Java - How to find count of items in a list in another list

Question

Say I have two lists:

List<String>products = new ArrayList<String>(); 
products.add("computer"); 
products.add("phone"); 
products.add("mouse"); 
products.add("keyboard"); 

List<String>cart = new ArrayList<String>(); 
cart.add("phone"); 
cart.add("monitor"); 

I need to find how many items in the cart list exist in the products list. For the lists above, the answer would be 1 (as phone is in products and cart). If the cart list was:

List<String>cart = new ArrayList<String>(); 
cart.add("desk"); 
cart.add("chair"); 

The result would be 0. If cart contained computer, mouse, desk, chair, the result would be 2 (for computer and mouse).

Is there something in the Apache Commons Collections or the Google Collections API? I've looked through them and see ways to get a bag count, but not from another list, although it's possible I'm missing something. Right now, the only way I can think of is to iterate over the cart items and see if products contains the individual item and keep a count. I can't use containsAll as I need the count (not a boolean), and that would fail if all items in cart didn't exist in the product list (which can happen).

I'm using Java 1.6 if that matters.

Answer 1

If you're willing to create another collection, you could do the following:

List<String> productsInCart = new ArrayList<String>(products);
productsInCart.retainAll(cart);

That'll give you all of the entries that appear in both cart and products.

Answer 2

int count = 0;
for (String item : cart) {
   if (products.contains(item))
    count++;
   }
}

Answer 3

For one extra line of code to sort (plus the run-time cost) and then calling a different method to do the containment check, you can get something that may very efficient for a list of products that is very long (as long as the list is efficient for random access and the list order of products isn't significant to the app):

Collections.sort(products);  // or maintain products in sort order at all times
int count = 0;
for(String i: cart) {
  if (Collections.binarySearch(products, i) >= 0) {
    count++;
  }
}