Object-Oriented Design. A Historical Perspective Programs Procedures to manipulate data Earlier: procedural Think actions/procedures first, data second

Object-Oriented Design

A Historical Perspective

Programs Procedures to manipulate data

Earlier: procedural Think actions/procedures first, data second

1970’s: “object-oriented” Think data/objects first, procedures/actions second

Different perspectives on how to organize problem solving and hence how to write programs

Object-oriented (OO) Design

1. What are the different kinds of objects/data? Design decision on how data are organized OO terminology: class, object Example:

Class Person: object mary, object john Class Building: object empireStateBuilding, object olinEngineering

Object-oriented (OO) Design

1. What are the different kinds of objects/data? Design decision on how data are organized OO terminology: class, object Example:

Class Person: object mary, object john Class Building: object empireStateBuilding, object olinEngineering

2. How can the objects/data be manipulated? Design decision on how data can be manipulated OO terminology: method (action) Example:

Class Person: method getAddress, method changeAddress Class Building: method getAddress empireStateBuilding.getAddress() john.changeAddress(newAddress)

OO vs Procedural Design

Depending on the problems OO is more appropriate

Many different types of data Many interactions among different types of data

Procedural is more appropriate Few types of data Few interactions among different types of data

OO and Java

Defining a class In one .java file Class name is the same as the file name

E.g. class Person is defined in Person.java Each class has attributes (instance variables)

E.g. class Person has attributes: name, address, phone

OO and Java

Defining a class In one .java file Class name is the same as the file name

E.g. class Person is defined in Person.java Each class has attributes (instance variables)

E.g. class Person has attributes: name, address, phone

Defining methods Methods are defined within a class

I.e. can’t define a method without a class Name, parameters, return type

class Person

public class Person{ private String _name; // attributes (instance variables) private String _address; private int _phone;

}

class Person


public Person(String name, String address, int phone)//constructor { _name = name; _address = address; _phone = phone; }

}

class Person


public Person(String name, String address, int phone)//constructor { _name = name; _address = address; _phone = phone; } public String getName() // method { return _name; }}

Class PersonTest

import java.util.*;class PersonTest{ public static void main(String[] args) {

// object jane created from class PersonPerson jane = new Person(”Jane Doe”, ”Melbourne”, 5551234 );

// get name from object jane and print itSystem.out.println(jane.getName());

}}

Exercise

Create a Person class Attributes (instance variables)

name, height (inches), salary (annual salary) Methods

“constructor”, getName, getSalary, getHeightInInches // return inches in int getHeightInFeet // return feet in double getHeightInMeters // return meters in double getHeightInFeetAndInches // return an int array with feet and inches

Create a PersonTest class Jane Doe, 61 inches, $55000 Mike Smith, 71 inches, $44000 Print Jane’s and Mike’s hourly rate--52 weeks, 40 hours/week Print Jane’s height in feet, meters, and feet and inches. Same

for Mike

Abstraction (Information Hiding)

The Person class Hides the internal attributes

private Provides external access via its methods

public

Hiding Some Info about Height

Look closer at height What information are we hiding?

Hiding some info about Height


how height is stored – different methods to get height



how height is stored – different methods to get height What can be changed in Person without changing

PersonTest? Imagine:

Person is written by a team in Florida PersonTest is written by a team in California Changes by the Florida team don’t affect the team in

California



how height is stored – different methods to get height What can be changed in Person without changing

PersonTest? Imagine:

Person is written by a team in Florida PersonTest is written by a team in California Changes by the Florida team don’t affect the team in

California Attribute _height could be in feet and inches instead

of inches Changes in Person [by the Florida team] But No changes in PersonTest! [by the California team]

Hiding Height Better

The constructor seems to imply the height must be stored in inches

How can we improve this?

Hiding Height Better

The constructor seems to imply the height must be stored in inches

How can we improve this?

Multiple constructors

class Person

public class Person{ private String _name; // attributes (instance variables) private int _height;// inches private int _salary;// annual salary

public Person(String name, int heightInches, int salary)//constructor { } public Person(String name, int heightFeet, int heightInches, int salary) {

}}


Which constructor is more natural for external use?1. Person(String name, int heightInches, int salary)2. Person(String name, int heightFeet, int heightInches, int

salary)



salary)

Which internal representation of attribute height uses less memory? int height // in inches int heightFeet, heightInches



salary)

Which internal representation of attribute height uses less memory? int height // in inches int heightFeet, heightInches

Abstraction (information hiding) allows: a more natural interface for external use and a more memory-efficient internal representation

Wait a minute

Two constructors with the same name Can we do that?

Wait a minute

Two constructors with the same name Can we do that?

Yes, if the parameter types are different “overloading” same name, different behavior Applies to methods as well

Overloading—same name,different sequence of parameter types

Given Person(String name, int heightInches, int salary) Person(String name, int heightFeet, int heightInches, int

salary)

Can we also have1. Person(String name, int heightInches, int heightFeet, int

salary)2. Person(String name, int salary, int heightFeet, int

heightInches)3. Person(int heightFeet, int heightInches, String name, int

salary)

Exercise

class Person Add a constructor that allows feet and inches

class PersonTest Create a third Person object

Michael Jordan, 6 feet, 6 inches, $1,000,000 Print his hourly rate, height in feet, meters, and feet

and inches

“this” object

public int getHeightInInches(){ return _height; // return this._height;}

------PersonTest---jane.getHeightInInches();

Parameter is an object of the same class

Consider a method that compares two Person objects Whether one is taller than another

In procedural thinking, we would want isTaller(jane, mike)

In OO thinking, we think object first jane.isTaller(mike) // yep, it looks ugly, but closer to

English

“this” object

public boolean isTaller(Person other){ // _height refers to jane’s height // other._height refers to mike’s height

return _height > other._height; // return this._height > other._height;}

-------PersonTest---------

jane.isTaller(mike)

A Second Class

Non-simple problems usually involve objects from different classes.

Let’s say we want to see how many minutes Michael Jordan need to work to buy an iPhone.

What would be another class in this problem? What are the attributes? What are the methods?

Exercise

class Phone Attributes: model, price constructor, getPrice

class PersonTest Create a Phone object: “iPhone 5c 16 GB unlocked”,

$549 Create another Phone object: “iPhone 5s 16GB

unlocked”, $649 Print how many minutes Michael Jordan needs to

work to buy each of the iPhones

A Note on Checking Invalid Input

You probably have learned about checking for invalid input

Same with methods, particularly with constructor Invalid values for attributes cause problems later

Checking Invalid Parameter Values

public Person(String name, int height, int salary){ // if name is null, attribute _name should be what?

// if height is negative, attribute _height should be what?

// if salary is negative, attribute _salary should be what?

}

Checking Invalid Parameter Values

public Person(String name, int height, int salary){ if (name != null) _name = name; else _name = ”Unknown”;

if (height > 0) _height = height; else _height = 0; if (_salary > 0) _salary = salary; else _salary = 0;}

Remember we have another constructor

Person(String name, int heightFeet, int heightInches, int salary)

We want to check for invalid parameter values too Repeating instructions? How to not repeat instructions?

Remember we have another constructor

Person(String name, int heightFeet, int heightInches, int salary)

We want to check for invalid parameter values Repeating instructions! How to not repeat instructions?

Abstract the checking into a method

“set” method with checking

public Person(String name, int height, int salary){ set(name, height, salary);}

public Person(String name, int heightFeet, int heightInches, int salary){ set(name, heightFeet * 12 + heightInches, salary);}

private void set(String name, int height, int salary) // public if you allow changes externally{ if (name != null) _name = name; else _name = ”Unknown”;

if (height > 0) _height = height; else _height = 0; if (_salary > 0) _salary = salary; else _salary = 0;}

Documents

Object-Oriented Design. A Historical Perspective Programs Procedures to manipulate data Earlier: procedural Think actions/procedures first, data second