1-Week3-ObjectOorientedProgramming-design.pdf

Oops design

1. The philosophy of OO programming
Madhavan Mukund
https://www.cmi.ac.in/~madhavan
Programming Concepts using Java
Week 3

2. Algorithms + Data Structures = Programs
Title of Niklaus Wirth's introduction to Pascal
Traditionally, algorithms come rstStructured programming
Design a set of procedures for specic tasks
Combine them to build complex systems
Data representation comes later
Design data structures to suit procedural manipulations
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

3. Algorithms + Data Structures = Programs
Title of Niklaus Wirth's introduction to Pascal
Traditionally, algorithms come rstStructured programming
Design a set of procedures for specic tasks
Combine them to build complex systems
Data representation comes later
Design data structures to suit procedural manipulations
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

4. Algorithms + Data Structures = Programs
Title of Niklaus Wirth's introduction to Pascal
Traditionally, algorithms come rstStructured programming
Design a set of procedures for specic tasks
Combine them to build complex systems
Data representation comes later
Design data structures to suit procedural manipulations
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

5. Algorithms + Data Structures = Programs
Title of Niklaus Wirth's introduction to Pascal
Traditionally, algorithms come rstStructured programming
Design a set of procedures for specic tasks
Combine them to build complex systems
Data representation comes later
Design data structures to suit procedural manipulations
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

6. Object Oriented design
Reverse the focus
First identify the data we want to maintain and manipulateThen identify algorithms to operate on the dataClaim: works better for large systemsExample: simple web browser
2000 procedures manipulating global data
. . . vs 100 classes, each with about 20 methodsMuch easier to grasp the designDebugging: an object is in an incorrect stateSearch among 20 methods rather than 2000 procedures
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

7. Object Oriented design
Reverse the focus
First identify the data we want to maintain and manipulateThen identify algorithms to operate on the dataClaim: works better for large systemsExample: simple web browser
2000 procedures manipulating global data
. . . vs 100 classes, each with about 20 methodsMuch easier to grasp the designDebugging: an object is in an incorrect stateSearch among 20 methods rather than 2000 procedures
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

8. Object Oriented design
Reverse the focus
First identify the data we want to maintain and manipulateThen identify algorithms to operate on the dataClaim: works better for large systemsExample: simple web browser
2000 procedures manipulating global data
. . . vs 100 classes, each with about 20 methodsMuch easier to grasp the designDebugging: an object is in an incorrect stateSearch among 20 methods rather than 2000 procedures
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

9. Object Oriented design
Reverse the focus
First identify the data we want to maintain and manipulateThen identify algorithms to operate on the dataClaim: works better for large systemsExample: simple web browser
2000 procedures manipulating global data
. . . vs 100 classes, each with about 20 methodsMuch easier to grasp the designDebugging: an object is in an incorrect stateSearch among 20 methods rather than 2000 procedures
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

10. Object Oriented design
Reverse the focus
First identify the data we want to maintain and manipulateThen identify algorithms to operate on the dataClaim: works better for large systemsExample: simple web browser
2000 procedures manipulating global data
. . . vs 100 classes, each with about 20 methodsMuch easier to grasp the designDebugging: an object is in an incorrect stateSearch among 20 methods rather than 2000 procedures
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

11. Object Oriented design
Reverse the focus
First identify the data we want to maintain and manipulateThen identify algorithms to operate on the dataClaim: works better for large systemsExample: simple web browser
2000 procedures manipulating global data
. . . vs 100 classes, each with about 20 methodsMuch easier to grasp the designDebugging: an object is in an incorrect stateSearch among 20 methods rather than 2000 procedures
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

12. Object Oriented design
Reverse the focus
First identify the data we want to maintain and manipulateThen identify algorithms to operate on the dataClaim: works better for large systemsExample: simple web browser
2000 procedures manipulating global data
. . . vs 100 classes, each with about 20 methodsMuch easier to grasp the designDebugging: an object is in an incorrect stateSearch among 20 methods rather than 2000 procedures
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

13. Object Oriented design
Reverse the focus
First identify the data we want to maintain and manipulateThen identify algorithms to operate on the dataClaim: works better for large systemsExample: simple web browser
2000 procedures manipulating global data
. . . vs 100 classes, each with about 20 methodsMuch easier to grasp the designDebugging: an object is in an incorrect stateSearch among 20 methods rather than 2000 procedures
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

14. Object Oriented design
Reverse the focus
First identify the data we want to maintain and manipulateThen identify algorithms to operate on the dataClaim: works better for large systemsExample: simple web browser
2000 procedures manipulating global data
. . . vs 100 classes, each with about 20 methodsMuch easier to grasp the designDebugging: an object is in an incorrect stateSearch among 20 methods rather than 2000 procedures
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

15. Object Oriented design: Example
An order processing system typically involves
Items
Orders
Shipping addresses
Payments
Accounts
What happens to these objects?
Items are
Orders are,
Payments are
Nouns
Associate with each order, a method to add an item
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

16. Object Oriented design: Example
An order processing system typically involves
Items
Orders
Shipping addresses
Payments
Accounts
What happens to these objects?
Items are
Orders are,
Payments are
Nouns
Associate with each order, a method to add an item
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

17. Object Oriented design: Example
An order processing system typically involves
Items
Orders
Shipping addresses
Payments
Accounts
What happens to these objects?
Items are
Orders are,
Payments are
Nouns
Associate with each order, a method to add an item
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

18. Designing objects
Behaviour | what methods do we need to operate on objects?
State | how does the object react when methods are invoked?
State
Encapsulation | should not change unless a method operates on it
Identity | distinguish between dierent objects of the same class
State may be the same | two orders may contain the same item
These features interact
State will typically aect behaviour
Cannot add an item to an order that has been shipped
Cannot ship an empty order
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

19. Designing objects
Behaviour | what methods do we need to operate on objects?
State | how does the object react when methods are invoked?
State
Encapsulation | should not change unless a method operates on it
Identity | distinguish between dierent objects of the same class
State may be the same | two orders may contain the same item
These features interact
State will typically aect behaviour
Cannot add an item to an order that has been shipped
Cannot ship an empty order
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

20. Designing objects
Behaviour | what methods do we need to operate on objects?
State | how does the object react when methods are invoked?
State
Encapsulation | should not change unless a method operates on it
Identity | distinguish between dierent objects of the same class
State may be the same | two orders may contain the same item
These features interact
State will typically aect behaviour
Cannot add an item to an order that has been shipped
Cannot ship an empty order
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

21. Designing objects
Behaviour | what methods do we need to operate on objects?
State | how does the object react when methods are invoked?
State
Encapsulation | should not change unless a method operates on it
Identity | distinguish between dierent objects of the same class
State may be the same | two orders may contain the same item
These features interact
State will typically aect behaviour
Cannot add an item to an order that has been shipped
Cannot ship an empty order
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

22. Relationship between classes
Dependence
OrderneedsAccountto check credit status
Itemdoes not depend onAccount
Robust design minimizes dependencies, or
Aggregation
OrdercontainsItemobjects
Inheritance
One object is a specialized versions of another
ExpressOrderinherits fromOrder
Extra methods to compute shipping charges, priority handling
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

23. Relationship between classes
Dependence
OrderneedsAccountto check credit status
Itemdoes not depend onAccount
Robust design minimizes dependencies, or
Aggregation
OrdercontainsItemobjects
Inheritance
One object is a specialized versions of another
ExpressOrderinherits fromOrder
Extra methods to compute shipping charges, priority handling
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

24. Relationship between classes
Dependence
OrderneedsAccountto check credit status
Itemdoes not depend onAccount
Robust design minimizes dependencies, or
Aggregation
OrdercontainsItemobjects
Inheritance
One object is a specialized versions of another
ExpressOrderinherits fromOrder
Extra methods to compute shipping charges, priority handling
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java

25. Summary
An object-oriented approach can help organize code in large projects
This course is
Nevertheless, useful to know the motivation underlying OO programming to
understand design choices in a programming language like Java
Madhavan Mukund The philosophy of OO programming Programming Concepts using Java