title: Create mutable collections (Map.of and List.of)
body: <p>Looks like <code>Map.of()</code> and <code>List.of()</code> both create immutable collections. That's useful, but I am looking for a way to create mutable collections sometimes using a factory method.</p>
  
  <p>When I try: <code>HashMap.of()</code> I get this error:</p>
  
  <blockquote>
    <p>Static method may be invoked on containing interface class only</p>
  </blockquote>