OptionalのflatMap

Main.java

public class Main {
    public static void main(String[] args) {
        System.out.println(flatMapRoutine());
        System.out.println(nonFlatMapRoutine());
    }

    private static Optional<Pole> flatMapRoutine() {
        return new Pole(0, 0).landLeft(1)
                .flatMap(x -> x.landRight(4))
                .flatMap(x -> x.landLeft(-1))
                .flatMap(x -> x.landRight(-2));
    }

    private static Optional<Pole> nonFlatMapRoutine() {
        Optional<Pole> result;
        Optional<Pole> pole1 = new Pole(0, 0).landLeft(1);
        if (pole1.isPresent()) {
            Optional<Pole> pole2 = pole1.get().landRight(4);
            if (pole2.isPresent()) {
                Optional<Pole> pole3 = pole2.get().landLeft(-1);
                if (pole3.isPresent()) {
                    result = pole3.get().landRight(-2);
                } else {
                    result = Optional.empty();
                }
            } else {
                result = Optional.empty();
            }
        } else {
            result = Optional.empty();
        }
        return result;
    }
}

Pole.java

public class Pole {
    private int left;
    private int right;

    public Pole(int left, int right) {
        this.left = left;
        this.right = right;
    }

    public Optional<Pole> landLeft(int n) {
        if (Math.abs(left + n - right) < 4) {
            left = left + n;
            return Optional.of(this);
        } else {
            return Optional.empty();
        }
    }

    public Optional<Pole> landRight(int n) {
        if (Math.abs(right + n - left) < 4) {
            right = right + n;
            return Optional.of(this);
        } else {
            return Optional.empty();
        }
    }

    @Override
    public String toString() {
        return "Pole{" +
                "left=" + left +
                ", right=" + right +
                '}';
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Pole pole = (Pole) o;

        if (left != pole.left) return false;
        if (right != pole.right) return false;

        return true;
    }

    @Override
    public int hashCode() {
        int result = left;
        result = 31 * result + right;
        return result;
    }
}