1. CSS161: Fundamentals of Computing
Defining Classes - Day 1
This slide set was compiled from the Absolute Java textbook slides (Walter Savitch) and the professor’s own class materials.
CSS161: Fundamentals of Computing

2. CSS161: Fundamentals of Computing
Class and Objects
nextInt( )
next( )
nextLine( ) …
keyboard object
main( )
Scanner.class
Scanner keyboard = new Scanner( System.in );
Scanner inFile = null;
try {
Scanner inFile = new Scanner( new FileInputStream( “data.txt” ) );
} catch ( FileNotFoundException e ) {
System.exit( 0 ); }
int data1 = keyboard.nextInt( );
String message = keyboard.nextLine( );
Int data2 = inFile.nextInt( );
String message = inFile.nextLine( );
instantiated
nextInt( ) {
do some action }
next( )
nextLine( ) …
nextInt( )
next( )
nextLine( ) …
inFile object
instantiated
123 how are you?
98765 yet alive!
CSS161: Fundamentals of Computing

3. Primitive Data Types versus Classes
a single piece of data
byte, char, short, int, long, float, double, boolean
Variables declared before their use:
int a = 10, b = 20;
Classes
a collection of multiple pieces of data + methods
Objects instantiated before their use
Including the same methods
Including the same set of data, but
Maintaining different values in each piece of data
MyClass object1 = new MyClass( );
MyClass object2 = new MyClass( );
object1.data = 10;
object2.data = 20; 10 a 20 b 10
data
method( )
object1 20
data
method( )
object2
CSS161: Fundamentals of Computing

4. CSS161: Fundamentals of Computing
Class Example
public class CourseDemo {
public static void main( String[] args ) {
Course myCourse1 = new Course( );
Course myCourse2 = new Course( );
myCourse1.department = "CSS";
myCourse1.number = 161;
myCourse1.section = 'A';
myCourse1.textPrice1 = ;
myCourse1.textPrice2 = 37.95;
myCourse1.courseFee = 15;
myCourse2.department = "CSS";
myCourse2.number = 451;
myCourse2.section = 'A';
myCourse2.textPrice1 = 88.50;
myCourse2.textPrice2 = 67.50;
myCourse2.courseFee = 0;
System.out.println( "myCourse1(" + myCourse1.department +
myCourse1.number + myCourse1.section +
") needs $" + myCourse1.totalExpenditure( ) );
System.out.println( "myCourse2(" + myCourse2.department +
myCourse2.number + myCourse2.section +
") needs $" + myCourse2.totalExpenditure( ) ); }
public class Course {
public String department; // CSS, IAS, BUS, NRS, EDU
public int number; // course number
public char section; // A, B, C ...
public double textPrice1; // primary textbook
public double textPrice2; // secondary textbook
public double courseFee; // laboratory fee
public double totalExpenditure( ) {
return textPrice1 + textPrice2 + courseFee; }
Multiple pieces of data
Method
instantiated
instantiated
myCourse2
myCourse1
department: CSS
number: 451
section: A
textPrice1: $88.50
textPrice2: $67.50
courseFee: $0
totalExpenditure( )
department: CSS
number: 161
section: A
textPrice1: $111.75
textPrice2: $37.95
courseFee: $15
totalExpenditure( )
164.7 =
156.0 =
myCourse1(CSS161A) needs $164.7
myCourse2(CSS451A) needs $156.0
CSS161: Fundamentals of Computing

5. Members, Instance Variables, and Methods
Class name
public class Course {
public String department; // CSS, IAS, BUS, NRS, EDU
public int number; // course number
public char section; // A, B, C ...
public int enrollment; // the current enrollment
public int limit; // the max number of students
public double textPrice1; // primary textbook
public double textPrice2; // secondary textbook
public double courseFee; // laboratory fee
public int availableSpace( ) {
return limit - enrollment; }
public double capacity( ) {
return ( double )enrollment / limit;
public double totalExpenditure( ) {
return textPrice1 + textPrice2 + courseFee;
Instance variables:
(data members)
Each object has its own
values in these variables.
members
Methods:
Each object has the same methods
(actions, computations).
CSS161: Fundamentals of Computing

6. Object Instantiation with new
Declare a reference variable (a box that contains a reference to a new instance.)
ClassName object;
object has no reference yet, (= null).
Create a new instance from a given class.
object = new ClassName( );
object has a reference to a new instance.
Declare a reference variable and initialize it with a reference to a new instance created from a given class
ClassName object = new ClassName( );
CSS161: Fundamentals of Computing

7. Accessing Instance Variables
Declaring an instance variable in a class
public type instanceVariable; // accessible from any methods (main( ))
private type instanceVariable; // accessible from methods in the same class
Examples:
public String department;
public int number;
public char section;
public int enrollment;
Assigning a value to an instance variable of a given object
objectName.instanceVariable = expression;
myCourse1.department = "CSS";
myCourse1.number = 161;
myCourse1.section = 'A';
myCourse1.enrollment = 24;
Reading the value of an instance of a given object
Operator objectName.instanceVariable operator
Example:
System.out.println( "myCourse1 = " + myCourse1.department +
myCourse1.number + ")" );
CSS161: Fundamentals of Computing

8. Defining and Accessing Methods
Defining a method in a class
public type method( ) { // heading
// method body
//code to perform some action and/or compute a value }
Example
public double totalExpenditure( ) {
return textPrice1 + textPrice2 + courseFee;
Accessing a method in a class
objectName.method( );
System.out.println( "myCourse1(" + myCourse1.department +
myCourse1.number + myCourse1.section +
") needs $" + myCourse1.totalExpenditure( ) );
Public: accessible from any other methods (including main( ))
Private: accessible from a method within the same class
If a method takes only a certain action, this should be void
and “return” is unnecessary.
CSS161: Fundamentals of Computing

9. CSS161: Fundamentals of Computing
Class Names and Files
Each class should be coded in a separate file whose name is the same as the class name + .java postfix.
Example
Source code
Course.java
CourseDemo.java
Compilation (from DOS/Linux command line.)
javac Course.java javac CourseDemo.java
javac CourseDemo.java javac Course.java
Compiled code
Course.class
CourseDemo.class
Execution (from DOS/Linux command line.)
java CourseDemo
An either way is fine. Compiling CourseDemo.java first
automatically compile Course.java, too.
Start with the class name that includes main( ).
CSS161: Fundamentals of Computing

10. CSS161: Fundamentals of Computing
Textbook Example - 4.1
public class DateFirstTry {
public String month;
public int day;
public int year; //a four digit number.
public void writeOutput( )
System.out.println(month + " " + day +
", " + year); }
public class DateFirstTryDemo {
public static void main(String[] args)
DateFirstTry date1, date2;
date1 = new DateFirstTry( );
date2 = new DateFirstTry( );
date1.month = "December";
date1.day = 31;
date1.year = 2007;
System.out.println("date1:");
date1.writeOutput( );
date2.month = "July";
date2.day = 4;
date2.year = 1776;
System.out.println("date2:");
date2.writeOutput( ); }
date1:
December 31, 2007
date2:
July 4, 1776
CSS161: Fundamentals of Computing

11. CSS161: Fundamentals of Computing
Self-Test Exercises
Work on textbook p170’s exercises 1 ~ 2.
CSS161: Fundamentals of Computing

12. CSS161: Fundamentals of Computing
More About Methods
Two kinds of methods
Methods that only perform an action
public void methodName( ) { /* body */ }
Example
public void writeOutput( ) {
System.out.println(month + " " + day + ", " + year); }
Methods that compute and return a result
pubic type methodName( ) {
/* body */
return a_value_of_type;
public double totalExpenditure( ) {
return textPrice1 + textPrice2 + courseFee;
Nothing to return
The same data type
A variable, a constant, an expression, or an object
CSS161: Fundamentals of Computing

13. CSS161: Fundamentals of Computing
return Statement
Method to perform an action
void Method( )
Return is not necessary but may be added to end the method before all its code is ended
Example
public void writeMesssage( ) {
System.out.println( “status” );
if ( ) {
System.out.println( “nothing” );
return; }
else if ( error == true )
System.out.print( “ab” );
System.out.println( “normal” );
Method to perform an action
type Method( )
Return is necessary to return a value to a calling method.
Example
public double totalExpenditure( ) {
return textPrice1 + textPrice2 + courseFee; }
CSS161: Fundamentals of Computing

14. Any Method Can Be Used As a void Method
A method that returns a value can also perform an action
If you want the action performed, but do not need the returned value, you can invoke the method as if it were a void method, and the returned value will be discarded:
objectName.returnedValueMethod();
CSS161: Fundamentals of Computing

15. CSS161: Fundamentals of Computing
public and private
Public
Methods and instance variables accessible from outside of their class
Private
Methods and instance variables accessible within their class
public class Course
css263.method1( )
public type method1( ) { }
css263.method2( )
public type method2( ) { }
css263.utility( )
private type utility( ) { }
css263.department
public String department;
public int number;
private int enrollment;
private int gradeAverage;
css263.number
css263.enrollment
CSS161: Fundamentals of Computing

16. Encapsulating Data in Class
Which design is more secured from malicious attacks?
public class Course
public class Course
public type method1( ) { }
public type method1( ) { }
css161.method1( )
css263.method1( )
public type method2( ) { }
public type method2( ) { }
css161.utility( )
public type utility( ) { }
private type utility( ) { }
css161.number = 263
public String department;
public int number;
public int enrollment;
public int gradeAverage;
private String department;
private int number;
private int enrollment;
private int gradeAverage;
CSS161: Fundamentals of Computing

17. CSS161: Fundamentals of Computing
Textbook Example - 4.2
public int getMonth( ) {
if (month.equalsIgnoreCase("January"))
return 1;
else if (month.equalsIgnoreCase("February"))
return 2;
else if (month.equalsIgnoreCase("March"))
return 3;
else if (month.equalsIgnoreCase("April"))
return 4;
else if (month.equalsIgnoreCase("May"))
return 5;
else if (month.equals("June"))
return 6;
else if (month.equalsIgnoreCase("July"))
return 7;
else if (month.equalsIgnoreCase("August"))
return 8;
else if (month.equalsIgnoreCase("September"))
return 9;
else if (month.equalsIgnoreCase("October"))
return 10;
else if (month.equalsIgnoreCase("November"))
return 11;
else if (month.equalsIgnoreCase("December"))
return 12;
else
System.out.println("Fatal Error");
System.exit(0);
return 0; //Needed to keep the compiler happy }
import java.util.Scanner;
public class DateSecondTry {
private String month;
private int day;
private int year; //a four digit number.
public void writeOutput( )
System.out.println(month + " " + day + ", " + year); }
public void readInput( )
Scanner keyboard = new Scanner(System.in);
System.out.println("Enter month, day, and year.");
System.out.println("Do not use a comma.");
month = keyboard.next( );
day = keyboard.nextInt( );
year = keyboard.nextInt( );
public int getDay( )
return day;
public int getYear( )
return year;
CSS161: Fundamentals of Computing

18. CSS161: Fundamentals of Computing
Textbook Example - 4.3
public class DemoOfDateSecondTry {
public static void main(String[] args)
DateSecondTry date = new DateSecondTry( );
date.readInput( );
int dayNumber = date.getDay( );
System.out.println("That is the " + dayNumber
+ "th day of the month."); }
Enter month, day, and year.
Do not use a comma.
July
That is the 4th day of the month
CSS161: Fundamentals of Computing

19. CSS161: Fundamentals of Computing
Self-Test Exercises
Work on textbook p176’s exercise 3
CSS161: Fundamentals of Computing