title: What does "optional operation" mean in Javadoc of for example Set#add(E)?
body: <p>When in the java documentation for <a href="http://docs.oracle.com/javase/6/docs/api/java/util/Set.html" rel="noreferrer">Set</a> it says in the specification of a method <code>Optional Operation</code> e.g. (emphasis by me) </p>
  
  <blockquote>
    <p>add(E e)<br>
              Adds the specified element to this set if it is not already present <strong>(optional operation)</strong>.  </p>
  </blockquote>
  
  <p>What does the optional mean here?  </p>
  
  <p>That if I use a JVM other than SUN/Oracle, this operation may not be provided by that implementation of Java?  </p>