Iterator Pattern

Traversing two different collections  When bringing two previously developed objects together, it can be difficult to change an implementation  Diner merger with PancakeHouse  Menu items -- same for both collections -- this is key  When bringing two previously developed objects together, it can be difficult to change an implementation  Diner merger with PancakeHouse  Menu items -- same for both collections -- this is key

public class DinerMenu implements Menu { static final int MAX_ITEMS = 6; int numberOfItems = 0; MenuItem[] menuItems; public DinerMenu() { menuItems = new MenuItem[MAX_ITEMS]; addItem("Steamed Veggies and Brown Rice", "Steamed vegetables over brown rice", true, 3.99); addItem(…) } public void addItem(String name, String description, boolean vegetarian, double price) { MenuItem menuItem = new MenuItem(name, description, vegetarian, price); if (numberOfItems >= MAX_ITEMS) { System.err.println("Sorry, menu is full! Can't add item to menu"); } else { menuItems[numberOfItems] = menuItem; numberOfItems = numberOfItems + 1; } public MenuItem[] getMenuItems() { return menuItems; } // other menu methods here } public class DinerMenu implements Menu { static final int MAX_ITEMS = 6; int numberOfItems = 0; MenuItem[] menuItems; public DinerMenu() { menuItems = new MenuItem[MAX_ITEMS]; addItem("Steamed Veggies and Brown Rice", "Steamed vegetables over brown rice", true, 3.99); addItem(…) } public void addItem(String name, String description, boolean vegetarian, double price) { MenuItem menuItem = new MenuItem(name, description, vegetarian, price); if (numberOfItems >= MAX_ITEMS) { System.err.println("Sorry, menu is full! Can't add item to menu"); } else { menuItems[numberOfItems] = menuItem; numberOfItems = numberOfItems + 1; } public MenuItem[] getMenuItems() { return menuItems; } // other menu methods here }

public class PancakeHouseMenu implements Menu { ArrayList menuItems; public PancakeHouseMenu() { menuItems = new ArrayList(); addItem("K&B's Pancake Breakfast", "Pancakes with scrambled eggs, and toast", true, 2.99); … } public void addItem(String name, String description, boolean vegetarian, double price) { MenuItem menuItem = new MenuItem(name, description, vegetarian, price); menuItems.add(menuItem); } public ArrayList getMenuItems() { return menuItems; } // other menu methods here } public class PancakeHouseMenu implements Menu { ArrayList menuItems; public PancakeHouseMenu() { menuItems = new ArrayList(); addItem("K&B's Pancake Breakfast", "Pancakes with scrambled eggs, and toast", true, 2.99); … } public void addItem(String name, String description, boolean vegetarian, double price) { MenuItem menuItem = new MenuItem(name, description, vegetarian, price); menuItems.add(menuItem); } public ArrayList getMenuItems() { return menuItems; } // other menu methods here }

public class Waitress { PancakeHouseMenu pancakeHouseMenu; ArrayList breakfastItems; DinerMenu dinerMenu; MenuItems[] lunchItems; public Waitress(PancakeHouseMenu pancakeHouseMenu, DinerMenu dinerMenu) { this.pancakeHouseMenu = pancakeHouseMenu; breakfastItems = pancakeHouseMenu.getMenuItems(); this.dinerMenu = dinerMenu; lunchItems = dinerMenu.getMenuItems(); } public void printMenu(){ for (int i = 0; breakfastItems.size(); i++) { MenuItem menuItem = (MenuItem)breakfastItems.get(i); System.out.println(menuItem.getName()+” “+menuItem.getPrice()); System.out.println(menuItem.getDescription()); } for (int i = 0; lunchItems.length; i++) { MenuItem menuItem = lunchItems[i]; System.out.println(menuItem.getName()+” “+menuItem.getPrice()); System.out.println(menuItem.getDescription()); } public class Waitress { PancakeHouseMenu pancakeHouseMenu; ArrayList breakfastItems; DinerMenu dinerMenu; MenuItems[] lunchItems; public Waitress(PancakeHouseMenu pancakeHouseMenu, DinerMenu dinerMenu) { this.pancakeHouseMenu = pancakeHouseMenu; breakfastItems = pancakeHouseMenu.getMenuItems(); this.dinerMenu = dinerMenu; lunchItems = dinerMenu.getMenuItems(); } public void printMenu(){ for (int i = 0; breakfastItems.size(); i++) { MenuItem menuItem = (MenuItem)breakfastItems.get(i); System.out.println(menuItem.getName()+” “+menuItem.getPrice()); System.out.println(menuItem.getDescription()); } for (int i = 0; lunchItems.length; i++) { MenuItem menuItem = lunchItems[i]; System.out.println(menuItem.getName()+” “+menuItem.getPrice()); System.out.println(menuItem.getDescription()); }

Waitress code  Need two loops to go through the two collections  Will need to add another loop if we add another collection later  waitress has to know intimate details of the collection code -- bad form!  Need two loops to go through the two collections  Will need to add another loop if we add another collection later  waitress has to know intimate details of the collection code -- bad form!

Let’s Encapsulate!  We can encapsulate the iteration to avoid sharing too much information

Iterator Interface public interface Iterator { boolean hasNext(); Object next(); } public interface Iterator { boolean hasNext(); Object next(); }

public class DinerMenuIterator implements Iterator { MenuItem[] items; int position = 0; public DinerMenuIterator(MenuItem[] items) { this.items = items; } public Object next() { MenuItem menuItem = items[position]; position = position + 1; return menuItem; } public boolean hasNext() { if (position >= items.length || items[position] == null) { return false; } else { return true; } public class DinerMenuIterator implements Iterator { MenuItem[] items; int position = 0; public DinerMenuIterator(MenuItem[] items) { this.items = items; } public Object next() { MenuItem menuItem = items[position]; position = position + 1; return menuItem; } public boolean hasNext() { if (position >= items.length || items[position] == null) { return false; } else { return true; }

public class DinerMenu implements Menu { static final int MAX_ITEMS = 6; int numberOfItems = 0; MenuItem[] menuItems; //constructor here //addItem here /* don’t need getMenuItems now that we have an Iterator! public MenuItem[] getMenuItems() { return menuItems; }*/ public Iterator createIterator() { return new DinerMenuIterator(menuItems); } // other menu methods here } public class DinerMenu implements Menu { static final int MAX_ITEMS = 6; int numberOfItems = 0; MenuItem[] menuItems; //constructor here //addItem here /* don’t need getMenuItems now that we have an Iterator! public MenuItem[] getMenuItems() { return menuItems; }*/ public Iterator createIterator() { return new DinerMenuIterator(menuItems); } // other menu methods here }

New Waitress Code

public class Waitress { PancakeHouseMenu pancakeHouseMenu; DinerMenu dinerMenu; public Waitress(PancakeHouseMenu pancakeHouseMenu, DinerMenu dinerMenu) { this.pancakeHouseMenu = pancakeHouseMenu; this.dinerMenu = dinerMenu; } public void printMenu() { Iterator pancakeIterator = pancakeHouseMenu.createIterator(); Iterator dinerIterator = dinerMenu.createIterator(); System.out.println("MENU\n----\nBREAKFAST"); printMenu(pancakeIterator); System.out.println("\nLUNCH"); printMenu(dinerIterator); } private void printMenu(Iterator iterator) { while (iterator.hasNext()) { MenuItem menuItem = (MenuItem)iterator.next(); System.out.print(menuItem.getName() + ", "); System.out.print(menuItem.getPrice() + " -- "); System.out.println(menuItem.getDescription()); } //other methods } public class Waitress { PancakeHouseMenu pancakeHouseMenu; DinerMenu dinerMenu; public Waitress(PancakeHouseMenu pancakeHouseMenu, DinerMenu dinerMenu) { this.pancakeHouseMenu = pancakeHouseMenu; this.dinerMenu = dinerMenu; } public void printMenu() { Iterator pancakeIterator = pancakeHouseMenu.createIterator(); Iterator dinerIterator = dinerMenu.createIterator(); System.out.println("MENU\n----\nBREAKFAST"); printMenu(pancakeIterator); System.out.println("\nLUNCH"); printMenu(dinerIterator); } private void printMenu(Iterator iterator) { while (iterator.hasNext()) { MenuItem menuItem = (MenuItem)iterator.next(); System.out.print(menuItem.getName() + ", "); System.out.print(menuItem.getPrice() + " -- "); System.out.println(menuItem.getDescription()); } //other methods }

Compare and contrast Waitresses Oldnew Waitress must know about menu implementation Menus are encapsulated Needs 2 loopsNeed 1 loop Waitress bound to concrete classes (MenuItem[] and ArrayList) Waitress now uses an interface (Iterator)

Java.util.iterator  Now that we’ve built an iterator from scratch, we can learn about Java’s Iterator interface  hasNext()  Next()  remove()  Optional -- in unsupported must throw  Java.lang.unsupportedOperationExc eption  Now that we’ve built an iterator from scratch, we can learn about Java’s Iterator interface  hasNext()  Next()  remove()  Optional -- in unsupported must throw  Java.lang.unsupportedOperationExc eption

Updating menus

import java.util.ArrayList; import java.util.Iterator; public class PancakeHouseMenu implements Menu { ArrayList menuItems; //constructor here public Iterator createIterator() { return menuItems.iterator(); } // other menu methods here } ArrayList already implements the Iterator interface import java.util.ArrayList; import java.util.Iterator; public class PancakeHouseMenu implements Menu { ArrayList menuItems; //constructor here public Iterator createIterator() { return menuItems.iterator(); } // other menu methods here } ArrayList already implements the Iterator interface

import java.util.Iterator; public class DinerMenuIterator implements Iterator { MenuItem[] list; int position = 0; public DinerMenuIterator(MenuItem[] list) {//constructor here } public Object next() { //current implemenation } public boolean hasNext() { //current implementation } public void remove() { if (position <= 0) { throw new IllegalStateException ("You can't remove an item until you've done at least one next()"); } if (list[position-1] != null) { for (int i = position-1; i < (list.length-1); i++) { list[i] = list[i+1]; } list[list.length-1] = null; } import java.util.Iterator; public class DinerMenuIterator implements Iterator { MenuItem[] list; int position = 0; public DinerMenuIterator(MenuItem[] list) {//constructor here } public Object next() { //current implemenation } public boolean hasNext() { //current implementation } public void remove() { if (position <= 0) { throw new IllegalStateException ("You can't remove an item until you've done at least one next()"); } if (list[position-1] != null) { for (int i = position-1; i < (list.length-1); i++) { list[i] = list[i+1]; } list[list.length-1] = null; }

Updating waitress with menu interface public interface Menu{ public Iterator createIterator(); } Add “implements Menu” to DinerMenu and PancakeHouseMenu Then… public interface Menu{ public Iterator createIterator(); } Add “implements Menu” to DinerMenu and PancakeHouseMenu Then…

import java.util.Iterator; public class Waitress { Menu pancakeHouseMenu; Menu dinerMenu; public Waitress(Menu pancakeHouseMenu, Menu dinerMenu) { this.pancakeHouseMenu = pancakeHouseMenu; this.dinerMenu = dinerMenu; } public void printMenu() { Iterator pancakeIterator = pancakeHouseMenu.createIterator(); Iterator dinerIterator = dinerMenu.createIterator(); System.out.println("MENU\n----\nBREAKFAST"); printMenu(pancakeIterator); System.out.println("\nLUNCH"); printMenu(dinerIterator); } private void printMenu(Iterator iterator) { while (iterator.hasNext()) { MenuItem menuItem = (MenuItem)iterator.next(); System.out.print(menuItem.getName()+", ”+menuItem.getPrice()+ " -- "); System.out.println(menuItem.getDescription()); } //other methods } import java.util.Iterator; public class Waitress { Menu pancakeHouseMenu; Menu dinerMenu; public Waitress(Menu pancakeHouseMenu, Menu dinerMenu) { this.pancakeHouseMenu = pancakeHouseMenu; this.dinerMenu = dinerMenu; } public void printMenu() { Iterator pancakeIterator = pancakeHouseMenu.createIterator(); Iterator dinerIterator = dinerMenu.createIterator(); System.out.println("MENU\n----\nBREAKFAST"); printMenu(pancakeIterator); System.out.println("\nLUNCH"); printMenu(dinerIterator); } private void printMenu(Iterator iterator) { while (iterator.hasNext()) { MenuItem menuItem = (MenuItem)iterator.next(); System.out.print(menuItem.getName()+", ”+menuItem.getPrice()+ " -- "); System.out.println(menuItem.getDescription()); } //other methods }

definitions  The Iterator Pattern provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.  Aggregate object - an object that is a collection of other objects of the same type, intended primarily for storage  The Iterator Pattern provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.  Aggregate object - an object that is a collection of other objects of the same type, intended primarily for storage

> Aggregate createIterator() Client > Iterator hasNext() next() remove() ConcreteAggregate createIterator() ConcreteIterator createIterator()

Iterator faq  Backwards iteration  Yes, but you need to add some methods. Which?  Java also has ListIterator, similar  Ordering  Iterator does not imply ordering  Backwards iteration  Yes, but you need to add some methods. Which?  Java also has ListIterator, similar  Ordering  Iterator does not imply ordering

Design Principle sinlge responsibility  A class should have only one reason to change  An aggregate that manages its own collection AND iteration has two opportunities to change.  Where?  A class should have only one reason to change  An aggregate that manages its own collection AND iteration has two opportunities to change.  Where?

Cohesion  A measure of how closely a class supports a single purpose or responsibility  High cohesion:designed around a set of related functions  Low cohesion: designed around unrelated functions  A measure of how closely a class supports a single purpose or responsibility  High cohesion:designed around a set of related functions  Low cohesion: designed around unrelated functions

Cohesion examples  Book pg. 340

Iterator and Hashtable  Hash table accesses Iterator indirectly

import java.util.*; public class CafeMenu implements Menu { Hashtable menuItems = new Hashtable(); public CafeMenu() { //constructor code here } public void addItem(String name, String description, boolean vegetarian, double price) { MenuItem menuItem = new MenuItem(name, description, vegetarian, price); menuItems.put(menuItem.getName(), menuItem); } public Iterator createIterator() { return menuItems.values().iterator(); } import java.util.*; public class CafeMenu implements Menu { Hashtable menuItems = new Hashtable(); public CafeMenu() { //constructor code here } public void addItem(String name, String description, boolean vegetarian, double price) { MenuItem menuItem = new MenuItem(name, description, vegetarian, price); menuItems.put(menuItem.getName(), menuItem); } public Iterator createIterator() { return menuItems.values().iterator(); }

Iterators and Collections  Java Colleections Framework  Vector, LinkedLIST, Stack, etc.  Java.util.collection interface  Each collection object knows how to create its own iterator! Sweet!  Java Colleections Framework  Vector, LinkedLIST, Stack, etc.  Java.util.collection interface  Each collection object knows how to create its own iterator! Sweet!

> Collection add() addAll() clear() contains() containsAll() equals() hashCode() isEmpty() iterator() remove() removeAll() reatainAll() size() toArray()

For/in is the new for  With java5 we have a new syntax for iterating over collections  For(Object obj: collection)  No need to get the iterator  Generics  Type security  With java5 we have a new syntax for iterating over collections  For(Object obj: collection)  No need to get the iterator  Generics  Type security

Composite  Iterator is related to the Composite Pattern  We are not going to cover it this semester, but you can read about it in your book.  Iterator is related to the Composite Pattern  We are not going to cover it this semester, but you can read about it in your book.