Ch4: Software Architecture and Design

1 Object-oriented paradigm  Object-oriented decomposition:  Agents comprised of two parts:  Hidden implementation:  Public interface:  An Agent can only be modified by operations defined in either the hidden implementation or public interface.

2 Core object oriented concepts  Class  Information:  Method  Object  Message

3 Class class name attributes: operations: external entities things occurrences roles organizational units places structures A class is a generalized description of a collection of similar objects. Object is an instance of a class. Class exports: -Operations used to manipulate instances -May export attributes.

4 Two representations of a class

5 Operations (or methods or services) An executable procedure that is a part of a class and operates on the data attributes defined as part of the class. A method operates on all instances of a class A method is invoked via message passing.

6 Messages Messages - the means by which objects invoke each other’s methods.

7 An example Employee class Class Employee { } //Hidden Implementation Private: //Instance Vars char[30] name; float salary; //Public Interface Public: void print_name(); void print_salary(); void update_salary(float i); Employee(char *n, float s); Main() { //Declare Objects Employee emp1(A,100.0); Employee emp2(B,120.0); //Pass Messages //Invoke Methods emp1.print_name(); emp1.print_salary(); emp2.update_salary(10); emp2.print_name(); emp2.print_salary(); } What’s Output of Main()? A B Conclusion: Each Object (emp1,emp2) has Own Independent State that is Accessible via Shared Public Interface of Class

8 Classes vs. ADTs  Classes extend ADTs as follows:  Message passing:  Inheritance:  Polymorphism:  ADT/OO benefits:

9 How to choose objects and classes  The first and most often raised concern for newcomers to OO concepts  Typical answers:  Better answer:

10 How to choose objects and classes (contd..)  Employee class  Private data:  Public interface:  Based on an information perspective, focusing on the idea that to track Employees a set of standard data and operations are needed.

11 How to choose objects and classes (contd..)  ATM_log class:  Private data:  Public interface:  Embodies the functions that take place to authenticate an individual to an ATM session.  Even with a functional view, information is needed to capture user input for verifying status.

12 How to choose objects and classes (contd..)  ATM_User:  Private data:  Public interface:  User interface by capturing the different interactions between the ATM and the user.

13 How to choose objects and classes (contd..) An appointments system that will allow telephone callers to book an appointment with a doctor. The caller will specify the day and the time when he wishes to be seen by a doctor.  Tentative classes could be:

14 How to choose classes and objects (contd..)  Redundancy:  Discard nouns outside the system domain  Vagueness:  Attributes:

15 How to choose classes and objects (contd..)  Operations:

16 How to choose objects and classes (contd..)  Attributes are properties of individual objects  Can be  Nouns followed by “of the” (E.g. day “of the” appointment)  Adjectives - color, number, state (on/off)  May not be fully described  Guidelines for identifying attributes:  Attributes that are directly relevant to the problem. Something can be an attribute in one context and an object in another e.g city.  Give them meaningful names.  Avoid attributes that are purely involved in implementation e.g an id number that is generated by the machine and has meaning only within the application.  Avoid attributes that can be derived from existing information e.g. age can be derived from date of birth  Different and unrelated attributes in a class may suggest that the class is a composite of a number of classes. Useful to divide such a class into a number of separate classes.

17 How to choose objects and classes (contd..)  Identifying operations:  Attributes:  Events in the scenarios:  A scenario consists of interactions (events exchanged) that have to take place among the objects to achieve the functionality.  Identify common and rare scenarios.  Events passed to and from the objects implies operation on the object or message from it.

18 How to choose objects and data (contd..)  Real world can also suggest the operations needed to support a class :  Operations should not overlap each other:  Number of operations that have access to the data should be reduced to a minimum.  Operations may refer to verbs in the problem description

19 High-Tech Supermarket System (HTSS)  Automate the functions and actions:  Cashiers and inventory updates  User friendly grocery item locator  Fast-track deli orderer  Inventory control  User system interfaces  Cash register/UPC scanner  GUI for inventory control  Shopper interfaces locator and orderer  Deli interface for deli workers

20 HTSS (contd..) IC IC CR CR CR CR IL IL IL SDO SDOEDO EDO Order Payment Item ItemDBLocalServer Non-Local Client Int. InventoryControl ItemDBGlobalServerOrderDB SupplierDB CreditCardDB ATM-BanKDB IL: Item Locator CR: Cash Register IC: Invent. Control DO: Deli Orderer for Shopper/Employee Shopper/Employee

21 Classes in the HTSS  Nouns:  Noun extraction:  Do we need classes for customers/shoppers?  Nouns such as aisle, shelf, UPC, etc. do not have any independent existence, in fact, they represent attributes of item.

22 Classes in HTSS  A class based on knowledge of the problem domain:  Receipt  There are other kinds of classes, mostly in the solution domain (do not represent any physical entity or a concept in the problem domain), that noun extraction does not reveal.  Classes to represent GUIs.  Collection classes such as linked lists, queues, stacks  Attributes based on domain knowledge:  Retail cost, whole sale cost, etc.

23 Item class in HTSS  Item class  Attributes:  Operations: