title: How to enable enum inheritance
body: <p>I'm writing a library, which has a predefined set of values for an enum. 
  Let say, my enum looks as below.</p>
  
  <pre><code>public enum EnumClass {
      FIRST("first"),
      SECOND("second"),
      THIRD("third");
  
      private String httpMethodType;
  
  }
  </code></pre>
  
  <p>Now the client, who is using this library may need to add few more values. Let say, the client needs to add <code>CUSTOM_FIRST</code> and <code>CUSTOM_SECOND</code>. This is not overwriting any existing values, but makes the enum having 5 values.</p>
  
  <p>After this, I should be able to use something like <code>&lt;? extends EnumClass&gt;</code> to have 5 constant possibilities.</p>
  
  <p>What would be the best approach to achieve this?</p>