Visitor en Java

Visitor es un patrón de diseño de comportamiento que permite añadir nuevos comportamientos a una jerarquía de clases existente sin alterar el código.

Lee por qué el patrón Visitor no puede simplemente sustituirse por la sobrecarga de métodos, en nuestro artículo Visitor y envío doble.

Aprende más sobre el patrón Visitor

Complejidad:

Popularidad:

Ejemplos de uso: El patrón Visitor no es muy habitual debido a su complejidad y limitada aplicabilidad.

Here are some examples of pattern in core Java libraries:Aquí tienes algunos ejemplos del patrón en las principales bibliotecas Java:

Exportar formas a XML

En este ejemplo, queremos exportar un grupo de formas geométricas a XML. La idea es que no queremos cambiar directamente el código de las formas o, al menos, debemos intentar mantener los cambios al mínimo.

Al fin y al cabo, el patrón Visitor establece una infraestructura que nos permite añadir cualquier comportamiento a la jerarquía de formas sin cambiar el código existente de esas clases.

shapes

shapes/Shape.java: Interfaz común de las formas

package refactoring_guru.visitor.example.shapes;

import refactoring_guru.visitor.example.visitor.Visitor;

public interface Shape {
    void move(int x, int y);
    void draw();
    String accept(Visitor visitor);
}

shapes/Dot.java: Un punto

package refactoring_guru.visitor.example.shapes;

import refactoring_guru.visitor.example.visitor.Visitor;

public class Dot implements Shape {
    private int id;
    private int x;
    private int y;

    public Dot() {
    }

    public Dot(int id, int x, int y) {
        this.id = id;
        this.x = x;
        this.y = y;
    }

    @Override
    public void move(int x, int y) {
        // move shape
    }

    @Override
    public void draw() {
        // draw shape
    }

    @Override
    public String accept(Visitor visitor) {
        return visitor.visitDot(this);
    }

    public int getX() {
        return x;
    }

    public int getY() {
        return y;
    }

    public int getId() {
        return id;
    }
}

shapes/Circle.java: Un círculo

package refactoring_guru.visitor.example.shapes;

import refactoring_guru.visitor.example.visitor.Visitor;

public class Circle extends Dot {
    private int radius;

    public Circle(int id, int x, int y, int radius) {
        super(id, x, y);
        this.radius = radius;
    }

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

    public int getRadius() {
        return radius;
    }
}

shapes/Rectangle.java: Un rectángulo

package refactoring_guru.visitor.example.shapes;

import refactoring_guru.visitor.example.visitor.Visitor;

public class Rectangle implements Shape {
    private int id;
    private int x;
    private int y;
    private int width;
    private int height;

    public Rectangle(int id, int x, int y, int width, int height) {
        this.id = id;
        this.x = x;
        this.y = y;
        this.width = width;
        this.height = height;
    }

    @Override
    public String accept(Visitor visitor) {
        return visitor.visitRectangle(this);
    }

    @Override
    public void move(int x, int y) {
        // move shape
    }

    @Override
    public void draw() {
        // draw shape
    }

    public int getId() {
        return id;
    }

    public int getX() {
        return x;
    }

    public int getY() {
        return y;
    }

    public int getWidth() {
        return width;
    }

    public int getHeight() {
        return height;
    }
}

shapes/CompoundShape.java: Una forma compuesta

package refactoring_guru.visitor.example.shapes;

import refactoring_guru.visitor.example.visitor.Visitor;

import java.util.ArrayList;
import java.util.List;

public class CompoundShape implements Shape {
    public int id;
    public List<Shape> children = new ArrayList<>();

    public CompoundShape(int id) {
        this.id = id;
    }

    @Override
    public void move(int x, int y) {
        // move shape
    }

    @Override
    public void draw() {
        // draw shape
    }

    public int getId() {
        return id;
    }

    @Override
    public String accept(Visitor visitor) {
        return visitor.visitCompoundGraphic(this);
    }

    public void add(Shape shape) {
        children.add(shape);
    }
}

visitor

visitor/Visitor.java: Interfaz visitante común

package refactoring_guru.visitor.example.visitor;

import refactoring_guru.visitor.example.shapes.Circle;
import refactoring_guru.visitor.example.shapes.CompoundShape;
import refactoring_guru.visitor.example.shapes.Dot;
import refactoring_guru.visitor.example.shapes.Rectangle;

public interface Visitor {
    String visitDot(Dot dot);

    String visitCircle(Circle circle);

    String visitRectangle(Rectangle rectangle);

    String visitCompoundGraphic(CompoundShape cg);
}

visitor/XMLExportVisitor.java: Visitante concreto, exporta todas las formas a XML

package refactoring_guru.visitor.example.visitor;

import refactoring_guru.visitor.example.shapes.*;

public class XMLExportVisitor implements Visitor {

    public String export(Shape... args) {
        StringBuilder sb = new StringBuilder();
        sb.append("<?xml version=\"1.0\" encoding=\"utf-8\"?>" + "\n");
        for (Shape shape : args) {
            sb.append(shape.accept(this)).append("\n");
        }
        return sb.toString();
    }

    public String visitDot(Dot d) {
        return "<dot>" + "\n" +
                "    <id>" + d.getId() + "</id>" + "\n" +
                "    <x>" + d.getX() + "</x>" + "\n" +
                "    <y>" + d.getY() + "</y>" + "\n" +
                "</dot>";
    }

    public String visitCircle(Circle c) {
        return "<circle>" + "\n" +
                "    <id>" + c.getId() + "</id>" + "\n" +
                "    <x>" + c.getX() + "</x>" + "\n" +
                "    <y>" + c.getY() + "</y>" + "\n" +
                "    <radius>" + c.getRadius() + "</radius>" + "\n" +
                "</circle>";
    }

    public String visitRectangle(Rectangle r) {
        return "<rectangle>" + "\n" +
                "    <id>" + r.getId() + "</id>" + "\n" +
                "    <x>" + r.getX() + "</x>" + "\n" +
                "    <y>" + r.getY() + "</y>" + "\n" +
                "    <width>" + r.getWidth() + "</width>" + "\n" +
                "    <height>" + r.getHeight() + "</height>" + "\n" +
                "</rectangle>";
    }

    public String visitCompoundGraphic(CompoundShape cg) {
        return "<compound_graphic>" + "\n" +
                "   <id>" + cg.getId() + "</id>" + "\n" +
                _visitCompoundGraphic(cg) +
                "</compound_graphic>";
    }

    private String _visitCompoundGraphic(CompoundShape cg) {
        StringBuilder sb = new StringBuilder();
        for (Shape shape : cg.children) {
            String obj = shape.accept(this);
            // Proper indentation for sub-objects.
            obj = "    " + obj.replace("\n", "\n    ") + "\n";
            sb.append(obj);
        }
        return sb.toString();
    }

}

Demo.java: Código cliente

package refactoring_guru.visitor.example;

import refactoring_guru.visitor.example.shapes.*;
import refactoring_guru.visitor.example.visitor.XMLExportVisitor;

public class Demo {
    public static void main(String[] args) {
        Dot dot = new Dot(1, 10, 55);
        Circle circle = new Circle(2, 23, 15, 10);
        Rectangle rectangle = new Rectangle(3, 10, 17, 20, 30);

        CompoundShape compoundShape = new CompoundShape(4);
        compoundShape.add(dot);
        compoundShape.add(circle);
        compoundShape.add(rectangle);

        CompoundShape c = new CompoundShape(5);
        c.add(dot);
        compoundShape.add(c);

        export(circle, compoundShape);
    }

    private static void export(Shape... shapes) {
        XMLExportVisitor exportVisitor = new XMLExportVisitor();
        System.out.println(exportVisitor.export(shapes));
    }
}

OutputDemo.txt: Resultado de la ejecución

<?xml version="1.0" encoding="utf-8"?>
<circle>
    <id>2</id>
    <x>23</x>
    <y>15</y>
    <radius>10</radius>
</circle>

<?xml version="1.0" encoding="utf-8"?>
<compound_graphic>
   <id>4</id>
    <dot>
        <id>1</id>
        <x>10</x>
        <y>55</y>
    </dot>
    <circle>
        <id>2</id>
        <x>23</x>
        <y>15</y>
        <radius>10</radius>
    </circle>
    <rectangle>
        <id>3</id>
        <x>10</x>
        <y>17</y>
        <width>20</width>
        <height>30</height>
    </rectangle>
    <compound_graphic>
       <id>5</id>
        <dot>
            <id>1</id>
            <x>10</x>
            <y>55</y>
        </dot>
    </compound_graphic>
</compound_graphic>