将项目“递归”添加到 ArrayList

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.

Answer 1

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<>();
}