Enums as special classes

public enum Operation {
    PLUS("+")  { public int apply(int a, int b) { return a + b; } },
    TIMES("*") { public int apply(int a, int b) { return a * b; } };

    private final String symbol;

    Operation(String symbol) { this.symbol = symbol; }

    public abstract int apply(int a, int b);

    public String symbol() { return symbol; }
}

Constant-specific bodies compile to anonymous subclasses, which is why getClass() on PLUS is not Operation.class — use getDeclaringClass() when you need the enum type itself.

EnumSet and EnumMap

Both exploit the fact that the constant set is known and ordinal-indexed. EnumSet is a bit vector, EnumMap a plain array — dramatically smaller and faster than the hash-based equivalents, and iteration follows declaration order.

EnumSet<Day> weekend = EnumSet.of(Day.SATURDAY, Day.SUNDAY);

Ordinal is a trap

ordinal() changes the moment someone reorders the declarations. Never persist it and never key business logic on it — store the name() or an explicit field instead.