java - Conveniently map between enum and int / String

Question

When working with variables/parameters that can only take a finite number of values, I try to always use Java's enum, as in

public enum BonusType {
  MONTHLY, YEARLY, ONE_OFF
}

As long as I stay inside my code, that works fine. However, I often need to interface with other code that uses plain int (or String) values for the same purpose, or I need to read/write from/to a database where the data is stored as a number or string.

In that case, I'd like to have a convenient way to associate each enum value with a an integer, such that I can convert both ways (in other words, I need a "reversible enum").

Going from enum to int is easy:

public enum BonusType {
  public final int id;

  BonusType(int id) {
    this.id = id;
  }
  MONTHLY(1), YEARLY(2), ONE_OFF(3);
}

Then I can access the int value as BonusType x = MONTHLY; int id = x.id;.

However, I can see no nice way for the reverse, i.e. going from int to enum. Ideally, something like

BonusType bt = BonusType.getById(2); 

The only solutions I could come up with are:

• Put a lookup method into the enum, which uses BonusType.values() to fill a map "int -> enum", then caches that and uses it for lookups. Would work, but I'd have to copy this method identically into each enum I use :-(.
• Put the lookup method into a static utility class. Then I'd only need one "lookup" method, but I'd have to fiddle with reflection to get it to work for an arbitrary enum.

Both methods seem terribly awkward for such a simple (?) problem.

Any other ideas/insights?

Answer 1

enum → int

yourEnum.ordinal()

---

int → enum

EnumType.values()[someInt]

---

String → enum

EnumType.valueOf(yourString)

---

enum → String

yourEnum.name()

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A side-note:
As you correctly point out, the ordinal() may be "unstable" from version to version. This is the exact reason why I always store constants as strings in my databases. (Actually, when using MySql, I store them as MySql enums!)