title: Number in parentheses after enum instance
body: <p>I am reading a book on programming and I met such thing:</p>
  
  <pre><code>public static enum Month {
  JANUARY(1),
  FEBRUARY(2),
  MARCH(3),
  APRIL(4),
  MAY(5),
  JUNE(6),
  JULY(7),
  AUGUST(8),
  SEPTEMBER(9),
  OCTOBER(10),
  NOVEMBER(11),
  DECEMBER(12);
  Month(int index) {
  this.index = index;
  }
  </code></pre>
  
  <p>What does a number in parentheses mean after an enum instance? Is it an enum constructor?</p>