Lecture 8: Object-Oriented Design

Lecture 8:

Object-Oriented Design

8-2MicrosoftIntroducing CS using .NETJ# in Visual Studio .NET

Objectives

“Good object-oriented programming is really about good object-oriented design. And good OOD is all about making things easier for ourselves, as well as for our class users…”

• Class design• Inheritance• Interfaces


Part 1

• Class design…


Class design rules #1 – #4

• These rules you probably already know:

1. "Classes should provide constructors" – to ensure initialization of fields

2. "Classes should provide methods to manipulate class fields"– e.g. bank Customer's getFormattedBalance( ) or makeDeposit(amt)

3. "Classes should practice data-hiding, and control access otherwise"– use of private keyword, get & set methods for field access

4. "Classes should override toString( )"– to return appropriate string-based representation


Class design rule #5

• "Classes should override Equals( )"– enables object equality to be determined based on fields

• Example:– students are equal if their IDs are the same

public class Student{ private int id; . . . public boolean equals(Object obj) { // make sure we are comparing apples to apples… if (obj == null || !(obj.getClass().equals(this.getClass()))) return false;

// we know obj is a student but J# doesn't, so type-cast to Student so we can compare IDs… Student other = (Student) obj; return this.id == other.id; }

public class Student{ private int id; . . . public boolean equals(Object obj) { // make sure we are comparing apples to apples… if (obj == null || !(obj.getClass().equals(this.getClass()))) return false;


if (s1.equals(s2)) ...if (s1.equals(s2)) ...


Class design rule #6

• "Classes that override Equals( ) should also override hashCode( )"– Java rule: equal objects must have equal hash codes

– Solution? Base hashCode( ) on same field(s) as equals( )

• Example:– student's hash code is their ID!

public class Student{ private int id; . . .

public int hashCode() { return this.id; // equal students will have same hash code! }

public class Student{ private int id; . . .

public int hashCode() { return this.id; // equal students will have same hash code! }


Part 2

• Inheritance…


Inheritance

• One class may extend another class, inheriting all fields & methods:

• Why do this?

– Code reuse! Class B doesn't have to reinvent class A…

– Design reuse! Class B "is-a" class A, and treated as such…

public class B extends A{ . . .}

public class B extends A{ . . .}


Example

Student

firstName : StringlastName : Stringid : intgpa : double

Student(fn, ln, id, gpa)get_firstName( ) : String . . .

Freshman

Sophomore

Junior

Senior

super / base / parent

sub / derived / child

• Student class hierarchy:

extends


Superclass design

• Superclass can define members as private, protected, or public • Superclass can define common functionality for its subclasses

• Superclass can define abstract methods for subclasses to implement

public abstract class Student{ protected String firstName, lastName; protected int id; protected double gpa; . . .

public String getFormattedGPA() { java.text.DecimalFormat formatter; formatter = new java.text.DecimalFormat("0.00"); return formatter.format(this.gpa); }

public abstract boolean meetsMinGPA();}

public abstract class Student{ protected String firstName, lastName; protected int id; protected double gpa; . . .

public String getFormattedGPA() { java.text.DecimalFormat formatter; formatter = new java.text.DecimalFormat("0.00"); return formatter.format(this.gpa); }

public abstract boolean meetsMinGPA();}


Subclass design

• Subclasses are responsible for:– calling their superclass's constructor– implementing any abstract methods

public class Freshman extends Student{ public Freshman(String fn, String ln, int id, double gpa) { super(fn, ln, id, gpa); }

public boolean meetsMinGPA() { // freshman must maintain a GPA of 1.5 or greater... return this.gpa >= 1.5; }}

public class Freshman extends Student{ public Freshman(String fn, String ln, int id, double gpa) { super(fn, ln, id, gpa); }

public boolean meetsMinGPA() { // freshman must maintain a GPA of 1.5 or greater... return this.gpa >= 1.5; }}


Polymorphism

• Poly-whatism?• Polymorphism is about writing code in a general way

– the more general the code, the more reusable in new situations

• More formally:– Definition: polymorphism is when different types support the

same operation, allowing the types to be treated similarly.– this allows the same code to work on a wider range of types

• Example?– Student objects and meetsMinGPA( )

– next page…


Example: meetsMinGPA( )

• meetsMinGPA() is an example of a polymorphic method– defined in super class so all students support it– implemented in subclasses so each implementation may differ

• Advantage?– the following code correctly checks *all* types of students to

ensure they are meeting minimum GPA requirements!

java.util.ArrayList students = new java.util.ArrayList();students.add( new Freshman(…) ); // a Freshman is-a Student…students.add( new Junior(…) ); // a Junior is-a Student… . . .

for (int i = 0; i < students.size(); i++){ Student s = (Student) students.get(i); if (!s.meetsMinGPA()) System.out.println("Min GPA violation: " + s);}//for

java.util.ArrayList students = new java.util.ArrayList();students.add( new Freshman(…) ); // a Freshman is-a Student…students.add( new Junior(…) ); // a Junior is-a Student… . . .

for (int i = 0; i < students.size(); i++){ Student s = (Student) students.get(i); if (!s.meetsMinGPA()) System.out.println("Min GPA violation: " + s);}//for


.

.

.

for (int i = 0; i < students.size(); i++){ Student s = (Student) students.get(i); if (!s.meetsMinGPA())

.

.

.

for (int i = 0; i < students.size(); i++){ Student s = (Student) students.get(i); if (!s.meetsMinGPA())

Dynamic binding

• Dynamic binding is the technology that enables polymorphism• When you make a method call:

1. Java dynamically determines the type of the object, then

2. Java binds to ("calls") the correct method based on object's type

(1) What type of object: Freshman, Sophomore, Junior, or Senior?

(2) Based on answer to (1), dynamically bind to (i.e. "call") the correct implementation of the meetsMinGPA( ) method in Freshman, Sophomore, Junior, or Senior.


Overriding equals, part 2

• Problem:– suppose we override equals( ) as discussed earlier, which

requires that objects be exactly the same type to be equal– in this case, how do we search for students?

java.util.ArrayList students = new java.util.ArrayList(); . . .

int loc, id;Student s;id = ...; // id of student we are looking for…s = new ????????? ("", "", id, 0.0); // create temp object for search

loc = students.indexOf(s); // linear search for actual student…


int loc, id;Student s;id = ...; // id of student we are looking for…s = new ????????? ("", "", id, 0.0); // create temp object for search

loc = students.indexOf(s); // linear search for actual student…

What class do we use?


Overriding equals again…

• Instead of requiring exactly the same types, check to make sure both objects inherit from Student…

– we know "this" object does, just a matter of checking "obj"…

public abstract class Student{ private int id; . . . public boolean equals(Object obj) { // make sure we are comparing apples to apples… if (obj == null || !(obj.getClass().equals(this.getClass()))) return false;


public abstract class Student{ private int id; . . . public boolean equals(Object obj) { // make sure we are comparing apples to apples… if (obj == null || !(obj.getClass().equals(this.getClass()))) return false;


!(obj instanceof Student))


Linear search again…

• Now you can search by creating any kind of object you want:– except for Student, which won't work since it's abstract


int loc, id;Student s;id = ...; // id of student we are looking for…s = new Senior("", "", id, 0.0); // create temp object for search

loc = students.indexOf(s); // linear search for actual student…if (loc >= 0) System.out.println("Found at location " + loc);



loc = students.indexOf(s); // linear search for actual student…if (loc >= 0) System.out.println("Found at location " + loc);


Part 3

• Interfaces & design by contract…


Motivation

• We program by relying on "contracts"– i.e. agreements between objects– "Object X agrees to do Y, so I'll call X when I need to do Y"

java.io.FileReader file = new java.io.FileReader("…");java.io.BufferedReader reader = new java.io.BufferedReader(file);

s = reader.readLine();

java.io.FileReader file = new java.io.FileReader("…");java.io.BufferedReader reader = new java.io.BufferedReader(file);

s = reader.readLine();

public class BufferedReader{ . . .

public String readLine() { . . . }

public class BufferedReader{ . . .

public String readLine() { . . . }


Design by contract

• Design by contract formalizes this process– design first– implement second

• Advantages?

– encourages "think before you code"

– ensures integration of classes in a large program

– different classes supporting the same contract offer another means of polymorphic programming!


Interfaces

• Interfaces are a formal way of specifying contracts• An interface = a set of method signatures

– representing the contract between two or more parties– pure design, no data / code

public interface SomeInterfaceName{ public void Method1(); public int Method2(int x, String y); . . .}

public interface SomeInterfaceName{ public void Method1(); public int Method2(int x, String y); . . .}


Conceptual view

• Here's how to visualize what's going on:

"server"(object performing

service according to interface contract)

"client"(object requesting

service)

Interface

– "client" uses interface to communicate with server– "server" implements interface to fulfill contract


Example

• Sorting and binary search• These algorithms are a part of the Java Class Library

– but use an interface to communicate with objects…

sort( ) objectobjectobject

binarySearch( ) objectobjectobject


(1) Comparable interface

• Defines the comparison of two objects, returning an integer:– < 0 means this object < obj– = 0 means this object = obj– > 0 means this object > obj

public interface Comparable{ public int compareTo(Object obj);}

public interface Comparable{ public int compareTo(Object obj);}

if (s1.compareTo(s2) < 0) ...if (s1.compareTo(s2) < 0) ...


(2) Objects implement Comparable

• To fulfill contract, objects must implement Comparable:

public abstract class Student implements Comparable{ private int id; . . .

public int compareTo(Object obj) { // make sure we are comparing apples to apples… Student other = (Student) obj; if (this.id < other.id) return -1; else if (this.id == other.id) return 0; else return 1; }

public abstract class Student implements Comparable{ private int id; . . .

public int compareTo(Object obj) { // make sure we are comparing apples to apples… Student other = (Student) obj; if (this.id < other.id) return -1; else if (this.id == other.id) return 0; else return 1; }

studentstudentstudent


(3) Call sort() and binarySearch()

• Now we can take advantage of these built-in algorithms:

java.util.ArrayList students = new java.util.ArrayList();students.add( new Freshman(…) );students.add( new Junior(…) ); . . .

java.util.Collections.sort(students);


loc = java.util.Collections.binarySearch(students, s);if (loc >= 0) System.out.println("Found at location " + loc);

java.util.ArrayList students = new java.util.ArrayList();students.add( new Freshman(…) );students.add( new Junior(…) ); . . .

java.util.Collections.sort(students);


loc = java.util.Collections.binarySearch(students, s);if (loc >= 0) System.out.println("Found at location " + loc);


Summary

• Good OOP is all about good Object-Oriented Design• Java, J# and .NET all employ a great deal of OOD

– effective use requires a solid understanding of OOD

– e.g. linear search is built-in, but you need to implement equals()

– e.g. sorting & binary search are built-in, but you need to implement Comparable and compareTo()

• Inheritance & interfaces are important OOD tools

– look for ways to create / exploit polymorphism

Lecture 8: Object-Oriented Design

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