Add item to ArrayList "recursively"

Question

I have an static ArrayList which I can only access from a class ("Sale"). In that ArrayList I want to add items of the same class. How can I add them from the constructor?

I've done it this way and it doesn't work:

public class Sale{
     private static ArrayList<Sale> sales;
     private Buyer buyer;
     private Item item;

     public Sale(Buyer buyer, Item item){
          this.buyer=buyer;
          this.item=item;
          sales.add(this);
     }
.....

Thanks in advance, I'm starting programming in Java.

Why would you want even to do that recursively. You do not need that. Since the variable is static, you can easily created a method and add the `Collection` there. I do not see any value in having this logic, when you can have a for loop and add the values there, through the static method, which references the static variable `sales`. — Jernej K, Mar 11 '16 at 18:42
I would also ask: why do you want to do this? In the sense of: this smells like BAD design. Why should the Sale class ... maintain a list of Sales?! If your "shopping system" needs to keep lists of Sales objects; then heck; they should be in other places. — GhostCat, Mar 11 '16 at 18:43
I know this solution is quite awful, but I was asked to do so (it is an exercise). Thanks for the comments. — jari, Mar 11 '16 at 18:53
Are you _sure_ you understood the exercise? An exercise is supposed to teach correct principles, and this is all wrong. As an exercise it is recommending very bad practice. — Jim Garrison, Mar 11 '16 at 18:57
Yes, I am _sure_. Thank you for your concern. I know it is not a good solution, but professors at university ... — jari, Mar 11 '16 at 19:42

score 3 · Accepted Answer · edited May 23 '17 at 11:59

You haven't initialized your sales ArrayList. You can do so by changing

private static ArrayList<Sale> sales;

to something like

private static ArrayList<Sale> sales = new ArrayList<Sale>();

or using the diamond operator <> (Java 7+) like

private static ArrayList<Sale> sales = new ArrayList<>();

But I would suggest you program to the List interface. Something like

private static List<Sale> sales = new ArrayList<>();

Another possible option is a static initializer, like

private static List<Sale> sales;
static {
    sales = new ArrayList<>();
}

Add item to ArrayList "recursively"

1 Answers1