Object-Oriented Design Part 2

Responsibility-Driven Design Frames object design as deciding How to assign responsibilities to objects How objects should collaborate – What role each each object should play in a collaboration

What is meant by responsibilities An object’s obligations (and thus behavior) Two types: Doing responsibilities, e.g.: – Creating an object – Calculating something – Initiating action on other objects – Coordinating activities among other objects Knowing responsibilities, e.g.: – Knowing about data – Knowing about related objects – Knowing about things it can derive or calculate

Domain Model attributes inspire knowing responsibilities Design Analysis Register knows its IDSale knows its time

What is meant by collaboration? Objects interact (via messages) to fulfill responsibilities Example: Register collaborates with Sale and Payment to process a payment (its responsibility)

Responsibilities vary in granularity “Big” responsibilities may require collaborations of many objects “Small” responsibilities may be fulfilled by a single object

How to assign responsibilities and design collaborations? No mechanical method – Requires expert human judgment! But there’s hope: patterns!

Patterns Repeatable solution for a commonly occurring software problem Codify existing tried-and-true knowledge Provide vocabulary

Point of Sale Example Think of a cashier’s register – Cashier scans or manually enters the price of items – All items sold make up a sale – Payment is received, change is given – A receipt is generated

What object should be responsible for creating a SalesLineItem? POS Design Question

Creator Pattern Assign class B responsibility of creating instances of class A if 1+ of the following: B “contains” A B records A B closely uses A B has initializing data for A – B is an expert with respect to creating A

What object should be responsible for creating a SalesLineItem? The Creator Pattern says that Sale should create SalesLineItem POS Design Question

How a Sale object might create a SalesLineItem object : Register : Sale

What object should be responsible for knowing the grand total of a sale? POS Design Question

Information Expert Pattern Assign a knowing responsibility to the class that has the information necessary to fulfill the responsibility

What object should be responsible for knowing the grand total of a sale? POS Design Question The Information Expert Pattern says that Sale should know the grand total

How Sale might calculate the grand total Sale will need a method for computing the total

How Sale might calculate the grand total Sales will get the subtotal from each SalesLineItem

How Sale might calculate the grand total SalesLineItem will need to get the price from ProductDescription

How Sale might calculate the grand total These three objects collaborate to compute the grand total Each one must fulfill its own set of responsibilities

What class should be responsible for this? POS Design Question Assume we need to create a Payment instance and associate it with a Sale instance

One possible design: Register creates Payment

Another possible design: Sale creates Payment

Which design is better?

Low Coupling Pattern Assign responsibilities so that coupling stays low Coupling: measure of how strongly one element is connected to others has knowledge of others relies on others

Common types of coupling in OO languages Class C is coupled to class D if C has instance variable that refers to D C invokes method/function of D C method parameter or local variable references D C is direct or indirect subclass of D C implements interface D

Problems with high coupling Given class C highly coupled to class D: Changes in D may force changes in C Harder to understand C in isolation Harder to reuse C because of dependencies on D

Which design has lower coupling? Assuming Sale will eventually need to be coupled with Payment First design adds extra coupling between Register and Sale so second design has lower coupling

Critique: Other considerations may override preference toward low coupling Strength of coupling should be balanced against other design considerations Critique: High coupling to stable/pervasive elements is seldom a problem Consider coupling to Java standard libraries

Review of Key OO Concepts Objects/Classes Information Hiding – The ability to protect class contents from external entities – Private/Protected Inheritance – The ability for a class to extend and override functionality of another class – Generalization/Specification Interface – A contract of functionality other classes can implement Polymorphism – The ability to create an entity that has more than one type

Design Advice Inputs for your design – Use Cases – Sequence Diagrams – Prototypes – Dataflow Diagrams – Architecture Diagrams

Best practices Use known design patterns – Creator – Information Expert – More to come on the next class Take advantage of tried-and-true libraries – Don’t reimplement something that’s already been done Ex: Use known encryption libraries, they have been thoroughly tested and much less likely to have bugs

Vision Statement Next week we’ll start Agile Everyone will create a new vision statement with a new topic using the same specification as HW 0 Consider a solution that lends itself to an iterative design These will be due next Monday

What’s next for you? HW 3 is due tomorrow by midnight HW 4 is posted Think about your new vision statements