Thursday, February 07, 2013

The Java enum factory

This comes up in interviews with intermediate programmers (and some seniors), the "enum factory" in Java:

public final class Money {
    // Infrequent that new currencies created,
    // old currencies never vanish, just fade away
    public enum Currency {
        USD, PHP; // And many more

        // Additional fields as needed, e.g., locale

        public Money print(final BigDecimal amount) {
            return new Money(this, amount);

    private final Currency currency;
    private final BigDecimal amount;

    private Money(final Currency currency, final BigDecimal amount) {
        this.currency = currency;
        this.amount = amount;

    // Appropriate methods

And elsewhere with static imports:

Money pocket = asList(USD.print(ONE), PHP.print(TEN));

The key observations:

  • Amounts of money are intimately attached to a particular currency
  • Currency has few qualities not attached to money
  • Ok to add more currencies through code
  • Code reads better with factory than without
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