Display result sqlite query in textview

public String calculrapport(Argent a)
   {
       db = this.getWritableDatabase();
           String rv = "0";
           db = this.getWritableDatabase();
           String query = "select sum(Entree) from Argent where date between \"strdatedebut\" and \"strdatefin\" ;";
           Cursor cursor = db.rawQuery(query , null) ;
           if (cursor.moveToFirst()) {
               rv = cursor.getString(0);
           }
           cursor.close();
           return rv;
       }

package com.example.pc.myapplication;
 
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
 
public class rapport extends Activity {
 
    DatabaseHelper helper = new DatabaseHelper(this);
 
 
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.rapport);
 
 
    }
    public void onOKClick ( View v ) {
 
        if (v.getId() == R.id.okrapport) {
 
            EditText datedebut = (EditText) findViewById(R.id.datedebut);
            EditText datefin = (EditText) findViewById(R.id.datefin);
            TextView edt1 = (TextView) findViewById(R.id.resultatrapport);
 
            String strdatedebut = datedebut.getText().toString();
            String strdatefin = datefin.getText().toString();
 
            Argent a = new Argent();
           String sum =  helper.calculrapport(a);
          edt1.setText(sum);
 
       }
 
    }
}