COMP 110: Introduction to Programming

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

2. Announcements • Program 2 due Wednesday by midnight • Lab 4 • Part 2 is now extra credit

3. Lab 2

4. Questions?

5. Today in COMP 110 • Classes & Methods • Read Section 5.1 in text

6. 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. 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. 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. Class Car Make Model Year Owner Location focus object Make = Ford Model = Focus Year = 2001 Owner = Samantha Smart Location = School Classes & Objects • camry object • Make = Toyota • Model = Camry • Year = 2000 • Owner = John Doe • Location = Garage

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

11. 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. 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. String Methods • string.length() • string.charAt(index) • string.substring(index1, index2) • string.toLowerCase() • string.equals(A_String)

14. UML • A UML class diagram can be useful in describing classes • UML stands for Universal Modeling Language

15. UML • A UML class diagram for the class “Car” Class name Attributes Methods (actions)

16. Defining Classes • You have already been defining your own classes! • Every program you have written defines a class //define the class TipCalculator publicclass TipCalculator { … }

17. 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. 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. The Student Class

20. Defining the Class Student public classStudent { publicString name; public intyear; public doubleGPA; publicStringmajor; // ... publicStringgetMajor() { return major; } public voidincreaseYear() { year++; } // … } Class name Data (instance variables) Methods

21. Instance Variables The data members of a class are called instance variables publicString name; public intyear; public doublegpa; publicStringmajor; public No restrictions on how these instance variables are used (more details later)

22. Creating Objects • Declaring/Initializing Primitives int i = 0; double area = 10.5; • Declaring/Creating Objects Scanner keyboard = new Scanner(System.in);

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

24. Using Instance Variables: Inside the Class Definition public classStudent { publicString name; public intyear; public doubleGPA; publicStringmajor; // ... publicStringgetMajor() { return major; } public voidincreaseYear() { year++; } // … }

25. Using public Instance Variables Outside a Class public static voidmain(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. Methods Two kinds of methods Methods that return a value Examples string.substring() string.charAt() Methods that return nothing Example System.out.println()

27. Methods public StringgetMajor() { return major; } public void increaseYear() { year++; } returns a String return type returns nothing

28. 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 voidincreaseYear() { year++; }

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

30. Methods • All method definitions must appear inside the definition of the class to which they belong! public classStudent { // … publicStringgetMajor() { return major; } public voidincreaseYear() { year++; } // … }

31. Methods • When you use a method such as keyboard.nextInt() you are said to call or invoke the method

32. 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); Output Jack’s class year is 3

33. 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 returnstatement publicStringgetMajor() { return major; }

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

35. Methods that Return a Value publicString getClassYear() { if(year == 1) return"Freshman"; else if(year == 2) return"Sophomore"; else if ... }

36. 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. 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. 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. The Main Method • The main method is a method like any other publicstatic void main(String[] args) { // … } • The main method is not invoked by you, but invoked automatically by the system when you run a program

40. 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. 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 publicstatic void main(String[] args) { int size; //a local variable // … } }

42. 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. 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. Counter Class • No methods should allow the count to become negative • Write a program to test the class

45. Programming Demo • Programming

46. Wednesday • More about Classes