Error with/using ArrayList

We are creating an ArrayList from a File, which contains both integers (odometer reading) and doubles (gallons per fill up). We're using them to make a mileage. We are then using a container class to access them, so on and so fourth.

The container looks like this:

package lab08;

public class FillUp{
   private final int odometer;
   private final double gallons;


  public FillUp(int givenOdometer, double givenGallons){
  odometer = givenOdometer;
  gallons = givenGallons;
  }

public int getOdometer(){
return odometer;
}

 public double getGallons(){
 return gallons;
 }

}

My code looks like the following:

public class MileageCalculator {    

    public static void main(String[] args) throws FileNotFoundException{
        Scanner scan = new Scanner(new File("/Users/Mobile_Hive/Desktop/Homework/mileage.txt"));
            double mileage=0.0;
            int i=0;
            System.out.println(fillUpArray(scan).get(0));
            /**
            while(i+1<fillUpArray(scan).size()){
                mileage=(((FillUp)fillUpArray(scan).get(i+1)).getOdometer()-((FillUp) fillUpArray(scan).get(i)).getOdometer())/((FillUp) fillUpArray(scan).get(i+1)).getGallons();
                System.out.println("Mileage: "+mileage);
                i++;
            }
           **/ 
        }
    private static ArrayList<FillUp> fillUpArray(Scanner scan){
        ArrayList<FillUp> list = new ArrayList<FillUp>();
        int odometer=0;
        double gallons=0.0;
        while(scan.hasNextLine()){
        String line = scan.nextLine();
        Scanner scans = new Scanner(line);
        while(scans.hasNext()){
            odometer=scans.nextInt();
            gallons=scans.nextDouble();
        }
        FillUp fill = new FillUp(odometer, gallons);
        list.add(fill);
        }
        return list;
    }

I am using the comments to occlude off my method for getting the actual mileage calculation and attempting to see if I can even access indexes of the ArrayList I have attempted to make.... When I run this in its current form I get:

lab08.FillUp@3bbf502d

When I remove the comments around "while(i+1< ) etc" leading to "i++" I get:

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 1, Size: 0
at java.util.ArrayList.rangeCheck(ArrayList.java:635)
at java.util.ArrayList.get(ArrayList.java:411)
at lab08.MileageCalculator.main(MileageCalculator.java:15)

I feel like in the first situation, I am not accessing the index of the array list, almost like it has not been filled. In the second attempt, I am not quite sure what is happening. I am not looking for straight forward answers, but maybe just explanation as to what is happening with very stripped down instructions on what needs to happen. I also would love information on what is happening so if someone can push me to some reading material that would be awesome.