COMP-202: Foundations of Programming. Lecture 9: Classes and Objects Sandeep Manjanna, Summer 2015

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1 COMP-202: Foundations of Programming Lecture 9: Classes and Objects Sandeep Manjanna, Summer 2015

2 Announcements Assignment 3: Due on 14 th of June at 11:30 pm.

Small Software Glitch Costing Huge The spacecraft encountered Mars at a lower than anticipated altitude and disintegrated due to atmospheric stresses.

3 Small Software Glitch Costing Huge The spacecraft encountered Mars at a lower than anticipated altitude and disintegrated due to atmospheric stresses. Mars Climate Orbiter (1998) Cost of the orbiter $327.6 million!!! $193.1 million for spacecraft development, $91.7 million for launching it, and $42.8 million for mission operations. Mismatch in the units used for trajectory correction. Software that calculated the total impulse produced by thruster firings calculated results in pound-seconds. The trajectory calculation software expected its inputs to be in newton-seconds.

4 This Lecture Classes and Objects Building Our Own Data types One step at a time.

5 Classes and Objects

6 int [] studentid; String [] studentlastname; String [] studentfirstname; String [] major_program; Sring [][] coursesregistered; No Classes The header of one simple method to compute GPA of all the students and print the details would look like : public double computegpa ( int[] sid, String[] slastname, String [] sfirstname, int [] numofcourses, Sring [][] coursesregistered, char [] grades,.) { }

7 Problem!! Each one of these methods needs a lot of information! There are too many variables to keep in our head. It's very easy to mix up which variable does what. Let's say we want to add extra features, for example add a new data indicating the annual grade. It will be very difficult to track all the places where GPA is manipulated and correct it.

8 Data: int studentid; String firstname; String lastname; String major_program; String[] coursestaken; Methods: Student Class double computegpa( ); void addcourse(string course); Here, computegpa( ) doesn t need any arguments as it has access to the members of the Student class.

9 Classes and Objects Class is a data type. Object is an instance of the class. Class : ID: Name: Grade: 4 Objects : ID: Name: Grade: Sarah A- ID: Name: Grade: Bob B- ID: Name: Grade: San F ID: Name: Grade: Nat A

10 What is a Class? Class is a structure that allows us to create complex data types, using primitive types, Encapsulating related data in a single type i.e. pieces of information that belong together A class includes: Data Methods Information relevant to this class Behaviours related to this class

11 Syntax of a Class public class Student { //define class PROPERTIES here public int id; public String first_name; public String last_name; public String major_program; } //define class METHODS here public double computegpa( ) { //Define the method}

12 Syntax of a Class public class Student { //define class PROPERTIES here public int id; public String first_name; public String last_name; public String major_program; public class <classname> Class Properties: public <data_type> variable; } //define class METHODS here public double computegpa( ) { //Define the method} Class Methods: Methods that act on the class properties.

13 Creating an Object Defining a Class is equivalent to defining our own DataType. Creating an object is similar to declaring a variable of the class. Declaring an object of type Student : Student s1 = new Student(); s1 is an instance of the class Student.

14 Constructor Method A special method that is executed when an object of that class is created. Within the Student class definition: public Student() { Properties of a Constructor Same name as the <classname> No return type /* this code is executed when new Student() is called */ }

15 Example Set up a Student class that prints Hello Student. Welcome to COMP202 when a Student instance is created.

16 Constructors with Parameters Sometimes we want to create objects and pass information to it at the time of creation. public class Student { } public String name; public int studentid; public Student(String name, int studentid) { } this.name = name; this.studentid = studentid;

17 Creating a Student Somewhere else in your program: Student s1 = new Student("Hack McMack", ); Student s2 = new Student("Codey Dodey", ); The constructor of Student is called, with two arguments.

18 Zooming into Constructor public Student(String name, int studentid) { this.name = name; this.studentid = studentid; } this.name = name; The name field of this object is set to be the newname String that is passed into the constructor. s1 and s2 each have their own name and studentid fields this refers to the current object being created.

19 Omitting this public Student(String name, int studentid) { this.name = name; this.studentid = studentid; } You can leave out this if there is no naming conflict: public Student(String myname, int mystudentid) { name = myname; studentid = mystudentid; }

20 Other Methods Just the same as Methods before, except: No need for static. We have access to all of the object's properties. In Student.java: public void printname() { } System.out.println(this.name); OR public void printname() { } System.out.println(name);

21 Somewhere Else: Student s1 = new Student("Hack McMack", ); Student s2 = new Student("Codey Dodey", ); s1.printname(); s2.printname(); Basic way to call methods (accessing properties using. operator): object.methodname() Just like we saw for String and Scanner objects What happens if we do this? System.out.println(s1.name);

22 Try it Out!! 1. Write a method that receives as input two objects s1 and s2 of the Student class. returns true if all of the properties of s1 are the same as the properties of s2. returns false otherwise. 2. Write a method that receives a Student object as an input, and prints all of its properties.

23 Objects are Reference Types Student s1 = new Student(); Memory Address Variable Type Variable name int id String first_name String last_name String major_program null

24 Objects are Reference Types Student s1 = new Student(); s1.id = 1; s1.first_name = "Banana"; s1.last_name = "Phone"; s1.major_program = "Math"; Memory Address Variable Type Variable name int Id String first_name String last_name String major_program "Math"

25 Using a class in Arrays We can now use Student as any other type. Declaring an array of elements of type Student. Student[] comp202_students = new Student[200]; Each element in comp202_students points to an instance of the Student class, in the computer's memory. Each position in the array is null by default. We need to initialize each position in the array before using it. Initialize each position in the array so that it points to the data of a new Student for (int i=0; i < comp202_students.length; i++){ comp202_students[i] = new Student(); }

26 Summary Classes group together related data and behaviour they often correspond to types in the real world, like Player and Student. Main components: Data We specify fields in the class definition Methods We define them within the class definition as well A constructor is a special method that is run when an instance of the class is created.

27 Keywords Let's look at these keywords in more detail: static private public

28 static Methods and data members of classes can be associated with the entire class, rather than instances of classes. In this case, they are static: public static void main(string[] args) static int STORE1 = 0; Calling methods/referring to data: double result = Math.pow(2.0, 4.0) ; double area = Math.PI * r * r ;

29 A Comparison Associated with static entire class (one per class) non-static instance of a class (one per object/instance) How to call (methods) from outside of the class ClassName.metho dname() obj.methodname() How to reference (data) from outside of the class ClassName.varNa me obj.varname Can it access the nonstatic fields belonging to this object? No Yes

30 static Public Class Student { public int ID; public String Name; public String Grade; public static int numstudents = 0; } numstudents static variable is shared by all the objects. 4 Objects : ID: Name: Grade: Sarah A- ID: Name: Grade: Bob B- ID: Name: Grade: San F ID: Name: Grade: Nat A

31 What is Externally Visible? We now have multiple files/classes, each with their own data and methods. We must decide: What should be visible OUTSIDE of the class? Should all methods be usable? Should all data be usable?

32 private The method or variable that uses private access-specifier, is only visible within the class. private void methodname() { // some code } private int donttouchme;

33 public The method or variable that uses public access-specifier, is visible everywhere. public static void main() { } public double everyonecanseeme;

34 Why So Shy? The idea behind private is to figure out what someone using your class needs to know about your object. Easier to debug and change; reduces unnecessary complexity Rule of thumb: Make all the fields private Write public methods to control which fields you can access and change

35 Getters A getter is a method the returns the value of a field. public class Student { private int studentid; public int getstudentid() { return this.studentid; } // constructor, other methods omitted }

36 Setters A setter is a method that sets the value of a field. public class Student { private int studentid; public void setstudentid(int newid) { this.studentid = newid; } // other methods omitted }

37 Controlling Access Control access to private fields by deciding which fields have getters and setters associated with them. e.g., if there is no getter for the studentid field, then only code from within the Student class can access the studentid.

38 Summary Classes and Objects Arrays of class types Keywords : static, private, and public

COMP-202: Foundations of Programming. Lecture 14: static, private, public Jackie Cheung, Winter 2015

COMP-202: Foundations of Programming Lecture 14: static, private, public Jackie Cheung, Winter 2015 Announcements Assignment 3 due Tue, Feb 24 at 11:59pm 2 Happy New Year! 恭喜發財! May all your code compile