Object-Oriented Design Part 2http://flic.kr/p/btp5ZK

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

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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 inspireknowing responsibilities

Design

Analysis

Register knows its ID Sale knows its time

What is meant by collaboration?

Objects interact (via messages) to fulfill responsibilities•Example: Register collaborates

with Sale and Paymentto 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!

http://flic.kr/p/4tTsQe

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 forcreating 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 createa SalesLineItem object

: Register : Sale

What object should be responsible forknowing 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 forknowing 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