Optional and switch

gistfile1.txt

public class TestOptional {
    private sealed interface MyOptional<T> {
        MyOptional<?> EMPTY = new MyEmpty<>();

        boolean hasValue();
        T get();

        static <T> MyOptional<T> of(T t) {
            return new MyJust<>(t);
        }

        static <T> MyOptional<T> empty() {
            return (MyOptional<T>) EMPTY;
        }
    }
    private static final class MyJust<T> implements MyOptional<T> {
        private final T t;

        private MyJust(T t) {
            this.t = t;
        }
        public boolean hasValue() {
            return true;
        }
        public T get() {
            return this.t;
        }
    }

    private static final class MyEmpty<T> implements MyOptional<T> {
        public boolean hasValue() {
            return false;
        }
        public T get() {
            throw new IllegalArgumentException();
        }
    }

    public static void main(String ... args) {
        var optionalValue = MyOptional.of("hello");
        var value = switch (optionalValue) {
            case MyJust<String> j -> j.get();
            case MyEmpty<String> w -> "nothing";
        };
    }
}