Effective Java 31 Use instance fields instead of ordinals
Principle
Never derive a value associated with an enum from its ordinal; store it in an instance field instead.
Bad practice Demo
// 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;}
}
// The right way
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; }
}
Disadvantages
- 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.
- You can‘t add a constant for an int value without adding constants for all intervening int values.
Summary
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 enum based data structures such as EnumSet and EnumMap." Unless you are writing such a data structure, you are best off avoiding the ordinal method entirely.
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