2. Visitor Design Pattern

The design pattern provides additional functionality to a class. The Visitor pattern allows us to create an external class to act on data in other classes. This is useful in those conditions if a fair number of instances are available of small number of classes and we have to perform some operations on all or on most of those classes.

Lets take an example that demonstrates that how the classes implement the Visitor interface. In the following example we are taking the two interfaces one is Visitor interface and the other is Colddrink interface and then we implement to these interfaces into our classes.

import java.util.*;

interface Visitor {
public void visit(Colddrink cld_drnk);
}
interface Colddrink {
public String Order();
}
class Pepsi implements Colddrink {
final String name = “Pepsi”;
public String Order() {
return name;
}
}
class CocaCola implements Colddrink {
final String name = “Coke”;
public String Order() {
return name;
}
}
class Pickup implements Visitor {
private String name;
private final String method = “Pick and
Go”;
public void visit(Colddrink cld_drnk) {
name = cld_drnk.Order();
}
public String toString() {
return name + “ “ + method;
}
}

class Drinkthere implements Visitor {
private String name;
private final String method = “Take the
drink over there”;
public void visit(Colddrink cld_drnk) {
name = cld_drnk.Order();
}
public String toString() {
return name + “ “ + method;
}
}
class GetByDelivery implements Visitor {
private String name;
private final String method = “Get it by
delivery”;
public void visit(Colddrink cld_drnk) {
name = cld_drnk.Order();
}
public String toString() {
return name + “ “ + method;
}
}
class Drink {
public Colddrink getDrink() {
switch ((int)(Math.random()*3)){
case 0: return new Pepsi();
case 1: return new CocaCola();
default: return null;
}
}
public Visitor howto() {
switch ((int)(Math.random()*3)){
case 0: return new Pickup();
case 1: return new Drinkthere();
case 2: return new GetByDelivery();
default: return null;
}
}
}
class TestVisitors {
public static void main(String[] args) {
List colddrinkList = new ArrayList();
colddrinkList.add(new Pepsi());
colddrinkList.add(new CocaCola());
Iterator it = colddrinkList.iterator();