Object oriented programming (OOP): Just an extension to modularity. Why objects? 1. What is an object? Prof. Dionne Aleman.

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1 Object oriented programming (OOP): Just an extension to modularity Prof. Dionne Aleman MIE250: Fundamentals of object-oriented programming University of Toronto MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 1 / 1 Why objects? 1 Reduce complexity Facilitate understanding and modification Help testing and debugging Promote reusability 1 Sound familiar? MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 2 / 1 What is an object? Objects sound mysterious and complex, but are in fact very simple. Objects are essentially glorified variables. They are just wrappers around other bits of information (variables called member fields). They can also have internal functions, called methods, to interact with member fields and do whatever we want. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 3 / 1

Can you think of any examples we have seen so far? String object Fields Probably an array of chars, maybe an int for length Methods.length(),.contains(),.equals(String s2), etc.

MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 4 / 1 What s the common theme? We don t really know what is happening inside those objects. Do we care?

2 Can you think of any examples we have seen so far? String object Fields Probably an array of chars, maybe an int for length Methods.length(),.contains(),.equals(String s2), etc. BufferedReader object Fields No idea Methods.readLine(), probably others that we haven t learned Math object Fields E, PI, maybe more Methods.sqrt(double x),.sin(double x),.random(), etc. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 4 / 1 What s the common theme? We don t really know what is happening inside those objects. Do we care? Nope. Is it easier for us that way? You betcha. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 5 / 1 Anatomy of an object: Encapsulation MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 6 / 1

3 Java program structure The.java file Header files The class Global variables (NEW) Function Function MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 7 / 1 Java class structure The.java file Header files The class Member variables (fields) Function (method) Function (method) MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 8 / 1 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. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 9 / 1

4 The Distance Calculator program Get a time and speed from the user and print out the distance traveled. Get speed input from user Get time input from user Calculate distance Print distance to screen Quit Get speed input from user Valid number? yes Get time input from user Valid number? yes Calculate distance no no Quit Print distance to screen MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 10 / 1 Remember this code? Get user input Do things with 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 (); } // end of main function 11 } // end of class src/distancecalculatoroop/distancev5.java MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 11 / 1 See it in action Let s re-build the Distance Calculator game as an OOP. We will go step-by-step: 1. Start with a simple object. 2. Slowly add in useful functionality. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 12 / 1

5 The steps to build an object oriented program 1. How do we want to the non-object part of the program (e.g., the main 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. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 13 / 1 Step 1: We already know what the main function should look like 1 public class DistanceV5 { 2 3 public static void main ( String [] args ) { 4 5 DistanceCalculator dc = new DistanceCalculator (); 6 dc. getuserinput (); 7 dc. printdistance (); } // end of main function 11 } // end of class src/distancecalculatoroop/distancev5.java MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 14 / 1 Step 2: What classes do we need? A class to hold all speed and time data and do the calculations. Let s call the class DistanceCalculator. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 15 / 1

6 Step 3: What data should go in it? Speed Time Anything else? MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 16 / 1 Step 3: The class with data Note the file name must be the class name, just like in a regular program. All the data are basically like global variables to the class. What is private? 1 public class DistanceCalculator { 2 3 // member fields ( variables ): everything we need to know 4 private double speed ; // speed from user in mph 5 private double time ; // time from user in min 6 7 } MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 17 / 1 Step 4: How do we want to access/manipulate the data? Action Get user input Print distance Method name getuserinput() printdistance() MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 18 / 1

7 this is the best practice Use this to preface member fields and other member methods in methods for clarity and specificity: 1 public class DistanceCalculator { 2 3 private double speed ; // speed from user in mph 4 private double time ; // time from user in min 5 6 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 } } MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 19 / 1 Step 5: Write the code To do together: 1. Write the DistanceCalculator class. 2. Write the main function. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 20 / 1 One thing missing... The code works, but there is one thing missing that is strongly recommended for reliable performance of object oriented programs. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 21 / 1

8 Constructors A constructor is a method with no return type that is called when the object is declared. It usually just initializes variable values. 1 public class DistanceCalculator { 2 private double speed ; 3 private double time ; 4 5 // a " void " constructor : 6 // DistanceCalculator dc = new DistanceCalculator (); 7 public DistanceCalculator () { 8 this. speed = 0; 9 this. time = 0.; 10 } // 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 } MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 22 / 1 Controlling data An important aspect of good OOP is controlling how other programs/functions can interact with our object s data. We don t want another program(mer) messing up the raw data inside a class we ve created. If someone needs access to an object s data, we should anticipate that need and write a function that 1. Makes that access easy. 2. Prevents someone else screwing up the data in our object. If data has to be accessed, it will be done on our terms. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 23 / 1 Access modifiers Access modifiers control the access we have to an object s data. Modifier Within class Outside class, in same package Outside class, outside package public protected private Unless you have a good reason to do otherwise, member variables should be private. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 24 / 1

9 Putting it together... Where have we seen public before? What does that mean? MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 25 / 1 public static void main We understand public now, but what is static? static means that particular member function (or variable) is the same across all objects of that class. In other words, we do not need to declare an object of that class in order to use a static function or variable. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 26 / 1 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 to run that line of code? MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 27 / 1

10 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 methods // a static method 6 public static double calcdist ( int speed, int time ) { 7 return speed * time ; 8 } 9 } Also works for members, not just methods MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 28 / 1 public or not? static or not? public static Math.sqrt() Math.PI str.equals() X MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 29 / 1 The Lucky Sevens program Let s re-build the Lucky Sevens game as an OOP. We will go step-by-step: 1. Start with a simple object. 2. Slowly add in useful functionality. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 30 / 1

11 The steps to build an object oriented program 1. How do we want to the non-object part of the program (e.g., the main 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. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 31 / 1 Step 1: The program should look something like this 1 public class LuckySevens_OOP { 2 3 public static void main ( String [] args ) { 4 5 int startingcash = getuserstartingcash (); 6 LuckySevens game = new LuckySevens ( startingcash ); 7 8 game. play (); 9 game. summarize (); 10 game. checkfairness (); } // end main } MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 32 / 1 Step 2: What classes do we need? A class to hold all of the Lucky Sevens game data and run the game. Let s call the class LuckySevens. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 33 / 1

12 Step 3: What data should go in it? Starting cash Number of rolls until bankruptcy Peak cash Roll distribution (to check for fairness) Amount won on 7 Amount lost on non-7 MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 34 / 1 Step 3: The class with data Note the file name must be the class name, just like in a regular program. All the data are basically like global variables to the class. What is private? 1 public class LuckySevens { 2 3 // member fields ( variables ): everything we want to know about a game 4 private int winvalue ; // amount won on roll of 7 5 private int losevalue ; // amount lost on non -7 roll 6 private int startingcash ; // starting cash value 7 private int nrolls ; // number of rolls until bankruptcy 8 private int maxcash ; // peak winnings in the game 9 private int [] rollfreq = new int [13]; // frequency of roll values } MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 35 / 1 Step 5: Write the code To do together: 1. Write the LuckySevens class. 2. Write the main function. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 36 / 1

13 Point class example Say we want a program that will store many points in a Cartesian coordinate system. We will want to do some calculations on these points, e.g., determine which is closest to the origin, determine which points are on a particular line, etc. Each point consists of an (x, y) coordinate. We could store an array of x-coordinates and an array of y-coordinates, but that would be cumbersome. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 37 / 1 Start simple We know our object will need two member variables to store the x-coordinate and y-coordinate. Let s start with the following capabilities: Initialize the point Modify the coordinates Print out the coordinates MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 38 / 1 What do we want our main program to look like? 1 public class GridSystem { 2 3 public static void main ( String [] args ) { 4 Point p1 = new Point (); 5 Point p2 = new Point (4,5); 6 7 System.out. print (" Point p1: "); 8 p1. print (); 9 10 System.out. print (" Point p2: "); 11 p2. print (); p1 = p2; 14 System.out. print ("New point p1: "); 15 p1. print (); 16 } } src/gridsystemoop/gridsystem.java MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 39 / 1

14 Let s build the Point class Again, building it TOGETHER. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 40 / 1 Good job! Now let s make it more interesting Any ideas? Translate point a certain amount Calculate distance to origin Function to determine which point is closest to the origin Calculate distance between points Function to determine which points are closest together Function to determine which points are furthest apart Determine if a point is on a line Function to determine how many and which points are on a line Function to determine how many and which points are not on a line MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 41 / 1 The new Point class Build it... MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 42 / 1

15 How about a Line class? Member variables: Point Slope Methods: Constructor by Point object and slope value, or maybe by two Points Find y-intercept Find root Check if a point is on the line MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 43 / 1 Points and Lines Let s build a program that uses both Point and Line objects. We may want to revise some of our Point code to use the Line class. MIE250: Fundamentals of object-oriented programming (Aleman) Object oriented programming (OOP) 44 / 1

CS 251 Intermediate Programming Methods and Classes

CS 251 Intermediate Programming Methods and Classes Brooke Chenoweth University of New Mexico Fall 2018 Methods An operation that can be performed on an object Has return type and parameters Method with