Recitation 02/6/2009

1. Recitation 02/6/2009 CS 180 Department of Computer Science, Purdue University

2. Announcements & Reminders • Project 1 grades out • Solution up & test cases on the Web • Project 2 was due on Wednesday • Project 3 is out • Mentoring program w/ Debbie will be in LWSN B131 on Tuesdays • Exam 1 is Feb. 18th (Less than a couple of weeks. Yikes! Better Start Studying!) • Expect 3 programming questions and multiple choice questions -- 100 points

3. Conventional Class Definition Structure Why Are Conventions Useful??

4. What Your Class Would Like import java.util.Date; /** * Book -- a book that knows only its name * * @author Henry David Thoreau **/ class Book { private String name; private Date dateMade; public Book( ) { name = “I have no name”; dateMade = new Date(); } public String getName( ) { return name; } public void setName(String newName) { name = newName; } } Import Statement Comments Class Name Data Member Constructor Methods

5. Class Definition and Object Usage Convention CookBook.java class CookBook { private int numRecipes; private String name; //Constructor(s) //Methods (e.g., getter & setter) public static void main(String[] args){ CookBook cookBook1; cookBook1.setName(“Cooking!”); System.out.println(cookBook1.getName()); cookBook1.setName(“Cooking part Deux!”); System.out.println(cookBook1.getName()); } } ColorBook.java class ColorBook { private int numImages; private String name; //Constructors(s) //Methods (e.g., getter & setter) public static void main( String[] args ) { ColorBook colorBook1; colorBook1 = new ColorBook( ); colorBook1.setName(“CB1”); colorBook1.setNumImages(35); ColorBook colorBook2 = new ColorBook(); colorBook2.setName(“CB2”); System.out.println(colorBook1.getName()); System.out.println(colorBook2.getName()); } } Why is it useful for each class to have its own main method? In what order would you develop these classes?

6. More On The Main Method... You can use the “java <className>” only if <className>.java has a main method “java <className>” runs only the main method that exists in <className>.java

7. Access Modifiers CookBook.java class CookBook { private int numRecipes; private String name; //Constructor(s) public int getNumRecipes(){ return numRecipes; } public void setNumRecipes(int num){ numRecipes = num; } //Rest of Methods publicstatic void main(String[] args){ //Statements } } Why? Why? Why? What’s “static” about?

8. Constructor class CookBook { private int numRecipes; private String name; public CookBook (){ numRecipes = 0; name = “Joe Blog”; } public CookBook(String newName){ name = newName; numRecipes = 0; } public CookBook(String newName, int num){ numRecipes = num; name = newName; } //Rest of Methods } Defining even ONE Constructor precludes you from getting the default Constructor Why have multiple Constructors?

9. Passing by Reference vs. Passing By Value CarDealer.java class CarDealer { private static Car lastCar; public static void lastCarSold(Car lCar){ lastCar = lCar; } public static void main( String [] arg ) { Car c1 = new Car(“Honda”); c1.setOwner(“Jonny B. Quick”); lastCarSold(c1); c1.setOwner(“Jonny’s Mama”); System.out.println(lastCar.getOwner); } } class objects are transferred as references when they are passed as parameters to a method. In contrast, basic data types like int and double are passed by value. Program output : Jonny’s Mama Note: Car class defined in another file in the same directory

10. Defining Class Constants class BookStore{ private static final int zipCode = 47906; private final String name = “Jays”; //rest of class } Why is this bad?

11. Calling Methods class Lion { public void eatYou( ) {System.exit(0);} public void finishingMove( ) { eatYou(); } } Class Jungle{ public void welcome( ) {System.out.println(“Welcome!”);} public void wildLife( ) { Lion l1 = new Lion( ); welcome(); l1.eatYou( ); } } When you call a method that’s within the same class, you can call the method by just using its name. If you call a method that is in a different class, then you must refer to that method using a . (dot) notation that first references the separate class object.

12. Identifier Types • Identifiers can be declared almost anywhere in a program. • There are three main types of declarations: • Data members of a class • Declared outside any method • Usually at the beginning of the class definition • As formal parameters of a method • Within a method -- local variables

14. Sample Matching

15. Sample Matching • Notice how one can hide data members bydeclaring a local variable with the same name

16. Things to Remember • A local variable can be declared just about anywhere! • Its scope (the area of code from where it is visible) is limited to the enclosing braces. • Statements within a pair of braces are called a block. • Local variables are destroyed when the block finishes execution. • Data members of a class are declared outside any method. Their scope is determined by public and private modifiers.

17. The Quiz • What’s the difference between a .class file and a class definition? • When you would make a function “static”? 17