title: Java - How to find count of items in a list in another list
body: <p>Say I have two lists:</p>
  
  <pre><code>List&lt;String&gt;products = new ArrayList&lt;String&gt;(); 
  products.add("computer"); 
  products.add("phone"); 
  products.add("mouse"); 
  products.add("keyboard"); 
  
  List&lt;String&gt;cart = new ArrayList&lt;String&gt;(); 
  cart.add("phone"); 
  cart.add("monitor"); 
  </code></pre>
  
  <p>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:</p>
  
  <pre><code>List&lt;String&gt;cart = new ArrayList&lt;String&gt;(); 
  cart.add("desk"); 
  cart.add("chair"); 
  </code></pre>
  
  <p>The result would be 0.  If cart contained computer, mouse, desk, chair, the result would be 2 (for computer and mouse).</p>
  
  <p>Is there something in the <a href="http://commons.apache.org/collections/userguide.html" rel="noreferrer">Apache Commons Collections</a> or the <a href="http://code.google.com/p/google-collections/" rel="noreferrer">Google Collections API</a>?  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 <a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Collection.html#contains(java.lang.Object)" rel="noreferrer">contains</a> the individual item and keep a count.  I can't use <a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Collection.html#containsAll(java.util.Collection)" rel="noreferrer">containsAll</a> 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).</p>
  
  <p>I'm using Java 1.6 if that matters.</p>