Item 31: Use instance fields instead of ordinals

Many enums are naturally associated with a single int value. All enums have an ordinal method, which returns the numerical position of each enum constant in its type. You may be tempted to derive an associated int value from the ordinal:

// Abuse of ordinal to derive an associated value - DON'T DO THIS

public enum Ensemble {

SOLO, DUET, TRIO, QUARTET, QUINTET,

SEXTET, SEPTET, OCTET, NONET, DECTET;

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

}

While this enum works, it is a maintenance nightmare. If the constants are reordered, the numberOfMusicians method will break. If you want to add a second enum constant associated with an int value that you’ve already used, you’re out of luck. For example, it might be nice to add a constant for double quartet, which, like an octet, consists of eight musicians, but there is no way to do it.

Luckily, there is a simple solution to these problems. Never derive a value associated with an enum from its ordinal; store it in an instance field instead:

public enum Ensemble {

SOLO(1), DUET(2), TRIO(3), QUARTET(4), QUINTET(5),

SEXTET(6), SEPTET(7), OCTET(8), DOUBLE_QUARTET(8),

NONET(9), DECTET(10), TRIPLE_QUARTET(12);

private final int numberOfMusicians;

Ensemble(int size) { this.numberOfMusicians = size; }

public int numberOfMusicians() { return numberOfMusicians; }

}

The Enum specification has this to say about ordinal: “Most programmers will have no use for this method. It is designed for use by general-purpose enumbased data structures such as EnumSet and EnumMap.” Unless you are writing such a data structure, you are best off avoiding the ordinal method entirely.

Reference: Effective Java 2nd Edition by Joshua Bloch