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 points

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 * 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 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()); } Class Definition and Object Usage Convention 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 ” only if.java has a main method “java ” runs only the main method that exists in.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 public static void main(String[] args){ //Statements } 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.

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

Sample Matching

Notice how one can hide data members by declaring a local variable with the same name

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