Workshop for CS-AP Teachers Chapter 4 Object-Oriented Design Georgia Institute of Technology
Learning Goals • Understand the basics of object-oriented design – – – Purpose of Design Goals of Object-Oriented Design Principles of Good Object-Oriented Design How to transition from analysis to design The design process Georgia Institute of Technology
Object-Oriented Design • The purpose of design is to build an abstract, high-level, description of HOW the system will work • In Structured Design the emphasis is on: – Top-down design – Successive refinement of functions • In OO Design the emphasis is on: – Refinement of the classes and the relationships between them – Mapping classes to the implementation environment • Choosing libraries, designing the user interface Georgia Institute of Technology
Goals of Object-Oriented Design • Decrease coupling: the degree to which two components depend on each other’s implementations (minimize the effect of changes) • Increase cohesion: the degree to which the responsibilities of a class are related (maximize the ability to combine objects) Georgia Institute of Technology
Good Object-Oriented Design • Is based on real world artifacts – That makes the design reusable • Information access is enough – Objects don't need information that they can't get to. – Objects can get to the information they need • directly or indirectly • Responsibility, control, and communication is distributed – No one object does everything • Attributes and operations are factored out as high in the class hierarchy as possible – Using inheritance • There should be little or no redundancy – Code should appear only once Georgia Institute of Technology
Good Design Exercise • Double click on battle. html and run the battleship game • Open the class battle. java in the folder exercises/battleship – What do you think of this design? – How easy or hard would it be to change this game? • adding 2 players • adding more ships • changing the size of the game board Georgia Institute of Technology
Moving from Analysis to Design • Determine the length of the next iteration – From 2 weeks to many months – Try to keep the iterations the same length • Review the use cases and determine which use cases will be implemented in the next iteration – Based on priority, risk, and time estimates – Tackle high priority and high risk first Georgia Institute of Technology
Getting Started on Design • Design can be partitioned into – Internal Design: • Converting conceptual models into specification models • Refinement of classes and the relationships between classes – Interface Design: Define the external interface to • Users • Persistent data • Other programs • OOD does not usually require – Pseudo-code for class operations – Class designs to be “frozen” before implementation commences Georgia Institute of Technology
Design Process • Review the class diagram • Create detailed use cases – Include user interface details and interface prototypes • Modify the class diagram to add attributes, operations, and new classes – Found in the detailed use cases – Add packages as the number of classes gets too large • Determine how to make the objects persistent Georgia Institute of Technology
Review the Class Diagram Georgia Institute of Technology
Example Class Diagram • This model is just one feasible static object model • At design time, we can refine the model – Is a Deck the same object as the Stock pile? – How do we handle moving piles of cards? Georgia Institute of Technology
Review the Class Diagram • Review classes and remove – vague classes • system – redundant classes • a class should appear only once in the diagram – irrelevant classes • outside the domain • • Review the class names Review attributes Review operations Review the relationships between classes Georgia Institute of Technology
Review the Class Names • Do the names reflect the domain? • Does the name reflect the purpose of the class? – Is a connector between shapes a line or a connector? • Are the names pronounceable? • Does each word in a class name start with a capital letter: Shape. Canvas? • Are the names clear and unambiguous? • Are the names singular? – Course not Courses Georgia Institute of Technology
Review Attributes • Add types to attributes – Using types in the chosen language (if needed) • Some attributes need to be expanded – A person’s name • Some attributes should be classes – An address • Some attributes can be grouped into a class – A date range Georgia Institute of Technology
Review Operations • Check the names of the operations – Are they easy to read and clear? • Add parameters and return types • Check that the operations are on the correct class – The class you expect would be doing the action • Check if several classes have similar operations that can be put in a parent class – Even if the names of the operations are different Georgia Institute of Technology
Review Inheritance • Use inheritance only when the child class is a “type of” the parent class – Use association if this isn’t true • Modify multiple inheritance to use interface realization – if you are using a language that doesn’t support multiple inheritance Georgia Institute of Technology
Add Classes to Clarify Relations • Doctor, Nurse, and Patient are all “a kind of” Person • But Doctor and Nurse are both Medical Personnel too – Introduce a new class Medical Person Georgia Institute of Technology
Delegation instead of Inheritance • Use delegation to simply use (buy) another class – An object handles a message by sending messages to other objects • User gets the person name from person – Allows an object to invoke other objects’ operations without adding the overhead of inherited operations Georgia Institute of Technology
Review Association • Use association when an object of a class “has a” object of another class – A catalog has items • Add navigability to show which side of an association cares about the relationship – A catalog will track the items in it but the items won’t care what catalog they are in Georgia Institute of Technology
Create Detailed Use Cases Georgia Institute of Technology
Create Detailed Use Cases • Expand the high-level use cases from analysis – Go into the details including the user interface • Elicit rare or minor use cases – People remember the “standard” things. • Consider future use cases – What things might we want the system to do in the near future? • Assign use cases to iterations – When will you implement the use case? Georgia Institute of Technology
A Detailed Use Case • Defines how as well as what – Names the user interface elements – Names parts of the system – Go into the details of how • Describes the success scenario – Sequence of steps when everything goes well • Describes alternative scenarios as well – Times when things go wrong or – Times when things are handled differently Georgia Institute of Technology
Scenarios versus Use Cases • A scenario is a sequence of interactions between a user and a system – About a user goal – Example for the goal of making a bulleted list: • User selects some text and then clicks on the icon to make the text a bulleted list. The system modifies the text to make it a bulleted list. • A use case is a set of scenarios that have the same user goal – Includes the success scenario “happy day” scenario – Includes failures and alternative success scenarios as well Georgia Institute of Technology
Detailed Use Case Format • There is no standard format • Two typical types – Use case with alternative scenarios included in the body of the success scenario • Indenting used to show alternatives – Use case with uninterrupted success scenario • Alternatives are listed after the success scenario • Numbering – Some use simple numbering 1, 2, 3, … – Others use decimal numbering 1, 1. 1 1. 2, 1. 2. 1, Georgia Institute of Technology
Detailed Use Case Example Name: Placing an Order Actors: customer (initiates), salesperson, Frequency: common, Priority: Essential 1. A customer calls in an order. The salesperson brings up an Order Entry page. The User Interface (UI) prompts the salesperson for the number on the catalog that identifies the catalog and customer. 2. The customer gives the number 3. The salesperson verifies the customer name and catalog name and asks for the first item 4. For each item the customer gives the item number 5. For each item the salesperson gives the item name, price and asks the customer for the options and quantity 6. For each item the customer gives the quantity and the salesperson reserves this quantity for the customer 7. When there are no more items in the order the salesperson gives the shipping choices, length of time, and cost 8. The customer picks a shipping method Georgia Institute of Technology
Detailed Use Case Example 9. 10. 11. 12. 13. 14. 15. The salesperson gives the total cost of the order and gets or verifies the credit card information. A check is made to make sure the credit card will allow the charge. The salesperson gives the order confirmation number (order number) and tells the customer when to expect the order. The order status is set to pending and after the credit card approval occurs it changes to approved The shipping clerk uses the Waiting to be Shipped page to see the items needed for the order. The shipping clerk uses the Item Locator page to find where the items are in the warehouse. The shipping clerk prints off an invoice, mailing label, and packs the order. The credit card is charged for the order. The shipping company picks up the order and delivers it. Georgia Institute of Technology
Use Case Alternatives • Alternative: New Customer – At step 1 this may be a new customer so use the use case of creating a new customer • Alternative: Expired Catalog – At step 1 this may be an expired catalog and if so notify the user that the prices may be out of date • Alternative: Backordered Item – At step 6 the item may indicate that it is on backorder. If so the salesperson will notify the customer and ask if they want to continue • Alternative: Cancel – At any point the customer cancel the order Georgia Institute of Technology
Detailed Use Case Example Use Case: Withdraw from ATM Actors: Customer, ATM (Customer Initiates) Frequency: Common Priority: Essential Description: Georgia Institute of Technology
Vending Machine Exercise Objective: Duration: Exercise: Practice Object-Oriented design 30 minutes Modify a class diagram for the following requirements. Then write a detailed use case describing getting an item from the vending machine. Create the next generation vending machine that can tell when a part of it is broken and can place a service call so that a repairman come and fix it. Georgia Institute of Technology
Vending Machine Class Diagram Georgia Institute of Technology
Revised Vending Machine Georgia Institute of Technology
Detailed Vending Use Case Name: Get item from machine Actors(s): Customer (initiates) Frequency: Common Priority: Essential Description: 1. The customer starts a transaction by putting money in the vending machine 2. The money holder validates the money entered and if valid adds it to the amount entered 3. The money holder updates the display to show the amount entered. 4. The customer pushes an item selector (sends a select to the item selector) 5. The item selector tells the item holder to dispense 6. The item holder gets the price from the item. 7. The item holder tells the display to display the item price. 8. The item holder checks if the money holder can make the required change. 9. The item holder dispenses the item 10. The item holder tells the money holder to dispense the change 11. The item holder tells item to decrement the quantity in stock 12. Item will decrement the quantity in stock and check if it is at the restock level and if so send a message to the supplier Georgia Institute of Technology
Detailed Vending Use Case - Alts • Alternative – Validate money fails – Step 2. If the validate money fails the money is returned to the user and the display tells the user • Alternative – Can’t make change – Step 8. If the money holder can’t make change then the display tells the user so the user cancel the transaction • Alternative – Can’t dispense item – Step 9. If the item isn’t dispensed the display is updated to tell the user so the user cancel the transaction • Alternative – Can’t reach supplier – Step 12. If the supplier can’t be reached try again every few hours and notify the vending machine company Georgia Institute of Technology
Revise the Class Diagram Georgia Institute of Technology
Adding Operations and Attributes • Creating detailed use cases will likely uncover attributes and operations that were not thought of in analysis. • Determining where the new attributes and operations should go depends on the class responsibilities. • The assignment of responsibilities is a crucial skill in object-oriented design. • Often new classes need to be created to refine the responsibilities. Georgia Institute of Technology
Responsibility Assignment • Where to put an attribute? – Assign attributes to the class that it belongs to or who you would expect to be able to ask about it. • Where to put an operation? – Assign an operation to the class that has the data needed for the operation or assign the operation to an intermediate class to reduce coupling (indirection). Georgia Institute of Technology
Responsibility Assignment • Who should create an object of a class? – A should create B if A contains B. – A should create B if A has the initializing data for B. • Who should handle an event? – The object that takes action based on the event or the container that holds the object that where the event occurred. Georgia Institute of Technology
Summary • The purpose of design is to build a high-level description of how the system will work • The goals of design are to reduce coupling and increase cohesion • During design we should – Build a class diagram of the solution • Add types and method parameters – Create detailed use cases – Create user interface prototypes Georgia Institute of Technology
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