Recitation 02/6/2009 CS 180 Department of Computer Science, Purdue - - PowerPoint PPT Presentation

Recitation 02/6/2009

CS 180 Department of Computer Science, Purdue University

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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

Conventional Class Definition Structure

Why Are Conventions Useful??

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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

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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?

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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

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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? Why? Why? What’s “static” about?

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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 ?

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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

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Defining Class Constants

class BookStore{ private static final int zipCode = 47906; private final String name = “Jays”; //rest of class }

Why is this bad?

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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

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

• utside any method. Their scope is

determined by public and private modifiers.

The Quiz

 What’s the difference between a .class file

and a class definition?

 When you would make a function “static”?

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