1. Announcements
Assignment 2 and Lab 4 due Wednesday

2. Questions?
Loops

3. Today in COMP 110
Classes & Methods

4. Classes
Java programs consists of objects of various class types interacting with one another
You have already been using classes
System, String, Scanner, Math

5. 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

6. 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

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

8. 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

9. Classes & Objects
Important: classes do not have data; individual objects have data
Classes specify what kind of data objects have 9

10. 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()

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

12. UML A UML class diagram for the class “Car” Class name Data
make: String
model: String
year: int
owner: String
- location: String
+ accelerate(double pedalPressure): void
+ brake(double pedalPressure): void
+ sell(): void
+ start(): void
Class name
Data
Methods (actions)

13. 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”

14. Class Files & Separate Compilation
What happens when you compile a .java file?
.java file gets compiled into a .class file
Car.java is compiled into Car.class
Contains Java bytecode
You can compile a Java class before you have a program that uses it

15. 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 { … }

16. 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

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

18. 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)

19. Creating Objects Create an object called “keyboard” of class “Scanner”
Create an object called “jack” of class “Student”
Student jack = new Student();
Create an object called “keyboard” of class “Scanner”
Scanner keyboard = new Scanner(System.in);
Assign memory address of object to variable
Return memory address of object
Create an object

20. 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++; // …

21. 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 instance variables because they belong to different objects

22. Methods Two kinds of methods Methods that return a value
Examples
string.substring()
string.charAt()
Methods that return nothing
Example
System.out.println()

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

24. 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++; }

25. 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); }

26. 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++;

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

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

29. 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; }

30. 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 30

31. Methods that Return a Value
public String getClassYear() { if(classYear == 1) return "Freshman"; else if(classYear == 2) return "Sophomore"; else if ... }

32. 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); 32

33. The Return Statement Can also be used in methods that return nothing
Terminates the method
Syntax:
return;
public void increaseYear() {
if(classYear >= 4)
return; //exit the method
classYear++; } 33

34. The Return Statement
Typically, you want to write your methods to contain only one return statement
It will make your programs easier to read

35. 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

36. The Main Method Classes that have a main method can be run a program
Not all classes have a main method
Some are used inside the main method of other classes

37. 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
// … }

38. Local/Instance variables
Declared in a class
Confined to the class
Can be used anywhere in the class that declares the variable, including inside the class’ methods
Local variables
Declared in a method
Confined to the method
Can only be used inside the method that declares the variable

39. 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

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

41. Programming Demo
Programming

42. Wednesday
More about Classes