1. Department of Computer Science
COM S 207
Classes and Objects
Instructor: Ying Cai
Department of Computer Science
Iowa State University

2. So far we have covered Memory allocation and data manipulation
Primitive data types
Array of primitive data types
Execution control
if, while-loop, for-loop, do-while-loop
Program structuring
methods
These are essential, but not enough
Experience shows that it is difficult to understand and update a program that consists of a large collection of methods

3. Object-Oriented Programming
A programming style in which tasks are solved by collaborating objects
Each object has its own set of data, together with a set of methods that act upon the data
We have experienced some, e.g., String, Scanner
Scanner in = new Scanner(System.in);
if (in.hasNextInt()) { … }
object
construct a Scanner object
class
invoke a method of Scanner

4. Classes and Objects className obj = new className(…);
class className {
// declare instance variables
type var1;
type var2;
:::
type varn;
//declare constructor
className(parameters) {}
//declare methods
type method1(parameters) {}
type method2(parameters) {}
type methodm(paramters) {}
} //end className
className obj = new className(…);

5. Definition of Class Vehicle (version1)
int passengers;
int fuelcap;
int mpg; }
public static void main(String[] args){
Vehicle minivan = new Vehicle();
Vehicel spartscar = new Vehicle();
minivan.passengers = 7; //use dot (.) to link
minivan.fuelcap = 16; //the name of an object
minivan.mpg = 21; //with the name of a member
sportscar.passengers = 2;
sportscar.fuelcap = 14;
sportscar.mpg = 12;
int range1 = minivan.fuelcap * minivan.mpg;
int range2 = sportscar.fuelcap * sportscar.mpg;
System.out.println(…);

6. Memory allocation minivan passenger 7 fuelcap 16 mpg 21 sportscar
class Vehicle {
int passengers;
int fuelcap;
int mpg; }
public static void main(String[] args){
Vehicle minivan = new Vehicle();
Vehicel spartscar = new Vehicle();
minivan.passengers = 7;
minivan.fuelcap = 16;
minivan.mpg = 21;
sportscar.passengers = 2;
sportscar.fuelcap = 14;
sportscar.mpg = 12;
:::
minivan
passenger 7
fuelcap 16
mpg 21
sportscar
passenger 2
fuelcap 14
mpg 12
Each object has its own copy of the instant variables of the class

7. Adding Constructor
class Vehicle {
int passengers;
int fuelcap;
int mpg;
Vehicle(int p, int f, int m) {
passengers = p;
fuelcap = f;
mpg = m; }
you can have multiple constructors, each having its own parameters

8. Adding Methods class Vehicle { int passengers; int fuelcap; int mpg;
int range() {…}; // without parameter
double fuelNeeded(int miles); // with a parameter }
int range1 = minivan.range();
int range2 = sportscar.range();
double gas1 = minivan.fuelNeeded(400);
double gas2 = sportscar.fuelNeeded(400);

9. Diagram of program structure
File
Class
Variables
Constructors
Variables
Statements
Methods
A program consists of one or more classes
Typically, each class is in a separate .java file

10. Example 1: Counter
Concept: private variables

11. Example 2: BankAccount Concept: static variables
public class BankAccount {
private int accountNumber;
private String SSN;
private String name;
private double balance;
private static lastAccountNumber;
// multiple constructors
BankAccount(String SSN, Name)
// methods: deposit, withdraw, etc. }

12. Example 3: Address
Implement a class Address. An address has a house number, a street, an optional apartment number, a city, a state, and a postal code.
Supply two constructors: one with an apartment number, one without.
Supply a print method that prints the address with the street on one line and the city, state, and zip code on the next line.

13. Example 4: Student
Implement a class Student. A student has a name and a total quiz score.
Supply an appropriate constructor.
Supply the following methods: addQuiz(int score), getTotalScore(), and getAverageScore(). To compute the latter, you need to store the number of quizzes that the student took.

14. Example 5: Bug
Implement a class Bug that models a bug moving along a horizontal line. The bug moves either to the right or left. Initially, the bug moves to the right, but it can turn to change its direction. In each move, its position changes by one unit in the current direction.
Supply a constructor that gives a bug’s initial position.
Supply the following methods: move(), turn(), getPosition();

15. Summary A class contains Code (methods) + Data (instance variables)
Members of a class are accessed through the DOT (.) operator
Each object has its own copy of the instance variables of the class
Constructors
Methods
Other notions: static, public, private