Static Interface Methods

// ── Before Java 8: interface + separate utility-class pairing ──────────
interface Shape {
    double area();
}
// Forced into a separate, unrelated-looking class:
final class Shapes {
    private Shapes() {}
    static Shape circle(double r) { return () -> Math.PI * r * r; }
    static Shape square(double s)  { return () -> s * s; }
}
Shape c = Shapes.circle(5);   // discoverability relies on knowing "Shapes" exists

// ── Java 8+: static factory methods live directly on the interface ─────
interface Shape2 {
    double area();

    static Shape2 circle(double r) { return () -> Math.PI * r * r; }
    static Shape2 square(double s)  { return () -> s * s; }
}
Shape2 c2 = Shape2.circle(5);   // IDE autocomplete on "Shape2." reveals factories directly

// ── Real JDK examples ───────────────────────────────────────────────────
Comparator<String> byLen = Comparator.comparingInt(String::length);
Function<String, String> id = Function.identity();
Predicate<String> notBlank = Predicate.not(String::isBlank);
List<Integer> nums = List.of(1, 2, 3);          // List's static factory
Optional<String> opt = Optional.of("value");     // Optional's static factory

interface Repo {
    static Repo inMemory() { return new InMemoryRepo(); }
}
class InMemoryRepo implements Repo {
    // InMemoryRepo.inMemory() does NOT exist — static methods are not inherited
}

Repo r = Repo.inMemory();          // OK — called through the interface
// InMemoryRepo.inMemory();        // COMPILE ERROR — not inherited by the implementor

// ── Calling through an instance is a compile error for interfaces ──────
Repo instance = Repo.inMemory();
// instance.inMemory();            // COMPILE ERROR: static method called on instance reference

// ── No override relationship, even with identical signatures ──────────
interface Factory {
    static String create() { return "from interface"; }
}
class Maker implements Factory {
    static String create() { return "from class"; }   // unrelated, separate method
}
System.out.println(Factory.create());  // "from interface"
System.out.println(Maker.create());    // "from class"
// Resolution is purely by which type name precedes the call — no polymorphism involved

interface Temperature {
    double celsius();

    // Default method — needs an instance (this) to operate on:
    default double toFahrenheit() {
        return celsius() * 9.0 / 5.0 + 32;
    }

    // Static method — constructs a new instance, no "this" involved:
    static Temperature ofCelsius(double c) {
        validate(c);
        return () -> c;
    }

    static Temperature ofFahrenheit(double f) {
        return ofCelsius((f - 32) * 5.0 / 9.0);
    }

    // Private static helper — shared validation logic, not part of public API:
    private static void validate(double c) {
        if (c < -273.15) {
            throw new IllegalArgumentException("Below absolute zero: " + c);
        }
    }
}

Temperature t1 = Temperature.ofCelsius(100);
Temperature t2 = Temperature.ofFahrenheit(32);
System.out.println(t1.toFahrenheit());   // 212.0
System.out.println(t2.celsius());        // 0.0
// Temperature.validate(-300);            // COMPILE ERROR — private to the interface