core15EnumsDemo

1package pkg1core;2 3/*4 * core15EnumsDemo.java5 * --------------6 * Enums: fixed sets of constants with fields, constructors, methods, and7 * per-constant behavior. Also enums in switch.8 *9 * EXPLANATION:10 *  - An enum is a type-safe set of named constants (each is a singleton).11 *  - Enums can have fields, constructors, and abstract methods overridden per12 *    constant (a clean alternative to switch-on-type).13 */14public class core15EnumsDemo {15 16    enum Planet {17        EARTH(5.976e24, 6.37814e6),18        MARS (6.421e23, 3.3972e6);19 20        private final double mass, radius;21        Planet(double mass, double radius) { this.mass = mass; this.radius = radius; }22        double gravity() { return 6.67300e-11 * mass / (radius * radius); }23    }24 25    // Per-constant behavior (constant-specific method bodies)26    enum Operation {27        ADD { int apply(int a, int b){ return a + b; } },28        SUB { int apply(int a, int b){ return a - b; } },29        MUL { int apply(int a, int b){ return a * b; } };30        abstract int apply(int a, int b);31    }32 33    public static void main(String[] args) {34        for (Planet p : Planet.values()) {35            System.out.printf("%s gravity = %.2f m/s^2%n", p, p.gravity());36        }37 38        for (Operation op : Operation.values()) {39            System.out.println("6 " + op + " 3 = " + op.apply(6, 3));40        }41 42        // Enum in switch + useful methods43        Planet here = Planet.EARTH;44        String note = switch (here) {45            case EARTH -> "home";46            case MARS  -> "the red planet";47        };48        System.out.println(here + " is " + note + " | ordinal=" + here.ordinal() + " name=" + here.name());49        System.out.println("valueOf(\"MARS\") = " + Planet.valueOf("MARS"));50    }51}