Visitor, a type of multiple dispatching : Visitor Pattern « Design Pattern « Java Tutorial

1. Java Tutorial
2. Design Pattern
3. Visitor Pattern

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

interface Visitor {
  void visit(FriendsInHighSchool g);

  void visit(FriendsFromNeighbour r);

  void visit(NewFriends c);
}

interface Friend {
  void accept(Visitor v);
}

class FriendsInHighSchool implements Friend {
  public void accept(Visitor v) {
    v.visit(this);
  }
}

class FriendsFromNeighbour implements Friend {
  public void accept(Visitor v) {
    v.visit(this);
  }
}

class NewFriends implements Friend {
  public void accept(Visitor v) {
    v.visit(this);
  }
}

class Party implements Visitor {
  String s;

  public String toString() {
    return s;
  }

  public void visit(FriendsInHighSchool g) {
    s = "FriendsInHighSchool";
  }

  public void visit(FriendsFromNeighbour r) {
    s = "FriendsFromNeighbour";
  }

  public void visit(NewFriends c) {
    s = "NewFriends";
  }
}

class Drink implements Visitor {
  public void visit(FriendsInHighSchool g) {
    System.out.println("Drink and FriendsInHighSchool");
  }

  public void visit(FriendsFromNeighbour r) {
    System.out.println("Drink and FriendsFromNeighbour");
  }

  public void visit(NewFriends c) {
    System.out.println("Drink and NewFriends");
  }
}

class FriendsGenerator {
  private static Random rand = new Random();

  public static Friend newFlower() {
    switch (rand.nextInt(3)) {
    default:
    case 0:
      return new FriendsInHighSchool();
    case 1:
      return new FriendsFromNeighbour();
    case 2:
      return new NewFriends();
    }
  }
}

public class BeeAndFlowers {
  public static void main(String args[]) {
    List<Friend> flowers = new ArrayList<Friend>();

    for (int i = 0; i < 10; i++)
      flowers.add(FriendsGenerator.newFlower());

    Party sval = new Party();
    Iterator it = flowers.iterator();
    while (it.hasNext()) {
      ((Friend) it.next()).accept(sval);
      System.out.println(sval);
    }
    Drink bee = new Drink();
    it = flowers.iterator();
    while (it.hasNext())
      ((Friend) it.next()).accept(bee);

  }

}