Classes and Object-Oriented Programming in C#

1. Classes and Object-Oriented Programming in C# Computers and Programming (01204111)

2. Outline • Array revisited • Data encapsulation in C# • Class and object creation • Array of objects • Member methods • Constructors

3. Array a a0=7 : a2=10 7 3 10 5 8 17 a5=17 0 1 2 3 4 5 a1=3 Arrays Revisited • Group multiple items of the same type into one "variable" or "object" • Make programming easier to manage • What if we want to keep a few things that are of different types together?

4. Example • Imagine that you have to write a program • To store 100 students' names • That's simple; just use an array • ...and their scores • Also simple; create another array • ...and also their ID, department, faculty, advisor, etc using System; class Scoring { public static void Main() { string [] name = new string[100]; double [] score = new double[100]; : } }

5. More Example • From the previous slide: • We want to store students' ID, name, score, department, faculty, advisor, etc • We could write a program like this: using System; class Scoring { public static void Main() { string [] name = new string[100]; int [] ID = new int[100]; double [] score = new double[100]; string [] dept = new string[100]; string [] faculty = new string[100]; string [] advisor = new string[100]; : } } What a mess...

6. Object studentInfo ID: 48500000 Name: Paula Dept: ME Advisor: Arthur Data Encapsulation • A mechanism that bundles multiple items of varying types into one item or "object" Advisor="Arthur" Dept="ME" ID=48500000 Name="Paula"

7. Encapsulation in C# • C# provides two kinds of data encapsulation: struct and class • This course will focus on classes only • Objects created from a class can store a fixed number of items • may be of different types • A class is defined by programmer

8. Using Class 1.Define a class 2.Create an object from the class 3.Access data in the object

9. Defining Class Must use "class" keyword Every class needs a name class StudentInfo { public int id; public string name; public string dept; } Members(or properties) of objects to be created Protection level – always use "public" for now

10. Defining Class (cont'd) • Where do we put the class definition? • Inside or outside a class • Outside a method • E.g., using System; class Test { class StdInfo { public int id; public string name; public string dept; } public static void Main() { : } } using System; class StdInfo { public int id; public string name; public string dept; } class Test { public static void Main() { : } } or

11. Creating Object from Class • Syntax: or • Example: class-nameobj-name; obj-name= newclass-name(); class-nameobj-name= newclass-name(); using System; class Test { class StdInfo { public int id; public string name; public string dept; } public static void Main() { StdInfo student = new StdInfo(); : } }

12. ?? Object studentInfo ID: ? Name: ? Dept: ? Advisor: ? Computer Memory Object Creation Process StdInfo student; student = newStdInfo(); just a reference, not an actual object student

13. Accessing Object's Members • Syntax: • Example: obj-name.member-name using System; class Test { class StdInfo { public int id; public string name; public string dept; } public static void Main() { StdInfo student = new StdInfo(); student.id = 49041234; student.name = "Paula"; student.dept = "ME"; Console.WriteLine("ID: {0}", student.id); } }

14. 7 3 10 5 8 17 5 8 17 7 3 10 0 1 2 3 4 5 6 7 8 9 10 11 ID: 49051234 obj-var Name: Paula Dept: ME ID: 49051234 ID: 49052345 ID: 49053456 Name: Paula Name: Lisa Name: Uma Dept: ME Dept: EE Dept: CPE 0 1 2 Array of Objects • Array of integers • Object of type StdInfo (with reference variable) • Array of (references to) objects of type StdInfo

15. Creating Array of Objects • Syntax: • Example: class-name[] array-name = new class-name[size]; using System; class Test { class StdInfo { public int id; public string name; public string dept; } public static void Main() { StdInfo [] students = new StdInfo[50]; for (int i = 0; i < 50; i++) students[i] = new StdInfo(); : } } Create an array for storing 50 references to StdInfo objects Create an actual object StdInfo for each reference in the array

16. Accessing Objects in Array • Syntax: • Example: • Set Student#2's name to "Ariya" • Display Student#3's department array-name[index].member students[2].name = "Ariya"; Console.WriteLine("Department: {0}", students[3].dept);

17. 0 ID: 49051234 1 Name: Paula Dept: ENVE 2 ID: 49052345 3 Name: Lisa Dept: ME ID: 49053456 Name: Uma Dept: CPE ID: 49054567 Name: Masha Dept: EE Accessing Details class StdInfo { public int ID; public string Name; public string Dept; } static void Main() { StdInfo[] students; students = new StdInfo[4]; : : students[2].Name = "Ariya"; } Ariya

18. Example: Student Records • Get N students' information with 3 fields • ID, Name, Score • Then output a table of information • First, we define a class as shown: class StdInfo { public int id; public string name; public int score; }

19. ReadInfo Method • Reads all information for each student • Returns an object of class StdInfo static StdInfo ReadInfo() { StdInfo info = new StdInfo(); Console.Write("ID: "); info.id = int.Parse(Console.ReadLine()); Console.Write("Name: "); info.name = Console.ReadLine(); Console.Write("Score: "); info.score = int.Parse(Console.ReadLine()); return info; }

20. ShowInfo Method • Takes a StdInfo and displays the information on screen • Returns nothing static void ShowInfo(StdInfo info) { Console.WriteLine("{0,3} {1,-10} {2,2}", info.id, info.name, info.score); }

21. Put Them All Together using System; class StdRecords { // Define class StdInfo here // Define ReadInfo() here // Define ShowInfo() here static void Main() { Console.Write("How many students? "); int n = int.Parse(Console.ReadLine()); StdInfo[] students = new StdInfo[n]; for (int i = 0; i < n; i++) students[i] = ReadInfo(); for (int i = 0; i < n; i++) ShowInfo(students[i]); } }

22. Object-Oriented Programming • Classes are more than just a mechanism to bundle data into objects • Objects may have its own behaviors (defined by classes) to perform on its properties • E.g., they know how to display their data on screen, or compute their properties • E.g., every circle object knows how to calculate its area • These are the concepts of Object-Oriented Programming (OOP)

23. Object-Oriented View of Classes • A class serves like a template to create objects of the same type • A class defines a list of properties its objects must have, but does not specify the values of these properties circle1 color = Yellow radius = 1 create circle2 color = Red radius = 1.5 create objectsof classCircle Class Circle Properties: radius, color create circle3 color = Blue radius = 2

24. OOP and Graphical User Interface • GUI components we have seen are objects of some classes • E.g., buttons are objects of class Button inside System.Windows.Forms namespace button1.Left = 60 button1.Top = 31 button1.Height = 56 button1.Width = 115 button1.Text = "OK" button2.Left = 144 button2.Top = 127 button2.Height = 75 button2.Width = 23 button2.Text = "Cancel" button2.Color = Color.Red

25. Member Methods • Class may contain methods • Allow objects to perform computation on its own data • Known as member methods • Each member method can access other members inside the same object

26. Example: Member Methods • Consider the following Person class • We can add a GetAge method to the class to calculate a person’s age class Person { public string name; public int birth_year; } In real program, 2010 should not be hard-coded like this! class Person { public string name; public int birth_year; public int GetAge() { return 2010 – birth_year; } }

27. Thinking Corner • Add two methods, Circumference and Area, into the Circle class below • So that each Circle object knows how to compute its own circumference length and area class Circle { public double radius; public double Circumference() { : } public double Area() { : } }

28. Constructors • A constructor is a special member method defined in a class • It allows us to specify how each object of this class gets constructed • It’s a method with the same name as the class and without return type (not even void) • E.g., class Person { public string name; public int birth_year; public Person() { birth_year = 1975; } } Person p = new Person(); Console.WriteLine(p.birth_year);

29. Constructors with Parameters • A constructor may also be defined to accept parameters • E.g., class Person { public string name; public int birth_year; public Person(string s) { name = s; birth_year = 1975; } } The new operation passes the parameter to the newly created object Person p = new Person("John"); Console.WriteLine(p.name); Console.WriteLine(p.birth_year);

30. Referencing Members • In the previous example, the parameter name s in the constructor is not so meaningful, so we change it to name class Person { public string name; public int birth_year; public Person(string name) { name = name; birth_year = 1975; } } Does nothing because both 'name's refer to the parameter of the constructor

31. Referencing Members: this Variable • To make a reference to the current object, the special keyword this can be used class Person { public string name; public int birth_year; public Person(string name) { this.name = name; this.birth_year = 1975; } }

32. Thinking Corner • Add a constructor to the Circle class so that its objects can be created with provided 'radius' parameter class Circle { public double radius; } static void Main() { Circle c1 = new Circle(30); Circle c2 = new Circle(2.5); }

33. a = (ax,ay) v = (vx,vy) v = (vx,vy) Example: Balls in 2D Space (1) • Let us write a program (OOP style) to simulate ball movement in 2D space • Each ball knows its own current acceleration and velocity (on both x- and y-axes) • Each ball knows its current position (x,y) • Each ball knows how to update its position and velocity after time t (seconds) has passed • Assuming constant acceleration (x,y) 3 seconds passed (x,y) + (0,0) +

34. Example: Balls in 2D Space (2) • Let us define the class Ball so that each Ball object has the following properties: • double sx,sy – current position on the x- and y- axes (in meters) • double vx,vy – current velocity on the x- and y- axes (in m/s) • double ax,ay – current acceleration on the x- and y- axes (in m/s2) class Ball { public double sx, sy; public double vx, vy; public double ax, ay; }

35. Example: Balls in 2D Space (3) • Add a method Update to update the position and velocity of the ball • High school physics applies here class Ball { : public void Update(double t) // t = time elapsed { sx = sx + 0.5*ax*t*t + vx*t; sy = sy + 0.5*ay*t*t + vy*t; vx = vx + ax*t; vy = vy + ay*t; } }

36. Example: Balls in 2D Space (4) • Finally, add a constructor to allow convenient creation of Ball objects class Ball { : public Ball(double sx, double sy, double vx, double vy, double ax, double ay) { this.sx = sx; this.sy = sy; this.vx = vx; this.vy = vy; this.ax = ax; this.ay = ay; } }

37. Example: Balls in 2D Space (5) • Test the program • Simulate two Ball objects • Ball b1 moves at constant velocity (ax = ay = 0) • Ball b2 moves under Earth's gravity (ax = 0, ay = 9.8) static void Main() { Ball b1 = new Ball(0,0,10,20,0,0); Ball b2 = new Ball(0,100,0,0,0,-9.8); b1.Update(10); b2.Update(10); Console.WriteLine("After 10 seconds…"); Console.Write("b1 is at position ({0},{1})", b1.sx, b1.sy); Console.WriteLine(" with velocity [{0} {1}]", b1.vx, b1.vy); Console.Write("b1 is at position ({0},{1})", b2.sx, b2.sy); Console.WriteLine(" with velocity [{0} {1}]", b2.vx, b2.vy); }

38. a = (0,-9.8) v = (50,50) Example: Projectile Motion (1) • Simulate projectile motion on earth • Cannon ball exits the cannon at position (0,0) • Ask user for initial velocity • Report the position of the cannon ball every second

39. Format the value to have 2 decimal places and width of 8 characters Example: Projectile Motion (2) static void Main() { Console.Write("Enter initial vx: "); double vx = double.Parse(Console.ReadLine()); Console.Write("Enter initial vy: "); double vy = double.Parse(Console.ReadLine()); Ball b = new Ball(0, 0, vx, vy, 0, -9.8); Console.WriteLine("Time sx sy vx vy"); Console.WriteLine("----------------------------------"); for (int i = 0; i <= 10; i++) // simulate for 10 seconds { Console.WriteLine("{0,2}{1,8:f2}{2,8:f2}{3,8:f2}{4,8:f2}", i, b.sx, b.sy, b.vx, b.vy); b.Update(1); } Console.ReadLine(); } Enter initial vx: 50 Enter initial vy: 50 Time sx sy vx vy ---------------------------------- 0 0.00 0.00 50.00 50.00 1 50.00 45.10 50.00 40.20 2 100.00 80.40 50.00 30.40 3 150.00 105.90 50.00 20.60 4 200.00 121.60 50.00 10.80 5 250.00 127.50 50.00 1.00 6 300.00 123.60 50.00 -8.80 7 350.00 109.90 50.00 -18.60 8 400.00 86.40 50.00 -28.40 9 450.00 53.10 50.00 -38.20 10 500.00 10.00 50.00 -48.00

40. Thinking Corner • Modify the program in the previous example to ask user for starting speed and angle of the cannon ball, instead of vx,vy s 

41. Challenging Corner • Write a GUI application that creates several Ball objects, then simulates their movements and draw them on a window • Use a Timer to update the time and draw the balls at new locations • Make balls bounce when they hit walls

42. Conclusion • Multiple related data items can be bundled into an object by defining a class for it • Object-Oriented Programming (OOP) allows programmers to view data as objects that have their own behaviors