Effective Java! Use Instance Fields Instead of Ordinals

There are times that there is a natural ordering and ordinals that could be assigned to our different enums. For example consider the following:

public enum Ensemble {
  SOLO, DUET, TRIO, QUARTET, QUINTET, SEXTET, SEPTET, OCTET, NONTET, DECTET; 

  public int numberOfMusicians( return ordinal() + 1; );
}

Using the ordinal built-in function may initially seem like a good idea as the ordinals follow the correct number that should be returned from our numberOfMusicians function. The problem lies in the maintenance of the enum. The ordering of the values in the enum are tied to their value. This can cause problems for if we want two values to return the same value or if we want to skip a value. For example consider a double quartet which should return 8 from the numberOfMusicians function. However an ordinal can only be connected to one value and OCTET is already connected with that ordinal. Also consider if we wanted to make a value for triple quartet which should return a 12 from the numberOfMusicians function. However ordinals go in order and the last value we have would match up with 11 and not 12. So we either need to make a useless value to fill up space or we can't do it.

So what we should reach for instead is using instance variables. Using this method we get way more flexibility without giving up much conciseness. Let's take a look at how this would look.

public enum Ensemble {
  SOLO(1), DUET(2), TRIO(3), QUARTET(4), QUINTET(5), SEXTET(6), SEPTET(7), OCTET(8), DOUBLE_QUARTET(8), NONTET(9), DECTET(10), TRIPLE_QUARTET(12); 

  private final int numberOfMusicians;

  public Ensemble(int numberOfMusicians) {
    this.numberOfMusicisians = numberOfMusicians;
  }

  public int numberOfMusicians( return numberOfMusicians; );
}

In this new and improved version we no longer use the ordinal function and instead use the instance fields. This has given the flexibility to easily add the double and triple quartet ensemble values.

So what good is the ordinal function if we basically are told never to use it. Well the documentation itself says: "Most programmers will have no use for this method. It is designed for use by general-purpose enum-based data structures such as EnumSet and EnumMap". Unless you are writing code like that you don't need to worry about this function.