Enum hufafanua aina yenye seti isiyobadilika ya constants zenye majina. Enums za Java zina nguvu zaidi kuliko orodha rahisi za constants katika lugha nyingine — ni classes kamili zinazoweza kuwa na fields, constructors, na methods.
{
MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}
Day.MONDAY;
(today) {
MONDAY -> System.out.println();
SATURDAY, SUNDAY -> System.out.println();
-> System.out.println();
}
Enums hutoa type safety — Day today = Day.MONDAY inaweza kushikilia tu thamani halali za Day, tofauti na kutumia ints au strings za kiholela kama constants (zinazoruhusu thamani batili na typos).
Day.MONDAY.name(); // "MONDAY"
Day.MONDAY.ordinal(); // 0 — position in the declaration
Day.values(); // array of all constants (iterate them)
Day.valueOf("FRIDAY"); // get a constant by name
for (Day d : Day.values()) { ... } // iterate all values
public enum Planet {
MERCURY(3.3e23, 2.4e6), // each constant calls the constructor with arguments
EARTH(5.97e24, 6.37e6),
JUPITER(1.9e27, 7.1e7);
private final double mass; // enums can have FIELDS
private final double radius;
Planet(double mass, double radius) { // constructor (implicitly private)
this.mass = mass;
this.radius = radius;
}
public double surfaceGravity() { // and METHODS
return 6.67e-11 * mass / (radius * radius);
}
}
Planet.EARTH.surfaceGravity(); // compute using EARTH's data
Kila enum constant ni instance, iliyosanidiwa kupitia constructor — ikikuwezesha kuhusisha data na tabia na kila constant. Hii ni ya kipekee na yenye nguvu ikilinganishwa na enums za lugha nyingi.
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; } };
public abstract int apply(int a, int b); // each constant implements it differently
}
Operation.PLUS.apply(2, 3); // 5
public enum Config { INSTANCE; /* fields/methods */ }
// enum singletons are thread-safe and serialization-safe by design
Enums za Java ni njia imara, salama kwa type ya kuwakilisha seti zisizobadilika za constants zinazohusiana (states, days, roles, operations) — bora zaidi kuliko mbadala wenye makosa wa constants za int/String, zinazoruhusu thamani batili na hazitoi type checking.
Nguvu yao ya kipekee — fields, constructors, methods, utekelezaji wa interface, na tabia ya kila constant — hukuwezesha kuambatanisha data na logic kwa kila constant, ukiunda dhana za domain kwa uzuri (mfano sayari zenye mass/gravity, operations zenye hesabu zao).
Pia hutoa idiom salama zaidi ya singleton.
Kuelewa enums zaidi ya orodha rahisi za constants — kama classes kamili zenye hali na tabia — huwezesha domain modeling safi, salama zaidi na ni kipengele cha Java chenye thamani lakini kinachotumika chini ya kiwango, na vilevile mada ya kawaida ya mahojiano.