How do enums work in Java?

Question

Accepted Answer

An **enum** defines a type with a **fixed set of named constants**. Java enums are far more powerful than simple constant lists in other languages — they're full-fledged classes that can have fields, constructors, and methods.

## Basic enum

```java
public enum Day {
    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}

Day today = Day.MONDAY;

// works great in switch
switch (today) {
    case MONDAY -> System.out.println("start of week");
    case SATURDAY, SUNDAY -> System.out.println("weekend");
    default -> System.out.println("weekday");
}
```

Enums provide **type safety** — `Day today = Day.MONDAY` can only hold valid `Day` values, unlike using arbitrary ints or strings as constants (which allow invalid values and typos).

## Built-in enum methods

```java
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
```

## Enums with fields, constructors, and methods (the powerful part)

```java
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
```

Each enum constant is an instance, configured via the constructor — letting you associate data and behavior with each constant. This is unique and powerful compared to most languages' enums.

## Enums can implement interfaces and have per-constant behavior

```java
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
```

## Enums are ideal singletons

```java
public enum Config { INSTANCE; /* fields/methods */ }
// enum singletons are thread-safe and serialization-safe by design
```

## Why it matters

Java enums are a robust, type-safe way to represent fixed sets of related constants (states, days, roles, operations) — far superior to the error-prone alternative of `int`/`String` constants, which allow invalid values and offer no type checking.

Their unique power — fields, constructors, methods, interface implementation, and per-constant behavior — lets you attach data and logic to each constant, modeling domain concepts elegantly (e.g. planets with mass/gravity, operations with their math).

They also provide the safest singleton idiom.

Understanding enums beyond simple constant lists — as full classes with state and behavior — enables cleaner, safer domain modeling and is a frequently-underused but valuable Java feature, as well as a common interview topic.