在android中的sqlite中求和

我是 java,android和sqlite的新手,我坚持这个.
我有列_id,type,amount(int),category,description.我正在尝试获取特定类别的金额总和并将它们存储在int中.
我知道我需要提供错误,但它只是给我力量关闭.请帮助

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();


}

}

如果详细解释,因为我是一个完整的菜鸟,我会尝试找到解决方案,但没有找到任何东西会有所帮助.谢谢提前@H_450_7@

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

这是日志.@H_450_7@

c.moveToFirst();

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();

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();