Visitor

Story
Image
UML
Implementation
Usage

Story

Allows for one or more operations to be applied to a set of objects at runtime, decoupling the operations from object structure.

Shopping in supermarket is example of the Visitor pattern. You pick a products and put them in shopping cart. When you get to the checkout, the cashier acts as a visitor, taking the disparate set of elements, some with prices and others that needs to be weighted, in order to provide you with total.

Image

alt text

Cashier in Supermarket, CC0 Public Domain

UML

Implementation

Element.java

package com.hundredwordsgof.visitor;

/**
 * Defines an Accept operation that takes a visitor as an argument.
 * 
 */
public interface Element {

  void accept(Visitor visitor);
}

ConcreteElementA.java

package com.hundredwordsgof.visitor;

/**
 * Implements accept operation.
 *
 */
public class ConcreteElementA implements Element {

  private int counter = 0;

  public void accept(Visitor visitor) {
    visitor.visitConcreteElementA(this);
  }

  public void operationA() {
    counter++;
  }

  protected int getCounter() {
    return counter;
  }
}

ConcreteElementB.java

package com.hundredwordsgof.visitor;

/**
 * Implements accept operation.
 *
 */
public class ConcreteElementB implements Element {

  private int counter = 0;

  public void accept(Visitor visitor) {
    visitor.visitConcreteElementB(this);
  }

  public void operationB() {
    counter++;
  }

  protected int getCounter() {
    return counter;
  }
}

Visitor.java

package com.hundredwordsgof.visitor;

/**
 * Declares a Visit operation for each class of ConcreteElement in the object
 * structure
 * 
 */
public interface Visitor {

  void visitConcreteElementA(ConcreteElementA concreteElementA);

  void visitConcreteElementB(ConcreteElementB concreteElementB);
}

ConcreteVisitor1.java

package com.hundredwordsgof.visitor;

/**
 * Implements operation declared by Visitor. Each operation implements a
 * fragment of the algorithm defined for the corresponding class of object in
 * the structure.
 *
 */
public class ConcreteVisitor1 implements Visitor {

  public void visitConcreteElementA(ConcreteElementA concreteElementA) {
    concreteElementA.operationA();
  }

  public void visitConcreteElementB(ConcreteElementB concreteElementB) {
    concreteElementB.operationB();
  }
}

ConcreteVisitor2.java

package com.hundredwordsgof.visitor;

/**
 * Implements operation declared by Visitor. Each operation implements a
 * fragment of the algorithm defined for the corresponding class of object in
 * the structure.
 *
 */
public class ConcreteVisitor2 implements Visitor {

  public void visitConcreteElementA(ConcreteElementA concreteElementA) {
    concreteElementA.operationA();
  }

  public void visitConcreteElementB(ConcreteElementB concreteElementB) {
    concreteElementB.operationB();
  }
}

ObjectStructure.java

package com.hundredwordsgof.visitor;

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

/**
 * Holds objects in structure. Provides interface to allow visitors to visit its
 * elements.
 * 
 */
public class ObjectStructure {

  private List<Element> children = new ArrayList<Element>();

  public void add(Element element) {
    children.add(element);
  }

  public void remove(Element element) {
    children.remove(element);
  }

  public Element getChild(int index) {
    return children.get(index);
  }

  public void acceptAll(Visitor visitor) {
    for (Iterator iterator = children.iterator(); iterator.hasNext();) {
      Element element = (Element) iterator.next();
      element.accept(visitor);
    }
  }
}

Usage

VisitorTest.java

package com.hundredwordsgof.visitor;

import static org.junit.Assert.assertEquals;
import org.junit.Test;

/**
 * Test implementation of the Visitor pattern.
 */
public class VisitorTest {

  @Test
  public void testVisitor() {

    // Setup structure
    ObjectStructure objectStructure = new ObjectStructure();

    ConcreteElementA concreteElementA = new ConcreteElementA();
    ConcreteElementB concreteElementB = new ConcreteElementB();

    objectStructure.add(concreteElementA);
    objectStructure.add(concreteElementB);

    // Create visitor objects
    ConcreteVisitor1 v1 = new ConcreteVisitor1();
    ConcreteVisitor2 v2 = new ConcreteVisitor2();

    // Structure accepting visitors
    objectStructure.acceptAll(v1);
    assertEquals(1, concreteElementA.getCounter());
    assertEquals(1, concreteElementB.getCounter());

    objectStructure.acceptAll(v2);
    assertEquals(2, concreteElementA.getCounter());
    assertEquals(2, concreteElementB.getCounter());

    // lets remove second element from objectStructure
    Element element = objectStructure.getChild(1);
    objectStructure.remove(element);
    // now visit all elements on objectStructure
    objectStructure.acceptAll(v1);
    assertEquals(3, concreteElementA.getCounter());

    // this element was removed from objectStructure so counter should remain
    // the same as before last invocation of the acceptAll
    assertEquals(2, concreteElementB.getCounter());

  }
}