Example of Functional Visitor Pattern using Vavr

FunctionalVisitor.java

package com.michaelpellegrini.example.functional;

import io.vavr.Function1;
import io.vavr.collection.List;

import static io.vavr.API.$;
import static io.vavr.API.Case;
import static io.vavr.API.Match;
import static io.vavr.Predicates.instanceOf;

public class FunctionalVisitor {
    static class Square {
        final double side;

        Square(double side) {
            this.side = side;
        }
    }

    static class Circle {
        final double radius;

        Circle(double radius) {
            this.radius = radius;
        }
    }

    static class Rectangle {
        final double width;
        final double height;

        Rectangle(double width, double height) {
            this.width = width;
            this.height = height;
        }
    }

    private static Function1<Object, Double> areaCalculatorVisitor = o ->
            Match(o).of(
                    Case($(instanceOf(Square.class)), s -> s.side * s.side),
                    Case($(instanceOf(Circle.class)), c -> Math.PI * c.radius * c.radius),
                    Case($(instanceOf(Rectangle.class)), r -> r.height * r.width)
            );

    private static Function1<Object, Double> perimeterCalculatorVisitor = o ->
            Match(o).of(
                    Case($(instanceOf(Square.class)), s -> 4 * s.side),
                    Case($(instanceOf(Circle.class)), c -> 2 * Math.PI * c.radius),
                    Case($(instanceOf(Rectangle.class)), r -> 2 * r.height + 2 * r.width)
            );

    public static void main(String[] args) {
        List<Object> figures = List.of(new Circle(4), new Square(5), new Rectangle(6, 7));

        double totalArea = figures.map(areaCalculatorVisitor).reduce(Double::sum);
        System.out.println("Total area = " + totalArea);

        double totalPerimeter = figures.map(perimeterCalculatorVisitor).reduce(Double::sum);
        System.out.println("Total perimeter = " + totalPerimeter);
    }
}