Object Oriented Design 2 Nelson Padua Perez Chau Wen Tseng Department of Computer Science University of Maryland College Park Overview Object oriented design Objects methods Classes inheritance Last lecture This lecture Applying object oriented design Elements of Object Oriented Design Objects Entities in program Methods Functions associated with objects Classes Groups of objects with similar properties Inheritance Relationship between classes Classes Definition Group of objects with same state behavior Abstract description of a group of objects Similar to data types Type is a set of data values their operations Example integer real boolean string Can view classes as types for objects Classes Properties Classes provides classification for objects Every object belongs to some class Objects instances instantiations of a class Example Class Given a class Car Objects can include MyHonda YourHonda HerMiniCooper Car HisSUV All Car objects Share same properties behavior May have different values for properties Inheritance Definition Relationship between classes when state and behavior of one class is a subset of another class Terminology Superclass parent More general class Subclass More specialized class Subclass Superclass Inheritance Properties Subclass inherits state behavior of superclass Is a relationship exists between inherited classes Example train is a type of transportation Inheritance Inheritance forms a hierarchy Helps organize classes Inheritance is transitive Class inherits state behavior from all ancestors Inheritance promotes code reuse Reuse state behavior for class Inheritance Hierarchy Example Classes Thermostat Analog thermostat Digital thermostat Programmable thermostat Forms of Inheritance Specification Defines behavior implemented only in subclass Guarantees subclasses implement same behavior Specialization Subclass is customized Still satisfies all requirements for parent class Specialization Example Forms of Inheritance Extension Adds new functionality to subclass Limitation Restricts behavior of subclass Combination Inherits features from multiple superclasses Also called multiple inheritance Not possible in Java Multiple Inheritance Example Combination AlarmClockRadio has two parent classes State behavior from both Radio AlarmClock Superclasses Applying Object Oriented Design 1 Look at objects participating in system Find nouns in problem statement requirements specifications Noun may represent class needed in design 2 Look at interactions between objects Find verbs in problem statement Verb may represent message between objects 3 Design classes accordingly Determine relationship between classes Find state methods needed for each class 1 Finding Classes Thermostat uses dial setting to control a heater to maintain constant temperature in room Nouns Thermostat Dial setting Heater Temperature Room Finding Classes Analyze each noun Does noun represent class needed in design Noun may be outside system Noun may describe state in class Analyzing Nouns Thermostat Central class in model Dial setting Thermostat Dial Setting State in class Thermostat Heater Class in model Heater Room Class in model Temperature State in class Room Room Temp Finding Classes Decision not always clear Possible to make everything its own class Approach taken in Smalltalk Overly complex 2 3 5 vs NUM2 add NUM3 NUM5 Impact of design More classes more abstraction flexibility Fewer classes less complexity overhead Choice somewhat depends on personal preference Avoid making functions into classes Examples class ListSorter NameFinder 2 Finding Messages Thermostat uses dial setting to control a heater to maintain constant temperature in room Verbs Uses Control Maintain Finding Messages Analyze each verb Does verb represent interaction between objects For each interaction Assign methods to classes to perform interaction Analyzing Verbs Uses Thermostat uses dial setting Thermostat SetDesiredTemp Control to control a heater Heater TurnOn Heater TurnOff Maintain to maintain constant temperature in room Room GetTemperature Example Messages SetDesiredTemp Thermostat GetTemperature Room TurnOn TurnOff Heater Resulting Classes Thermostat State DialSetting Methods SetDesiredTemp Heater State HeaterOn Methods TurnOn TurnOff Room State Temp Methods GetTemperature
View Full Document
Unlocking...