Objects, Classes & OOP Design

Enums

Java enums are full classes with a fixed set of instances — they can carry fields, methods, and per-constant behavior. Use them instead of int constants, and use EnumSet/EnumMap instead of bit fields and ordinal indexing.

Type-safe: you cannot pass an invalid value where an enum is expected (EJ 34)
Enums can have constructors, fields, methods, and constant-specific method bodies
Switch over enums is exhaustiveness-checked when all constants are covered
Never rely on ordinal() — use instance fields instead (EJ 35)
EnumSet replaces bit fields (EJ 36); EnumMap replaces ordinal-indexed arrays (EJ 37)
Single-element enum: the most robust singleton (EJ 3)

Enums with data and behavior (EJ Item 34)

public enum Planet {
    MERCURY(3.302e+23, 2.439e6),
    EARTH(5.975e+24, 6.378e6);

    private final double mass, radius;

    Planet(double mass, double radius) {
        this.mass = mass;
        this.radius = radius;
    }

    public double surfaceGravity() {
        return 6.67e-11 * mass / (radius * radius);
    }
}

Constant-specific behavior

public enum Operation {
    PLUS("+")  { public double apply(double x, double y) { return x + y; } },
    TIMES("*") { public double apply(double x, double y) { return x * y; } };

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

    public abstract double apply(double x, double y);
}

Every enum implicitly extends java.lang.Enum and provides values(), valueOf(String), name(), and ordinal(). Enums are serialization-safe and reflection-proof singletons per constant, which is why a one-constant enum is the sturdiest singleton implementation (EJ Item 3).