title: Using ArrayList on Observer patterns
body: <p>I am currently studying the observer patterns, but I am still confused on this set of codes:</p>
  
  <pre><code>public interface Observer {
      public void update(String availability);
  }  
  private ArrayList&lt;Observer&gt; observers = new ArrayList&lt;Observer&gt;();
  </code></pre>
  
  <p>Please help me to understand how <code>Observer</code> interface runs on the <code>ArrayList</code>.</p>