My presentation to the Code Foo 2012 interns. http://code.ign.com/foo

1. Object Oriented Principles and
Design
Manish Pandit

2. About me
• Joined IGN in Aug 2009 to build the Social API
Platform
• Worked at E*Trade, Accenture, WaMu, BEA
• Started working with Java ecosystem in ’96
• @lobster1234 on Twitter, Slideshare, Github and
StackOverflow

3. Programming
• What constitutes a ‘program’?
– Data
– Behavior

4. Procedural Code
#include<stdio.h>
int main(){
int what, result;
printf("nFactorial of what, fellow code-foo’er? t");
scanf("%d",&what);
result = function(what);
printf("nThe number you are looking for is %d n", result);
}
int function(int number){
int result;
if(number==1) return 1;
else result = number*function(number -1);
return result;
}

5. Lets analyze..
• Simple to write, as it is linear. Think machine.
• Can kick any OO language’s ass in performance
BUT
• No separation of data and behavior
• No concept of visibility of variables
• No encapsulation
• Hard to map real world problems as humans – great for
algorithms though.
• Global, shared data.
• Reusability achieved via copy paste, header files (.h) or
shared object files (.so files in Unix)

6. Objects – What?
• Abstraction over linear programming
• Keywords – reduce, reuse, recycle
• Modeling a problem around data and
behavior vs. a big block of code
• Identify patterns to model
– Classes
– Methods
• HUMAN!

7. Confused?
• Let me confuse you more with the cliché,
textbook examples of
– Shapes (Circle, Triangle, Rectangle, Square)
– Animals (Dog, Cat, Pig, Tiger)
– Vehicles (Car, Truck, Bike)

8. We can do better!
• Lets model IGN the OO-way
– We have games
– We have properties of games
– We have users (who have properties too)
– Users follow games
– Users follow other users
– Users rate games
– Users update status

9. Real world enough for you?
• Actors – Nouns – Users, Games, Activities,
Status
• Actions – Verbs – Rate, Follow, Update
• Properties
– User – nickname, age, gender
– Game – publisher, title, description

10. Connect the dots
• Actors, Nouns – are Classes
• Actions, Verbs – are functions or methods
• A function operates on data encapsulated within
a Class
• Every actor has certain properties
• A caller of this Class doesn’t give a shit to the
implementation details (abstraction, data hiding)
• Collectively, the properties and methods in a class
are called its members.

11. Going back to C
#include<stdio.h>
int main(){
char* title; char* publisher;
// save??
// describe??
}

12. Java
public class Game {
private String title;
private String publisher;
public String getTitle(){
return "Title is " + title;
}
public void setTitle(String title){
if(title!=null)
this.title = title;
else this.title = "";
}
public String getPublisher(){
return "Publisher is " + publisher;
}
public void setPublisher(String publisher){
this.publisher = publisher;
}
public void describe(){
System.out.println(“Title is “ + title + “ Publisher is “ +
publisher);
}
}

13. Principles so far..
• Objects are instances of classes and are created
by invoking the constructor which is defined in a
class.
• Objects have a state and identity, classes don’t.
• Some classes cannot be instantiated as objects
– Abstract classes
– Factories (getting a little ahead of ourselves here!)
• Properties are defined by “has-a” clause
• Classes are defined by “is-a” clause (more on this
later)

14. Inheritance
• Classes can extend other classes and interfaces
– is-a relationship
• A Game is a MediaObject, so is a DVD
• A User is a Person, so is an Editor
• Java does not support multiple inheritance per-
se, but you can mimic that via interfaces.
• A cluster of parent-child nodes is called a
hierarchy
• CS Students – Generalization vs. Specialization

15. Inheritance and Behavior
• Child classes can
– override behavior
– overload behavior
Provided the Parent has provided visibility to its
members

16. Association: Aggregation
• A game has a title
• A game has a publisher
– A Publisher can live without this game. He can
have other games. This is aggregation.

17. Association: Composition
• A game has a story
– A story cannot live outside of the game. This is
composition

18. Behavior : Interfaces
• An interface defines a behavior of the classes
that inherit it – as seen by the consumers
• Interfaces are actions on verbs – like
Observable, Serializable, Mutable,
Comparable. They add behavior.
• Interfaces do not define implementation
(that’s OO Power!). They just define behavior.
The implementation is encapsulated within
the class. This is polymorphism.

19. Behavior of saving data
• Interface:
public interface Persistable{
public void save(Object o);
}
• Implementations:
public class MySQLStore implements Persistable{
public void save(Object o){
//save in MySQL
}
}
public class MongoStore implements Persistable{
public void save(Object o){
//save in MongoDB
}
}

20. Packages
• Packages for logical organization

21. Visibility and Modifiers
• public/protected/package/private
– Scala has explicit package-private
– Accessors and Mutators
• Advanced for this session
– final
– static
– constructors (default, noargs..)
– super

22. Recap : Object Modeling
• Nouns, Verbs
• is-a, has-a
• Inheritance (is-a)
• Association (has-a)
– Composition
– Aggregation
• Data vs. Behavior

23. Design Patterns
• Singleton
• Adapter
• Decorator
• Proxy
• Iterator
• Template
• Strategy
• Factory Method

24. Advanced OO
• Polymorphism
– Dynamic/runtime binding
• Mix-ins
– Traits in Scala
• Frameworks
– Frameworks like Scalatra, Struts, Grails..
– Spring, Shindig

25. Case Study : IGNSocial
• Interfaces
• Implementation
• Packages
• Dependencies

26. Java Collections API
• Go over the javadoc of collections API and
identify
– Packages
– Inheritance
• Classes
• Interfaces

27. Collections Type Hierarchy

28. Factorial in Scala
def fact(x:Int):Int=if(x==1) x else x*fact(x-1)

29. Further Reading
• Apache Commons Library
– Connection Pool
– String and Calendar Utils
– JDBC
• Java or C++ Language Spec
• Wikipedia