title: ArrayList.add works, but not ArrayList.remove
body: <p>Creating an instance of an object (o) and adding it to an Arraylist (arrayList) works fine. However, the remove function doesn't work.</p>
  
  <pre><code>arrayList.add(o); // works
  arrayList.remove(o); // does nothing
  </code></pre>
  
  <p>What am I missing?</p>