Question

I’m having two lists of objects, users and products

users own products, and each products is associated to 1 user

but a product type can be multiple and be owned by separate users

• users: Ed, Rob
• products: Coca, Sprites(1), Sprites(2), Beer
• Ed has Coca and Sprites(1), Rob Sprites (2) and Beer

I need to generate an id for each unique (user+product)

It’s probably not a good idea to do

user.hashCode() + product.hashCode()

What could be a good way to proceed?

Answer 1

Your hashCode is not that bad if both user and product create pseudo-random hash-codes. If you are afraid of hash collisions because of bad hashCode implementations in either user or product, then multiply one of the source hash codes by a prime number:

public int hashCode() {
  final int prime = 31;
  int result = 1;
  result = prime * result + ((product == null) ? 0 : product.hashCode());
  result = prime * result + ((user == null) ? 0 : user.hashCode());
  return result;
}

Eclipse builds this very code when selecting Source | Generate hashCode() and equals().

As mentioned by Thilo, you can also simply use Arrays.hashCode(new Object[]{ user, product }); This call takes care of null values for user or product and also multiplies the result by 31 – same as the hand written code.
If you are using Google Guava, there is an Objects.hashCode(Object...) that makes your intent a little bit clearer and uses varargs, but it also only delegates to Arrays.hashCode.