Defining Classes - Day 1

1. 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. keyboard object nextInt( ) next( ) nextLine( ) … inFile object nextInt( ) next( ) nextLine( ) … Class and Objects 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( ) … instantiated 123 how are you? 98765 yet alive! CSS161: Fundamentals of Computing

3. b 20 a 10 object1 object2 data data 10 20 method( ) method( ) Primitive Data Types versus Classes • Primitive data types • 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; CSS161: Fundamentals of Computing

4. 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 myCourse2 myCourse1 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 = 111.75; 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( ) ); } } instantiated instantiated 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 = 117.75 + 37.95 + 15 156.0 = 88.50 +67.50 + 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; • Examples: 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. 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. 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( ); • Example System.out.println( "myCourse1(" + myCourse1.department + myCourse1.number + myCourse1.section + ") needs $" + myCourse1.totalExpenditure( ) ); CSS161: Fundamentals of Computing

9. 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. Textbook Example - 4.1 public classDateFirstTry { publicStringmonth; publicintday; publicintyear; //a four digit number. publicvoidwriteOutput( ) { System.out.println(month + " " + day + ", " + year); } } public classDateFirstTryDemo { public static void main(String[] args) { DateFirstTrydate1, date2; date1 = newDateFirstTry( ); date2 = newDateFirstTry( ); 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. Self-Test Exercises • Work on textbook p170’s exercises 1 ~ 2. CSS161: Fundamentals of Computing

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

13. 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; } return Statement 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. 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. Textbook Example - 4.2 publicint 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 classDateSecondTry { privateString month; privateintday; privateint year; //a four digit number. publicvoid writeOutput( ) { System.out.println(month + " " + day + ", " + year); } publicvoid readInput( ) { Scannerkeyboard = newScanner(System.in); System.out.println("Entermonth, day, and year."); System.out.println("Do not use a comma."); month = keyboard.next( ); day = keyboard.nextInt( ); year = keyboard.nextInt( ); } publicint getDay( ) { return day; } publicint getYear( ) { return year; } CSS161: Fundamentals of Computing

18. Textbook Example - 4.3 public classDemoOfDateSecondTry { public staticvoidmain(String[] args) { DateSecondTrydate = 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 4 1776 That is the 4th day of the month CSS161: Fundamentals of Computing

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