Q:

Getting cannot find symbol errors. ArrayLists, Iterator

A supermarket wants to reward the top customers of the day, that is, the topN customers with the largest sales, where topN is a value that the user of the program supplies, showing the customer’s name on a screen in the supermarket. For that purpose, the customer’s purchase amount is stored in an ArrayList

Implement a method: public static ArrayList

Write a program that prompts the cashier to enter all prices and names, adds them to two array lists, calls the method that you implemented, and displays the result. Use a price of 0 as a sentinel.
My errors when compiling are:
------ Compile ----------
hw1num2.java:44: error: cannot find symbol
                double check = iter.nextDouble();
                                   ^
  symbol:   method nextDouble()
  location: variable iter of type Iterator
hw1num2.java:45: error: cannot find symbol
                if (check>=sorted(topN-1))
                           ^
 symbol:   method sorted(int)
location: class hw1num2
hw1num2.java:47: error: no suitable method found for add(double)
                    topCust.add(check);
                           ^
   method ArrayList.add(int,String) is not applicable
    (actual and formal argument lists differ in length)
 method ArrayList.add(String) is not applicable
 (actual argument double cannot be converted to String by method invocatio>n conversion)
3 errors

Output completed (1 sec consumed) - Normal Termination

Here's what I have:
import java.util.*;

public class hw1num2
{
    public static void main(String[] args) 
    {
        Scanner in = new Scanner(System.in);
        ArrayList<Double> sales = new ArrayList<Double>();
        ArrayList<String> customers = new ArrayList<String>();

        boolean end = true;

        while (end)  //Continues to take input for customer names and purchases until done
        {
            System.out.println("Enter the first name of the customer.");
            String cust = in.next();
            System.out.println("Enter the purchase total of that customer.");
            Double total = in.nextDouble();
            sales.add(total);
            customers.add(cust);
            System.out.println("Enter 1 if you have another customer to add, or 2 if you are done.");
            int choice = in.nextInt();
            if (choice != 1)
            {
                end=false;
            }
        }
        System.out.println("How many top customers would you like to display?");
        int topN = in.nextInt();
        System.out.println(nameOfBestCustomers(sales, customers, topN));  //calls method that computes top custs
    }

    public static ArrayList<String> nameOfBestCustomers(ArrayList<Double> sales, ArrayList<String> customers, 
    int topN) //Finds out who the topN customers were
    {
        ArrayList<Double> sorted = new ArrayList<Double>(sales); //create copy of sales ArrayList
        ArrayList<String> topCust = new ArrayList<String>(); //create ArrayList to hold top cust names
        Collections.sort(sorted);  //sort the copied ArrayList.
        Iterator iter = sales.iterator();  
        while (iter.hasNext())  //iterate through sales ArrayList to find indexes of top purchases
        {
            for (int i=0;i<sales.size();i++)
            {
                double check = (double)iter.next(); 
                if (check>=sorted.get(topN-1)) //checks if each index is >= the top Nth customer
                {
                    topCust.add(customers.get(i)); //if so, adds it to topCust list
                }
            }
        }
        return topCust;  //returns the list with the top customer names
    }
}

A:

1- You are getting the error because the Iterator class doesn't have a method called nextDouble().
Please have a look at Iterator API for a list of supported methods.
2- topCust is a String ArrayList, you cannot add a double directly to that list.
3- 
P:S:your code here : 
int choice = 1;
if (choice != 1)
{
    end=false;
}

will cause an endless while loop in your main method.
Hope this helps.

