Java Visitor

Visitor desing pattern implemented in Java.

This pattern is used to add new operation(s) to existing class or set of classes, without the need of major code re-factorisation.

Main.java

package visitor;

// ? 2019 TheFlyingKeyboard and released under MIT License
// theflyingkeyboard.net
public class Main {
    public static void main(String[] args) {
        Shape circle = new Circle(2, 5, 4);
        Shape triangle = new Triangle(1, 3, 7, 10);
        Shape square = new Square(-1, 2, 8);

        Visitor basicInfoVisitor = new BasicInfoVisitor();
        Visitor areaVisitor = new AreaVisitor();

        System.out.println("BasicInfoVisitor");
        circle.accept(basicInfoVisitor);
        triangle.accept(basicInfoVisitor);
        square.accept(basicInfoVisitor);

        System.out.println();
        System.out.println("AreaVisitor");
        circle.accept(areaVisitor);
        triangle.accept(areaVisitor);
        square.accept(areaVisitor);
    }
}

After running the output should be:

BasicInfoVisitor
Circle at 2.0, 5.0 radius 4.0
Triangle at 1.0, 3.0 base 7.0 height 10.0
Square at -1.0, 2.0 side 8.0

AreaVisitor
Circle area 50.26548245743669
Triangle area 35.0
Square area 64.0

Shape.java

package visitor;

// ? 2019 TheFlyingKeyboard and released under MIT License
// theflyingkeyboard.net
public abstract class Shape {
    private double posX;
    private double posY;

    public Shape(double posX, double posY) {
        this.posX = posX;
        this.posY = posY;
    }

    public double getPosX() {
        return posX;
    }

    public double getPosY() {
        return posY;
    }

    public abstract void accept(Visitor visitor);
}

Circle.java

package visitor;

// ? 2019 TheFlyingKeyboard and released under MIT License
// theflyingkeyboard.net
public class Circle extends Shape {
    private double radius;

    public Circle(double posX, double posY, double radius) {
        super(posX, posY);
        this.radius = radius;
    }

    public double getRadius() {
        return radius;
    }

    @Override
    public void accept(Visitor visitor) {
        visitor.visitCircle(this);
    }
}

Triangle.java

package visitor;

// ? 2019 TheFlyingKeyboard and released under MIT License
// theflyingkeyboard.net
public class Triangle extends Shape {
    private double base;
    private double height;

    public Triangle(double posX, double posY, double base, double height) {
        super(posX, posY);
        this.base = base;
        this.height = height;
    }

    public double getBase() {
        return base;
    }

    public double getHeight() {
        return height;
    }

    @Override
    public void accept(Visitor visitor) {
        visitor.visitTriangle(this);
    }
}

Square.java

package visitor;

// ? 2019 TheFlyingKeyboard and released under MIT License
// theflyingkeyboard.net
public class Square extends Shape{
    private double side;

    public Square(double posX, double posY, double side) {
        super(posX, posY);
        this.side = side;
    }

    public double getSide() {
        return side;
    }

    @Override
    public void accept(Visitor visitor) {
        visitor.visitSquare(this);
    }
}

Visitor.java

package visitor;

// ? 2019 TheFlyingKeyboard and released under MIT License
// theflyingkeyboard.net
public interface Visitor {
    public void visitCircle(Circle circle);

    public void visitTriangle(Triangle triangle);

    public void visitSquare(Square square);
}

BasicInfoVisitor.java

package visitor;

// ? 2019 TheFlyingKeyboard and released under MIT License
// theflyingkeyboard.net
public class BasicInfoVisitor implements Visitor {
    @Override
    public void visitCircle(Circle circle) {
        System.out.println("Circle at " + circle.getPosX() + ", " + circle.getPosY() + " radius " + circle.getRadius());
    }

    @Override
    public void visitTriangle(Triangle triangle) {
        System.out.println("Triangle at " + triangle.getPosX() + ", " + triangle.getPosY() + " base " + triangle.getBase() + " height " + triangle.getHeight());
    }

    @Override
    public void visitSquare(Square square) {
        System.out.println("Square at " + square.getPosX() + ", " + square.getPosY() + " side " + square.getSide());
    }
}

AreaVisitor.java

package visitor;

// ? 2019 TheFlyingKeyboard and released under MIT License
// theflyingkeyboard.net
public class AreaVisitor implements Visitor {
    @Override
    public void visitCircle(Circle circle) {
        System.out.println("Circle area " + circle.getRadius() * circle.getRadius() * Math.PI);
    }

    @Override
    public void visitTriangle(Triangle triangle) {
        System.out.println("Triangle area " + 0.5f * triangle.getHeight() * triangle.getBase());
    }

    @Override
    public void visitSquare(Square square) {
        System.out.println("Square area " + square.getSide() * square.getSide());
    }
}