What is Enums & Enum Methods in Java?

An enum is a class with a fixed set of instances. Each constant is a singleton object, not a number. Which is the whole advantage over int constants. Constants

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Enums & Enum Methods

Use enums as type-safe constants with behavior — fields, methods, abstract methods, and implementing interfaces.

An enum is a class with a fixed set of instances. Each constant is a singleton object, not a number.

Enum = a deck of specific cards. You can't create new cards at runtime, and each card can have its own behavior (ace counts as 1 or 11).

Key Concepts

    enum Status {
      OPEN("Open"), CLOSED("Closed");

      private final String label;
      Status(String label) { this.label = label; }    // implicitly private
      String label() { return label; }
    }

Which is the whole advantage over int constants.

    static final int OPEN = 1;
    setStatus(47);              // compiles, and is nonsense

int

    setStatus(Status.OPEN);     // the only values that fit are the real ones

Constants can each have their own behaviour, which turns a switch into dispatch:

    enum Op {
      PLUS  { int apply(int a, int b) { return a + b; } },
      TIMES { int apply(int a, int b) { return a * b; } };
      abstract int apply(int a, int b);
    }

    Op.TIMES.apply(3, 4);       // 12

The methods you get for free.

    values()        every constant, in declaration order
    valueOf("OPEN") the constant with that exact name
    name()          the declared name
    ordinal()       the 0-based position
    compareTo       by ordinal

Do not store ordinal(). It changes the moment someone inserts a constant, and any persisted value then means something different. Store name(), or an explicit code field.

ordinal()name()

valueOf throws on a miss, so guard it:

valueOf

    try { return Status.valueOf(s); }
    catch (IllegalArgumentException e) { return null; }

An enum is the best singleton in Java. Constants are initialised once when the class loads, which gives you thread safety, and serialisation cannot create a second copy — two problems the hand-written singleton has to solve explicitly.

EnumSet and EnumMap are the right collections for enum keys. EnumSet is a bit vector and EnumMap a plain array, so both are far faster than HashSet or HashMap here.

EnumSetEnumMapHashSetHashMap

Enums can implement interfaces but cannot be extended, and new is not allowed.

new

What the interviewer is probing.1. "Why is an enum the best singleton in Java?" Probing: whether you know what it gives you for free. Stalls: "It is simple." Moves up: initialisation is thread-safe by class loading, and serialisation and reflection cannot create a second instance — the three problems a hand-written singleton must solve.

2. "Why should you never persist ordinal()?" Probing: the fragility. Stalls: "It is fine if the order is stable." Moves up: inserting or reordering a constant silently changes every stored value; persist name() or an explicit code field.

3. "How do you give each constant its own behaviour?" Probing: constant-specific method bodies. Stalls: "With a switch." Moves up: declare an abstract method on the enum and implement it per constant, which turns the switch into dispatch.

4. "Why are EnumSet and EnumMap faster than HashSet and HashMap?" Probing: whether you know the implementation. Stalls: "They are specialised." Moves up: EnumSet is a bit vector and EnumMap an array indexed by ordinal — no hashing, no collisions, no boxing.