1. Behavioral and Structural Patterns
11/20/2018
Programming Design Patterns

2. Computer Game with saved states
A computer game is installed in one of the high performance desktop machines in a lab
The game remembers the score and state of the user’s last play
The code has a manager and a game-engine modules
The game-engine knows the progress of a game at any given time, but the manager module manages saving/restoring progress of each player
Design code so that a player’s progress can be restored the next time (s)he plays the game
progress can be stored as score and state (both integers)
Programming Design Patterns
11/20/2018

3. Programming Design Patterns
Storing Mementos
public class MasterGameObject {
// data member to store state
Object getCurrentState() {
// specific code to gather state }
void restoreState(Object savedState) {
// specific code for the game to restore state
// rest of the game code
Programming Design Patterns
11/20/2018

4. Programming Design Patterns
Manager Code
Object savedState = masterGameObj.getCurrentState();
masterGameObj.restoreState(savedState)
Programming Design Patterns
11/20/2018

5. Programming Design Patterns
Memento Code
Notice that the manager just stores the memento object
How can you extend this code for checkpointing long-running code?
need a series of checkpoints at different stages of the computation
Use a stack for a series of checkpoints
Programming Design Patterns
11/20/2018

6. Memento Pattern objectives
Encapsulate a checkpoint capability
Allow undo to a previous state
Allow redo
Capture an object’s internal state so that an object can be returned to this state at a later stage
Note that Memento is a behavioral pattern
Programming Design Patterns
11/20/2018

7. Programming Design Patterns
Java Serialization
Used to persist Java objects to a file, database, or a remote location
Convert an object into a stream of bytes so that it can be re-created at a later time
Serialization does not occur for static or transient fields
transient is used for local information such as processID that does not need to be saved
Objects to be serialized should implement
java.io.Serializable or java.io.Externalizable
Whats’ the difference between these two?
Not in exam syllabus
Serializable objects can be written to a file for persistence
Java.io.serializable: if you do not define writeObject(…) then the responsibility is not yours to serialize the object (automatically taken care of). Even if writeObject(..) is defined, serializing the superclass is automatically taken care of
Java.io.externalizable: everything is up to the programmer on what is serialized and what is not. Programmer has to write the required code.
Programming Design Patterns
11/20/2018

8. Java Serialization Example
import java.io.Serializable;
public class Student implements Serializable {
// standard setX and getX methods }
No method needs to be implemented for implementing the Serializable interface
what is this kind of interface called?
why is it useful?
Tag interface
So that Java knows that this object needs to be serialized
Programming Design Patterns
11/20/2018

9. Programming Design Patterns
Another Pattern
Tag interface
So that Java knows that this object needs to be serialized
Programming Design Patterns
11/20/2018

10. There is a hierarchy of abstractions
One purpose is to implement the Window system on a variety of platforms (X, MS windows, …)
There is a hierarchy of abstract classes describing the components in a platform independent way:
Window
ApplWindow
IconWindow
TransientWindow
Programming Design Patterns
11/20/2018

11. Programming Design Patterns
Design Problem
There are three kinds of Windows
application windows, transient windows, icon windows
Each of these windows is implemented separately
LinuxWindowImpl, AppleWindowImpl, MicroWindowImpl
The Window abstraction has a method display
The client wants to invoke display() on a transient window and use the AppleWindowImpl
Draw an inheritance diagram for the various classes
Programming Design Patterns
11/20/2018

12. Subclassing mixes two hierarchies
It is not a clear enough layout if we use subclassing to provide the implementations (here only for X):
Window
ApplWindow
IconWindow
TransientWindow
WindowXImp
MicroWindowXImp
AppleWindowXImp
LinuxWindowXImp
Programming Design Patterns
11/20/2018

13. Programming Design Patterns
Obvious problem
The possible need to use multiple inheritance (not available in Smalltalk or Java), …
Also, it is not very clear when you have some subclasses being specialized abstractions and other subclasses that provide implementations
Programming Design Patterns
11/20/2018

14. Programming Design Patterns
Bridge Pattern
Decouple an abstraction from its implementation
There is an expectation that the abstraction needs one of several implementations, or that the current implementation will be replaced
Using subclassing--abstract parent defines interface, subclass defines implementation--is not satisfactory
The Bridge Pattern puts the abstract classes and the implementing classes into separate hierarchies
Bridge pattern is a structural pattern
Programming Design Patterns
11/20/2018

15. Separate the hierarchies
There can be one hierarchy for implementations
There can be one hierarch for window categories
has-a
Window
WindowXImpl
AppWindow
MacWindowXImp
IconWindow
LinuxWindowXImp
TransientWindow
MicroWindowXImp
Programming Design Patterns
11/20/2018

16. Programming Design Patterns
Bridge Pattern Code
CLASSWORK
Abstract Base Class Window
void display()
Code for Class AppWindow
Driver code that displays an AppWindow using the MacWindow’s X implementation
Programming Design Patterns
11/20/2018

17. ApplicationWindow uses delegation
public class AppWindow extends Window {
private WindowXImpl ximpl;
private Coordinates xyCoord;
// pass concerete implementation in the constructor
AppWindow(Coordinates coordIn, WindowXiml ximplementation) {
ximpl = ximpementation;
xyCoord = coordIn; }
void display() {
// delegation to the implementation hierarch
ximpl.drawWindow();
Programming Design Patterns
11/20/2018

18. Programming Design Patterns
Driver Code
// concrete implementation class
WindowXImpl macWindow = new MacWindowXImp();
// code to the abstraction
Window window =
new AppWindow(xyCoord, macWindow)
window.display();
Programming Design Patterns
11/20/2018

19. Relation with other patterns
“The Adapter pattern is geared toward making unrelated classes work together. It is usually applied to systems after they are designed
Bridge, on the hand, is used up-front in a design to let abstractions and implementations vary independently”
“Abstract Factory can create and configure a particular Bridge”
How? HW
Programming Design Patterns
11/20/2018

20. Programming Design Patterns
Design Problem
Binghamton University has students of different types: grad, undergrad, freshman, junior, .., foreign, domestic, funded, full-time, part-time, etc. Design a class hierarchy using a well-known Design pattern that will be useful to display all the details of any given student.
decorator
Programming Design Patterns
11/20/2018

21. Programming Design Patterns
Design Problem
There are many different shapes (circle, square, …)
All shapes can be drawn()
There are many different drawing tools, each with a different specialization. Assume they have the same API for drawing a given shape
Use a pattern to decouple the following:
abstraction of shapes
client codes to this abstraction to draw shapes
implementations using various tools: gimp, photoshop, etc.
bridge
Programming Design Patterns
11/20/2018

22. Programming Design Patterns
Design Problem
Extend the Iterator pattern, using all design principles learned so far, so that a sub-menu can be added for dessert, breakfast, etc. (support menu within a menu).
composite
Programming Design Patterns
11/20/2018