Java Tutorial/Design Pattern/Visitor Pattern

Use Visitor pattern to walk through file system

   <source lang="java">

import java.io.File; public class VisitorTester {

 public static void main(String[] args) {
   DirectoryNode node = new DirectoryNode(new File(".."));
   node.visit(new PrintVisitor());
 }

} class PrintVisitor {

 int level = 0;
 public void visitFileNode(FileNode node) {
   for (int i = 0; i < level; i++)
     System.out.print(" ");
   System.out.println(node.getFile().getName());
 }
 public void visitDirectoryNode(DirectoryNode node) {
   for (int i = 0; i < level; i++)
     System.out.print(" ");
   System.out.println(node.getDirectory().getName());
   level++;
   for (FileSystemNode c : node.getChildren())
     c.visit(this);
   level--;
 }

} interface FileSystemNode {

 void visit(PrintVisitor v);

} class FileNode implements FileSystemNode {

 public FileNode(File file) {
   this.file = file;
 }
 public File getFile() {
   return file;
 }
 public void visit(PrintVisitor v) {
   v.visitFileNode(this);
 }
 private File file;

} class DirectoryNode implements FileSystemNode {

 public DirectoryNode(File directory) {
   this.directory = directory;
 }
 public void visit(PrintVisitor v) {
   v.visitDirectoryNode(this);
 }
 public File getDirectory() {
   return directory;
 }
 public FileSystemNode[] getChildren() {
   File[] files = directory.listFiles();
   FileSystemNode[] children = new FileSystemNode[files.length];
   for (int i = 0; i < files.length; i++) {
     File f = files[i];
     if (f.isDirectory())
       children[i] = new DirectoryNode(f);
     else
       children[i] = new FileNode(f);
   }
   return children;
 }
 private File directory;

}</source>

Visitor, a type of multiple dispatching

   <source lang="java">

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);
 }

}</source>