Collections and Generics - SCJP Sun Certified Programmer for Java Platform SE6 - page 442

Product other = (Product) obj;
return this.id == other.id;
}
public int hashCode() {
return id;
}
public String toString() {
return id + “ “ + description;
}
}
Let's add some Product objects to a set. Using generics, the following statements create
a HashSet for Product objects. Study the code carefully and see if you can determine its
result:
7. Product one = new Product(“Laptop”, 1299.99, 101);
8. Product two = new Product(“Television”, 1099.00, 202);
9. Product three = new Product(“Cellphone”, 200.00, 303);
10. Product four = new Product(“PC”, 699.99, 101);
11. Set<Product> set = new HashSet<Product>();
12. set.add(one);
13. set.add(two);
14. set.add(three);
15. set.add(four);
16. set.add(null);
17. set.add(null);
18. for(Product p : set) {
19. System.out.println(p);
20. }
The four Product objects are added to set using the add method of Set . Notice that the
objects one and four have the same id and are therefore equal, so adding four on line 15
does not modify the set. A HashSet allows a null entry, but only once. Adding null on line
16 modifi es the set, while line 17 does not. Sets are not ordered, so the elements of the set
are displayed in the for-each loop in no particular order. The output of the code is
null
101 Laptop
202 Television
303 Cellphone