object oriented programming (oop): just an extension to...

Overview Distance Calculator OOP Access modifiers Lucky Sevens OOP Grid System OOP

Object oriented programming (OOP):Just an extension to modularity

Prof. Dionne Aleman

MIE250: Fundamentals of object-oriented programmingUniversity of Toronto

MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 1 / 44


Why objects?1

I Reduce complexity

I Facilitate understanding and modification

I Help testing and debugging

I Promote reusability

1Sound familiar?MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 2 / 44


What is an object?

I Objects sound mysterious and complex, but are in fact very simple.

I Objects are essentially glorified variables.

I They are just wrappers around other bits of information (variablescalled member fields).

I They can also have internal functions, called methods, to interactwith member fields and do whatever we want.



Can you think of any examples we have seen so far?

I String objectFields Probably an array of chars, maybe an int for length

Methods .length(), .contains(), .equals(String s2), etc.

I BufferedReader objectFields No idea

Methods .readLine(), probably others that we haven’t learned

I Math objectFields E, PI, maybe more

Methods .sqrt(double x), .sin(double x), .Random(), etc.



What’s the common theme?

I We don’t really know what is happening inside those objects.

I Do we care?I Nope.

I Is it easier for us that way?I You betcha.





I Do we care?

I Nope.







I Is it easier for us that way?

I You betcha.



Anatomy of an object: “Encapsulation”



Java program structureThe .java file

Header files

The class

Global variables (NEW)

Function

Function

One function is the main function



Java class structureThe .java file

Header files

The class

Member variables (fields)

Function

Function

No main function!



“Class” v. “Object”

Class The blueprint from which individual objects are made

Object An instance of a class

For example:1 String s;

Variable s is an instance (object) of type (class) String.



The Distance Calculator programGet a time and speed from the user and print out the distance traveled.

Get speedinput

from user

Get timeinput

from userCalculatedistance

Printdistanceto screen

Quit




Get speedinput

from user

Get timeinput



Quit

Get speedinput

from userValid

number?

no

yes Get timeinput

from userValid

number?

no

yes Calculatedistance


Quit




Get speedinput

from user

Get timeinput



Quit

Get speedinput

from userValid

number?

no

yes Get timeinput

from userValid

number?

no



Quit

Get user input




Get speedinput

from user

Get timeinput



Quit

Get speedinput

from userValid

number?

no

yes Get timeinput

from userValid

number?

no



Quit

Get user input

Do thingswith it



Remember this code?

Get userinput

Do thingswith it Quit

1 public class DistanceV5 {2

3 public static void main(String [] args) {4

5 DistanceCalculator dc = new DistanceCalculator ();6 dc.getUserInput ();7 dc.printDistance ();8

9 } // end of main function10

11 } // end of class

src/DistanceCalculatorOOP/DistanceV5.java



Remember this code?

Get userinput








Declare DistanceCalculatorobject dc



Remember this code?

Get userinput








Use dc toget user input



Remember this code?

Get userinput








Use dc todo things with it



See it in action

I Let’s re-build the Distance Calculator game as an OOP.

I We will go step-by-step:1. Start with a simple object.2. Slowly add in useful functionality.



The steps to build an object oriented program

1. How do we want to the non-object part of the program (e.g., themain function) to look?

2. What classes do we need?

3. What data should go in them?

4. What methods to access and manipulate the data do we need?

5. Write the code.



Step 1: We already know what the main function shouldlook like

1 public class DistanceV5 {23 public static void main(String [] args) {45 DistanceCalculator dc = new DistanceCalculator ();6 dc.getUserInput ();7 dc.printDistance ();89 } // end of main function





Step 2: What classes do we need?

I A class to hold all speed and time data and do the calculations.

I Let’s call the class DistanceCalculator.



Step 3: What data should go in it?

I Speed

I Time

I Anything else?



Step 3: The class with data

I Note the file name must be the class name, just like in a regularprogram.

I All the data are basically like global variables to the class.

I What is private?

1 public class DistanceCalculator {23 // member fields (variables): everything we need to know4 private double speed; // speed from user in mph5 private double time; // time from user in min67 }



Step 4: How do we want to access/manipulate the data?

Action Method nameGet user input getUserInput()Print distance printDistance()



this is the best practice

Use this to preface member fields and other member methods inmethods for clarity and specificity:

1 public class DistanceCalculator {23 private double speed; // speed from user in mph4 private double time; // time from user in min56 public void setValues(double speed , double time) {7 this.speed = speed; // No conflict thanks to "this"!8 this.time = time;9 System.out.println("New distance calculation values:");

10 this.printValues ();11 }1213 }



Step 5: Write the code

To do together:1. Write the DistanceCalculator class.2. Write the main function.



One thing missing ...

The code works, but there is one thing missing that is stronglyrecommended for reliable performance of object oriented programs.



ConstructorsA constructor is a method with no return type that is called when theobject is declared. It usually just initializes variable values.

1 public class DistanceCalculator {2 private double speed;3 private double time;45 // a "void" constructor:6 // DistanceCalculator dc = new DistanceCalculator ();7 public DistanceCalculator () {8 this.speed = 0;9 this.time = 0.;

10 }1112 // an alternate constructor:13 // DistanceCalculator dc = new DistanceCalculator (5.2, 20.75);14 public DistanceCalculator(double speed , double time) {15 this.speed = speed;16 this.time = time;17 }18 }



Controlling data

I An important aspect of good OOP is controlling how otherprograms/functions can interact with our object’s data.

I We don’t want another program(mer) messing up the raw datainside a class we’ve created.

I If someone needs access to an object’s data, we should anticipatethat need and write a function that:1. Makes that access easy.2. Prevents someone else screwing up the data in our object.

I If data has to be accessed, it will be done on our terms.



Access modifiers

Access modifiers control the access we have to an object’s data.

Modifier Within class Outside class, insame package

Outside class,outside package

public X X X

protected X X

private X

Unless you have a good reason to do otherwise, member variables shouldbe private.



Putting it together ...

I Where have we seen public before?

I What does that mean?



public static void main

I We understand public now, but what is static?

I static means that particular member function (or variable) is thesame across all objects of that class.

I In other words, we do not need to declare an object of that class inorder to use a static function or variable



static example

Say we want to use DistanceCalculator like this:

1 double dist = DistanceCalculator.calcDist(speed , time);

Do we need to declare an actual DistanceCalculator object in order torun that line of code?



static example

Say we want to use DistanceCalculator like this:

1 double dist = DistanceCalculator.calcDist(speed , time);

Do we need to declare an actual DistanceCalculator object in order torun that line of code?

No:There’s no object in that statement



When to use static

So, we use static when an actual object of a class isn’t needed:

1 public class DistanceCalculator {2 // all the other class members and methods3 ...45 // a static method6 public static double calcDist(int speed , int time) {7 return speed * time;8 }9 }

Also works for members, not just methods



public or not? static or not?

public static

Math.sqrt()

Math.PI

str.equals()




public static

Math.sqrt() X

Math.PI

str.equals()




public static

Math.sqrt() X X

Math.PI

str.equals()




public static

Math.sqrt() X X

Math.PI X

str.equals()




public static

Math.sqrt() X X

Math.PI X X

str.equals()




public static

Math.sqrt() X X

Math.PI X X

str.equals() X




public static

Math.sqrt() X X

Math.PI X X

str.equals() X X



The Lucky Sevens program

I Let’s re-build the Lucky Sevens game as an OOP.

I We will go step-by-step:1. Start with a simple object.2. Slowly add in useful functionality.



The steps to build an object oriented program

1. How do we want to the non-object part of the program (e.g., themain function) to look?

2. What classes do we need?

3. What data should go in them?

4. What methods to access and manipulate the data do we need?

5. Write the code.



Step 1: The program should look something like this

1 public class LuckySevens_OOP {23 public static void main(String [] args) {45 int startingCash = getUserStartingCash ();6 LuckySevens game = new LuckySevens(startingCash);78 game.play();9 game.summarize ();

10 game.checkFairness ();1112 } // end main1314 }



Step 2: What classes do we need?

I A class to hold all of the Lucky Sevens game data and run the game.

I Let’s call the class LuckySevens.



Step 3: What data should go in it?

I Starting cash

I Number of rolls until bankruptcy

I Peak cash

I Roll distribution (to check for fairness)

I Amount won on 7

I Amount lost on non-7



Step 3: The class with data

I Note the file name must be the class name, just like in a regularprogram.

I All the data are basically like global variables to the class.

I What is private?

1 public class LuckySevens {23 // member fields (variables): everything we want to know about a game4 private int winValue; // amount won on roll of 75 private int loseValue; // amount lost on non -7 roll6 private int startingCash; // starting cash value7 private int nRolls; // number of rolls until bankruptcy8 private int maxCash; // peak winnings in the game9 private int [] rollFreq = new int [13]; // frequency of roll values

1011 }



Step 5: Write the code

To do together:1. Write the LuckySevens class.2. Write the main function.



Point class example

I Say we want a program that will store many points in a Cartesiancoordinate system

I We will want to do some calculations on these points, e.g.,determine which is closest to the origin, determine which points areon a particular line, etc.

I Each point consists of an (x , y) coordinate

I We could store an array of x -coordinates and an array ofy -coordinates, but that would be cumbersome



Start simple

I We know our object will need two member variables to store thex -coordinate and y -coordinate

I Let’s start with the following capabilities:I Initialize the pointI Modify the coordinatesI Print out the coordinates



What do we want our main program to look like?

1 public class GridSystem {23 public static void main(String [] args) {4 Point p1 = new Point();5 Point p2 = new Point (4,5);67 System.out.print("Point p1: ");8 p1.print();9

10 System.out.print("Point p2: ");11 p2.print();1213 p1 = p2;14 System.out.print("New point p1: ");15 p1.print();16 }1718 }

src/GridSystemOOP/GridSystem.java



Let’s build the Point class

Again, building it TOGETHER.



Good job! Now let’s make it more interesting

Any ideas?

I Translate point a certain amount

I Calculate distance to originI Function to determine which point is closest to the origin

I Calculate distance between pointsI Function to determine which points are closest togetherI Function to determine which points are furthest apart

I Determine if a point is on a lineI Function to determine how many and which points are on a lineI Function to determine how many and which points are not on a line




Any ideas?


I Calculate distance to origin

I Function to determine which point is closest to the origin






Any ideas?








Any ideas?



I Calculate distance between points

I Function to determine which points are closest togetherI Function to determine which points are furthest apart





Any ideas?



I Calculate distance between pointsI Function to determine which points are closest together

I Function to determine which points are furthest apart





Any ideas?








Any ideas?




I Determine if a point is on a line

I Function to determine how many and which points are on a lineI Function to determine how many and which points are not on a line




Any ideas?




I Determine if a point is on a lineI Function to determine how many and which points are on a line

I Function to determine how many and which points are not on a line




Any ideas?







The new Point class

Build it ...



How about a Line class?

I Member variables:I PointI Slope

I Methods:I Constructor by Point object and slope value, or maybe by two PointsI Find y -interceptI Find rootI Check if a point is on the line




I Member variables:

I PointI Slope






I Methods:

I Constructor by Point object and slope value, or maybe by two PointsI Find y -interceptI Find rootI Check if a point is on the line





I Methods:I Constructor by Point object and slope value, or maybe by two Points

I Find y -interceptI Find rootI Check if a point is on the line





I Methods:I Constructor by Point object and slope value, or maybe by two PointsI Find y -intercept

I Find rootI Check if a point is on the line





I Methods:I Constructor by Point object and slope value, or maybe by two PointsI Find y -interceptI Find root

I Check if a point is on the line



Points and Lines

I Let’s build a program that uses both Point and Line objects.

I We may want to revise some of our Point code to use the Lineclass.


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