at this. I have columns namely _id,type,amount(int),category,description.what i'm trying is to fetch the sum of amount of a particular category and store them in int. i know i need to provide the errors but it is simply giving me force close.Please Help

 public class TransactionReport extends Activity {
private static SQLiteDatabase db;
Cursor c;
int amount;
DbCrud localDbCrud = new DbCrud(this);

@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    setContentView(R.layout.report);
    localDbCrud.open();                //localDatabase = localDbHelper.getWritableDatabase();  return this;
    c.moveToFirst();
    do{
        c = db.rawQuery("select sum(amount) from transaction_table where category = Salary ;", null);


      }while(c.moveToNext());

    amount = c.getInt(0);
    c.close();
    TextView ltxt = (TextView) findViewById(R.id.amt);
    ltxt.setText(""+amount);
    localDbCrud.close();  //localDatabase.close();


}

}

Would be helpful if explained in detail as i am a complete noob and i did try to find the solution but didnt find any.Thanks in advance

10-10 03:55:29.828: E/AndroidRuntime(10229): FATAL EXCEPTION: main
10-10 03:55:29.828: E/AndroidRuntime(10229): java.lang.RuntimeException: Unable to      start activity   ComponentInfo{com.hishighness.budgetracker/com.hishighness.budgetracker.TransactionReport}:     java.lang.NullPointerException
10-10 03:55:29.828: E/AndroidRuntime(10229):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1651)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1667)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at  android.app.ActivityThread.access$1500(ActivityThread.java:117)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:935)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at android.os.Handler.dispatchMessage(Handler.java:99)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at android.os.Looper.loop(Looper.java:130)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at android.app.ActivityThread.main(ActivityThread.java:3687)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at java.lang.reflect.Method.invokeNative(Native Method)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at java.lang.reflect.Method.invoke(Method.java:507)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:842)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at dalvik.system.NativeStart.main(Native Method)
10-10 03:55:29.828: E/AndroidRuntime(10229): Caused by: java.lang.NullPointerException
10-10 03:55:29.828: E/AndroidRuntime(10229):    at com.hishighness.budgetracker.TransactionReport.onCreate(TransactionReport.java:27)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at  android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1615)
10-10 03:55:29.828: E/AndroidRuntime(10229):    ... 11 more

Here's the log.

Answer 1

Addition
I believe you want to use localDBCrud.rawQuery(), not db.rawQuery().

Original
I'm guessing that you have a NullPointerException. Here:

c.moveToFirst();

You are try to use c before you have initialized it with db.rawQuery().

---

You have some more logic errors here:

c.moveToFirst();
do{
    c = db.rawQuery("select sum(amount) from transaction_table where category = Salary ;", null);
  }while(c.moveToNext());

amount = c.getInt(0);
c.close();

Try this instead:

c = db.rawQuery("select sum(amount) from transaction_table where category = Salary ;", null);
if(c.moveToFirst())
    amount = c.getInt(0);
else
    amount = -1;
c.close();

Also @toadsky has made a great observation, follow his advice too.