CMSC 132 Object Oriented Programming II Object Oriented Design Department of Computer Science University of Maryland College Park Applying Object Oriented Design 1 Look at objects participating in system 2 Find nouns in problem statement requirements specifications Noun may represent class needed in design Relationships e g has or belongs to may represent fields Look at interactions between objects 3 Find verbs in problem statement Verb may represent message between objects Design classes accordingly Determine relationship between classes Find state methods needed for each class Classes A class or interface defines and describes a set of objects It describes a set of methods or messages that the object responds to Not only the name and signature of the method but the contract the method respects Classes also provide describe fields and method implementations 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 Singleton classes A Singleton class is a class for which there will only ever be one instance Makes sense if the class is a subclass of another class For example you might have a class Person and a singleton subclass Elvis Avoid making verbs functions into classes Examples class ListSorter NameFinder Unless you might have multiple verb classes that all implement a common interface The Strategy design pattern 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 int degrees 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 Subtypes If a class Y extends class X and implements interface A then Y is a subtype of both X and A If Q is a subtype of P then Q satisfies P s contract Anyone who expects a P can be given a Q This is known as the Liskov Substitution Principle named for Prof Barbara Liskov Not always strictly followed but an ideal to approach For example some iterators don t support remove Which could be a subtype Class B Search for x in a return location of first occurrence 1 if not found int search int x int a Class C same as above uses binary search for speed the array a must be sorted int search int x int a Which could be a subtype Class B Search for x in a return location of first occurrence 1 if not found int search int x int a Class C Search for x in a return location of any occurrence 1 if not found int search int x int a is a vs has a Say we have two classes Engine and Car Two possible designs A Car object has a reference to an Engine object has a The Car class is a subtype of Engine is a Prefer Composition to Subtyping Generally prefer composition delegation hasa to subtyping is a Subtyping is very powerful but easy to overuse and can create confusion and lead to mistakes Using is a restricts you from having a car with more than one engine or with no engine Tempting to use subclassing in places where it doesn t really make conceptual sense to avoid having to delegate methods Don t Forms of Inheritance Extension Adds new functionality to subclass In Java new method Limitation Restricts behavior of subclass In Java override method throw exception Combination Inherits features from multiple superclasses Also called multiple inheritance Not possible in Java In Java implement interface instead Multiple Inheritance Example Combination AlarmClockRadio has two parent classes State behavior from both Radio AlarmClock Superclasses
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