1. Menu item at a restaurant
public class MenuItem {
String name, description;
double price;
boolean vegetarian;
public MenuItem(String name, …) { …}
public String getName() { …}
public String getDescription() {…}
public double getPrice() { …}
public boolean isVegetarian() { …} }
Programming Design Patterns
12/5/2018

2. Programming Design Patterns
PancakeHouse
public class PancakeHouseMenu {
ArrayList menuItems;
public PanckageHouseMenu() {
// initialize menuItems with various items }
public void addItem(…) { …}
public ArrayList getMenuItems() {…}
// other methods
Programming Design Patterns
12/5/2018

3. Programming Design Patterns
DinerMenu
public class DinerMenu {
MenuItems[] menuItems;
public DinerMenu() {
// initialize menuItems with various items }
public void addItem(…) { …}
public MenuItem[] getMenuItems() {…}
// other methods
Programming Design Patterns
12/5/2018

4. Combining the two menus
Write code to print the menu items of both the PancakeHouse and Diner
CLASSWORK: printCombinedMenu(…)
assume references pankItems and dinerItems
size of an arrayList: pankItems.size()
accessing element in an ArrayList: pankItems.get(i);
size of an array: dinerItems.length
accessing element in an array: dinerItems[i];
Programming Design Patterns
12/5/2018

5. Programming Design Patterns
printCombinedMenu()
ArrayList pankItems = pancakeHouseMenu.getMenuItems();
MenuItem[] dinerItems = dinerMenu.getMenuItems();
for (int i=0; i<pankItems.size(); i++) {
MenuItem menuItem = (MenuItem)pankItems.get(i);
// print details }
for (int i=0; i<dinerItems.length; i++) {
MenuItem menuItem = (MenuItem)dinerItems[i];
Combine the two for loops into one
Programming Design Patterns
12/5/2018

6. Combining the two menus
Why can’t the two for loops be combined into one?
What code will change if another menu list is added to this combined menu restaurant?
What design principles are violated in the printCombinedMenu() method?
The size of the two menus is different.
The way to step through an array and arrayList is different?
Need to add for loops for each new menu
Programming Design Patterns
12/5/2018

7. Combining the two menus
Why can’t the two for loops be combined into one?
The size of the two menus is different.
The way to step through an array and arrayList is different
What code will change if another menu is added to this combined menu restaurant?
a new for loop will have to be added
Design Principles?
Programming Design Patterns
12/5/2018

8. Combined Menu: Design principles
PancakeHouseMenu and DinerMenu are concrete implementations
should program to an interface
If the underlying storage of DinerMenu or PancakeHouseMenu changes, to a Hashtable for example, the combined menu code will also have to change
Bad design. Exposing the underlying implementation.
Duplicate code in the for loops.
encapsulate what varies in the for loop
Programming Design Patterns
12/5/2018

9. Program to an interface
How can the two for loops be combined into one?
What code will change if another menu is added to this combined menu restaurant?
Programming Design Patterns
12/5/2018

10. Program to an interface
Why can’t the two for loops be combined into one?
The size of the two menus is different.
The collections should implement an interface with a method hasNext()
The way to step through an array and arrayList is different?
Let another object be responsible for stepping through
What code will change if another menu is added to this combined menu restaurant?
a new for loop will have to be added
Use polymorphism (method overloading wherein the parameter is an interface) so that the same for loop can be used
Programming Design Patterns
12/5/2018

11. Programming Design Patterns
Iterator Interface
public interface Iterator {
boolean hasNext();
Object next(); }
Note
Collections: a group of objects stored in data structures such as lists, arrays, hashtables, etc.
Also called as Aggregates
Programming Design Patterns
12/5/2018

12. Programming Design Patterns
Idea of Iterators
Iterator iterator = pancakeHouseMenu.createIterator();
while(iterator.hasNext()) {
MenuItem menuItem = (MenuItem)iterator.next(); }
Iterator iterator = dinerMenu.createIterator();
Programming Design Patterns
12/5/2018

13. Programming Design Patterns
PancakeHouse
public class PancakeHouseMenu {
ArrayList menuItems;
public PanckageHouseMenu() {
// initialize menuItems with various items }
public void addItem(…) { …}
public ArrayList getMenuItems() {…}
// other methods
CLASSWORK: Modify code so that it returns an Iterator
Programming Design Patterns
12/5/2018

14. Programming Design Patterns
PancakeHouse
public class PancakeHouseMenu {
ArrayList menuItems;
public PanckageHouseMenu() {
// initialize menuItems with various items }
public void addItem(…) { …}
public ArrayList getMenuItems() {…}
public Iterator createIterator() {
return new PancakeHouseIterator(menuItems);
// other methods
CLASSWORK: Need to implement PancakeHouseIterator
Programming Design Patterns
12/5/2018

15. Programming Design Patterns
DinerMenu
public class DinerMenu {
MenuItems[] menuItems;
public DinerMenu() {
// initialize menuItems with various items }
public void addItem(…) { …}
public MenuItem[] getMenuItems() {…}
// other methods
CLASSWORK: Modify code so that it returns an Iterator
Programming Design Patterns
12/5/2018

16. Programming Design Patterns
DinerMenu
public class DinerMenu {
MenuItems[] menuItems;
public DinerMenu() {
// initialize menuItems with various items }
public void addItem(…) { …}
public MenuItem[] getMenuItems() {…}
public Iterator createIterator() {
return new DinerMenuIterator(menuItems);
// other methods
Programming Design Patterns
12/5/2018

17. Programming Design Patterns
Combined Menu Class
Write a combinedMenu class
In the constructor pass PancakeHouseMenu and DinerMenu
Write code for the printMenu(…) method
Page 328 in the book
Programming Design Patterns
12/5/2018

18. Programming Design Patterns
Combined Menu Class
public class CombinedMenus {
PancakeHouseMenu pMenu;
DinerMenu dMenu;
public CombinedMenus(PancakeHouseMenu pMenuIn,
DinerMenu dMenuIn) {
…// }
public void printMenu() {
// get iterator from each of the data members
// call another method that just takes an Iterator
Page 328 in the book
Programming Design Patterns
12/5/2018

19. Programming Design Patterns
printMenu
public void printMenu() {
Iterator pIterator = pMenu.getIterator();
Iterator dIterator = dMenu.getIterator();
printMenu(pIterator);
printMenu(dIterator) }
public void printMenu(Iterator iterator) {
// CLASSWORK: write code for this method
Page 328 in the book
Programming Design Patterns
12/5/2018

20. Programming Design Patterns
printMenu
public void printMenu(Iterator iterator) {
while (iterator.hasNext()) {
MenuItem mItem = (MenuItem)iterator.next();
// print name, description, … from mItem }
Page 328 in the book
Programming Design Patterns
12/5/2018

21. How are these questions answered by Iterators?
Why can’t the two for loops be combined into one?
What code will change if another menu is added to this combined menu restaurant?
What design principles are violated followed in the printCombinedMenu() method?
Programming Design Patterns
12/5/2018

22. Dependence on Concrete Implementations
The code (slide 19) still depends on concrete implementations
where?
Note the use of polymorphism/method-overloading for printMenu(…)
Is the waitres code decoupled from DinerMenu and PancakeHouseMenu classes?
page 328 from the book
The data members should be Menu pancakeHouseMenu;
The constructor should take two parameters of interface Menu
Yes, it is decoupled if the code is written to the interface Menu, not concrete implementations.
Programming Design Patterns
12/5/2018

23. CombinedMenus code: common interface
public class CombinedMenus {
PancakeHouseMenu pMenu;
DinerMenu dMenu;
// … }
Use a Menu interface to remove dependency on concrete implementations
Menu should have the createIterator() method
Programming Design Patterns
12/5/2018

24. Programming Design Patterns
Improved code
Homework
Study the code in page 344 of the book
Change data member to be an ArrayList of menus
Change printMenu() code accordingly
it should retrieve an iterator for the arrayList
for each element obtained from the iterator
retrieve another iterator
pass it to the printMenu() code
Programming Design Patterns
12/5/2018

25. Programming Design Patterns
Iterator
Provides a way to step through the elements of an aggregate
without forcing the aggregate to clutter its own interface with a bunch of methods for traversal
Implementation of the Iterator lives outside of the aggregate object
iterator encapsulates the iteration
In our examples, the aggregates are responsible for creating their own iterators
Programming Design Patterns
12/5/2018

26. Java’s Iterator Interface
Has 3 methods
hasNext()
next()
remove()
Multithreading: read the documentation on behavior when multiple threads are accessing the collection
some are internally synchronized
Programming Design Patterns
12/5/2018

27. Programming Design Patterns
Iterator Pattern
provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation
Uniform way of accessing aggregate objects
Can write polymorphic code that works with any aggregate
Responsibility of iteration is not with the aggregate object, but with the iterator
Design principle?
Next slide …
A class should have only one reason to change
Programming Design Patterns
12/5/2018

28. Programming Design Patterns
Design Principle
A class should have only one reason to change
aggregate should just manage its data
not be concerned about providing functionality to traverse
Cohesion
high cohesion: class is designed around a set of related functions
low cohesion: class is designed around a set of unrelated functions
a class that returns current temperature and stock quote
Programming Design Patterns
12/5/2018

29. Java Collection interface
add()
clear()
equals()
hashCode()
isEmpty()
iterator()
size()
toArray()
and a few more methods
Programming Design Patterns
12/5/2018

30. Programming Design Patterns
Java 5
Don’t have to directly use an Iterator
Has a new for/in construct
for (Object obj: collection) { }
for (MenuItem item: items) {
String description = item.getDescription();
System.out.println(“Item is: “ + description);
Programming Design Patterns
12/5/2018