1. COMP 110: Introduction to Programming Tyler Johnson Feb 16, 2009 MWF 11:00AM-12:15PM Sitterson 014

2. COMP 110: Spring 20092 Announcements Program 2 due Wednesday by midnight Lab 4 Part 2 is now extra credit

3. COMP 110: Spring 20093 Lab 2

4. COMP 110: Spring 20094 Questions?

5. COMP 110: Spring 20095 Today in COMP 110 Classes & Methods Read Section 5.1 in text

6. COMP 110: Spring 20096 Classes Java programs consists of objects of various class types interacting with one another You have already been using classes System, String, Scanner, Math

7. COMP 110: Spring 20097 Classes A class is a definition of a kind of object It describes a general template or blueprint for creating objects of that class A class “Car” is a definition of what a “Car” is and what it can do

8. COMP 110: Spring 20098 Classes & Objects Objects of the class Car might have the following attributes Make Model Year Owner Location All objects of the class Car have these attributes, but with potentially different values

9. COMP 110: Spring 20099 Classes & Objects Class Car Make Model Year Owner Location focus object Make = Ford Model = Focus Year = 2001 Owner = Samantha Smart Location = School camry object Make = Toyota Model = Camry Year = 2000 Owner = John Doe Location = Garage

10. COMP 110: Spring 200910 Classes & Objects Important: classes do not have data; individual objects have data Classes specify what kind of data objects have 10 Class Car Make Model Year Owner Location camry object Make = Toyota Model = Camry Year = 2000 Owner = John Doe Location = Garage

11. COMP 110: Spring 200911 Classes & Objects Both the “camry” and “ford” objects satisfy the definition of the class “Car” They are said to be instances of the “Car” class When we create an object of a certain class, we are said to instantiate it

12. COMP 110: Spring 200912 Methods Classes also define the actions that objects of its type can perform These are called methods All objects of the same class have the same methods The class “Car” might have the following methods accelerate() brake() sell() start()

13. COMP 110: Spring 200913 String Methods string.length() string.charAt(index) string.substring(index1, index2) string.toLowerCase() string.equals(A_String)

14. COMP 110: Spring 200914 UML A UML class diagram can be useful in describing classes UML stands for Universal Modeling Language

15. COMP 110: Spring 200915 UML Car - make: String - model: String - year: int - owner: String - location: String + accelerate(double pedalPressure): void + brake(double pedalPressure): void + sell(String newOwner): void + start(): void Class name Attributes Methods (actions) A UML class diagram for the class “Car”

16. COMP 110: Spring 200916 Defining Classes You have already been defining your own classes! Every program you have written defines a class //define the class TipCalculator public class TipCalculator { … }

17. COMP 110: Spring 200917 Class Files & Separate Compilation Each Java class definition goes in its own, SEPARATE.java file ClassName  ClassName.java The class “Car” must be saved in a file called “Car.java”

18. COMP 110: Spring 200918 Class Files & Separate Compilation What happens when you compile a.java file?.java file gets compiled into a.class file.class file contains Java bytecode Car.java is compiled into Car.class You can compile a Java class before you have a program that uses it

19. COMP 110: Spring 200919 The Student Class Student - name: String - year: int - gpa: double - major: String - credits: int - gpaSum: double + getName(): String + getMajor(): String + printData(): void + increaseYear(): void + calculateGPA(double grade): void

20. COMP 110: Spring 200920 Defining the Class Student public class Student { public String name; public int year; public double GPA; public String major; //... public String getMajor() { return major; } public void increaseYear() { year++; } // … } Class name Data (instance variables) Methods

21. COMP 110: Spring 200921 Instance Variables The data members of a class are called instance variables public String name; public int year; public double gpa; public String major; public No restrictions on how these instance variables are used (more details later)

22. COMP 110: Spring 200922 Creating Objects Declaring/Initializing Primitives int i = 0; double area = 10.5; Declaring/Creating Objects Scanner keyboard = new Scanner(System.in);

23. COMP 110: Spring 200923 Creating Objects Create an object called “jack” of class “Student” Student jack = new Student(); Create an object Assign memory address of object to variable

24. COMP 110: Spring 200924 Using Instance Variables: Inside the Class Definition public class Student { public String name; public int year; public double GPA; public String major; //... public String getMajor() { return major; } public void increaseYear() { year++; } // … }

25. COMP 110: Spring 200925 Using public Instance Variables Outside a Class public static void main(String[] args) { Student jack = new Student(); jack.name = "Jack Smith"; jack.major = "Computer Science"; System.out.println(jack.name + " is majoring in " + jack.major); Student sam = new Student(); sam.name = "Samantha Smart"; sam.major = "Biology"; System.out.println(sam.name + " is majoring in " + sam.major); } “jack.name” and “sam.name” are two different variables because they belong to different objects

26. COMP 110: Spring 200926 Methods Two kinds of methods Methods that return a value Examples – string.substring() – string.charAt() Methods that return nothing Example –System.out.println()

27. COMP 110: Spring 200927 Methods public String getMajor() { return major; } public void increaseYear() { year++; } returns a String returns nothing return type

28. COMP 110: Spring 200928 Defining Methods that Return Nothing Method heading: keywords public: no restriction on how to use the method (more details later) void: the method returns nothing Method body: statements executed when the method is called (invoked) Must be inside a pair of braces public void increaseYear() { year++; }

29. COMP 110: Spring 200929 Method printData As usual, inside a block (defined by braces), you can have multiple statements public void printData() { System.out.println("Name: " + name); System.out.println("Major: " + major); System.out.println("GPA: " + gpa); }

30. COMP 110: Spring 200930 Methods All method definitions must appear inside the definition of the class to which they belong! public class Student { // … public String getMajor() { return major; } public void increaseYear() { year++; } // … }

31. COMP 110: Spring 200931 Methods When you use a method such as keyboard.nextInt() you are said to call or invoke the method

32. COMP 110: Spring 200932 Calling Methods that Return Nothing Syntax object.method(); Use them like Java statements Student jack = new Student(); jack.year = 1; jack.increaseYear(); //year = 2 jack.increaseYear(); //year = 3 System.out.println("Jack’s class year is " + jack.year); Jack’s class year is 3 Output

33. COMP 110: Spring 200933 Defining Methods that Return a Value Method heading: keywords public: no restriction on how to use the method (more details later) Type: the type of value the method returns Method body: statements executed Must be inside a pair of braces Must have a return statement public String getMajor() { return major; }

34. COMP 110: Spring 200934 The Return Statement A method that returns a value must have at least one return statement Terminates the method, and returns a value to the caller Syntax: return Expression; Expression can be any expression that produces a value of the return type 34

35. COMP 110: Spring 200935 Methods that Return a Value public String getClassYear() { if(year == 1) return "Freshman"; else if(year == 2) return "Sophomore"; else if... }

36. COMP 110: Spring 200936 Calling Methods that Return a Value Syntax object.method(); Use as a variable of the method’s return type Student jack = new Student(); jack.name = "Jack Smith"; jack.major = "Computer Science"; String m = jack.getMajor(); System.out.println("Jack’s full name is " + jack.getName()); System.out.println("Jack’s major is " + m); 36

37. COMP 110: Spring 200937 The Return Statement Can also be used in methods that return nothing Terminates the method Syntax: return; public void increaseYear() { if(year >= 4) return; //exit the method year++; } 37

38. COMP 110: Spring 200938 The Return Statement Typically, you want to write your methods to contain only one return statement It will make your programs easier to read

39. COMP 110: Spring 200939 The Main Method The main method is a method like any other public static void main(String[] args) { // … } The main method is not invoked by you, but invoked automatically by the system when you run a program

40. COMP 110: Spring 200940 The Main Method Classes that have a main method can be run as a program Not all classes have a main method Some are used inside the main method of other classes

41. COMP 110: Spring 200941 Local Variables Variables declared inside a method are called local variables As opposed to instance variables, which are declared in the class definition public class Student { public double gpa; //an instance variable public static void main(String[] args) { int size; //a local variable // … }

42. COMP 110: Spring 200942 Local/Instance variables Instance variables Declared in a class Can be used anywhere in the class that declares the variable, including inside the class’ methods Local variables Declared in a method Can only be used inside the method that declares the variable

43. COMP 110: Spring 200943 Programming Demo Counter class A class used to count things Define a class that records a count as a non- negative whole number Include methods for the following Reset counter to 0 Increment the count Decrement the count Get the current count Display the current count

44. COMP 110: Spring 200944 Counter Class No methods should allow the count to become negative Write a program to test the class

45. COMP 110: Spring 200945 Programming Demo Programming

46. COMP 110: Spring 200946 Wednesday More about Classes