Spring 2008 Mark Fontenot CSE 1341 Principles of Computer Science I Note Set 2

Note Set 2 Overview ATM Case Study Introduction Use Case Diagrams What is an object? Declaring class Learn/refresh basic object terminology Instance Variables and Instance Methods – Different from 1340? Implementing the Class vs. Using the class

Car Analogy To Drive a Car, do we need to know: the details of power steering? intricacies of a internal combustion engine? how anti-lock breaks work?

What are we building? We are building a software system In a software system, and under OOP, objects interact Must determine which objects interact in the system and then create the Java classes that represent those objects Today’s First Example: Class GradeBook – shall display message to user welcoming them to grade-book app Class GradeBookTest – Will be used to test the Grade-book class

GradeBook Class Evolution Example 1 – simple method to display welcome message Example 2 – method to receive course name and use it in displaying message Example 3 – Add an instance variable (data member) to the class to store the name of the course name Example 4 – Simple Class constructor to initialize instance variables.

GradeBook - 1 Iteration 1 – contains a method displayMessage that prints a welcome message to the screen public class GradeBook { public void displayMessage () { System.out.println(“Welcome to the GradeBook App.”); } public – can be used by other classes class – what we’re declaring/creating identifier – name of the class that we are defining Must be placed in a file named ________________________.

GradeBookTest A client object that will contain a main method used to test the GradeBook class public class GradeBookTest { public static void main (String [] args) { //create a GradeBook object (with new) and assign it //reference variable gb1 GradeBook gb1 = new GradeBook(); //call the method displayMessage gb1.displayMessage (); } Must be placed in a file named ________________________.

Aside – Compiling from the Command Line What is going on “behind the scenes” when you compile and run from inside NetBeans? Assuming that you’ve created the 2 source files GradeBook.java GradeBookTest.java Compile the files with javac GradeBook.java GradeBookTest.java Run Program with java GradeBookTest starts the JVM and looks inside class GradeBookTest for a static main method That’s where execution begins in a program Can’t run GradeBook because it doesn’t contain a main method

The Class Diagram Use a class diagram to graphically represent the class/object plus sign (+) means public – can be accessed outside the class ex: that method is being accessed from GradeBookTest

GradeBook – 2 – Welcome with parameter public class GradeBook { //display welcome message including course name in msg public void displayMessage (String courseName) { System.out.printf(“Welcome to the GradeBook for %s!”, courseName); } This method accepts a parameter – a value that is sent to the method from the call. Similar to variable declaration. A method can specify that it requires multiple parameters by including each in the parameter list separated by a comma.

GradeBookTest – 2 import java.util.*; //What is this for??? public class GradeBookTest { public static void main (String [] args) { //create scanner object and store in kb so we can //read from keyboard Scanner kb = new Scanner(System.in); String nameOfCourse; GradeBook gb = new GradeBook (); //get the name of the course from the user System.out.println(“Please enter course name: “); nameOfCourse = kb.nextLine();//read a line of text System.out.println();//print blank line //pass the name of the course to dispalyMessage method gb.displayMessage(nameOfCourse); }

GradeBookTest – 2 public class GradeBookTest { public static void main (String [] args) { // --- other stuff --- //pass the name of the course to dispalyMessage method gb.displayMessage(nameOfCourse); } public class GradeBook { //display welcome message including course name in msg public void displayMessage (String courseName) { System.out.printf(“Welcome to the GradeBook for %s!”, courseName); } string stored in nameOfCourse is copied to the parameter courseName when method call is executed

New Class Diagram Indicate a parameter in this fashion Note: We’re doing this in reverse right now. Class Diagrams should be done first.

Quick Data Type Review Primitive Data Types – boolean, byte, char, short, int, long, float, double when declared, memory reserved for a piece of data of that type e.g. int mySpecialIntVariable; Reference Any other data type when declared, as in: GradeBook gb; only a memory location is reserved that can reference an actual object using new – creates an object then we store the reference somewhere GradeBook gb = new GradeBook();

Reference Variables GradeBook gb = new GradeBook(); Executed first – reserves place in memory for a GradeBook object. GradeBook Object GradeBook Object then, reference is stored in reference variable gb

GradeBook – 3 – Instance Variables Instance Variable attribute of an object that is being modeled e.g. Car object – color might be an attribute e.g. Bank Account Object – account number would be attribute declared inside the class but outside the body of any methods Book calls them a field GradeBook – courseName would be a valid attribute store it as instance variable public class GradeBook { private String courseName; //Other stuff to come } Instance variables are usually declared private. Can only be accessed from members of class GradeBook (and not GradeBookTest).

GradeBook – 3 – Instance Variables Instance Variables Always given a default value Object references are initialized to null meaning they don’t reference an object yet byte, char, short, int, long, float, double  Initialized to zero (0) boolean  initialized to false public class GradeBook { private String courseName; //Other stuff to come } Given an initial value of null because it doesn’t reference to any particular String object in memory yet.

GradeBook – 3 – Instance Variables Class provide interface to instance variables through methods 2 main categories for access to data members accessor methods – return the value stored in a field usually start with get e.g. getCourseName mutator methods – set the value of a field usually start with set e.g. setCourseName public class GradeBook { private String courseName; public void setCourseName(String newName) { //set parameter value to instance variable courseName = newName; } public String getCourseName () { return courseName; } Mutator Accessor

GradeBook – 3 – Full Version public class GradeBook { private String courseName; public void setCourseName(String newName) { //set parameter value to instance variable courseName = newName; } public String getCourseName () { return courseName; } public void displayMessage () { System.out.printf(“Welcome to %s!”, getCourseName()); } String is return type - indicates the data type of what the method is returning

GradeBook – 3 – Full Version public class GradeBook { private String courseName; public void setCourseName(String newName) { //set parameter value to instance variable courseName = newName; } public String getCourseName () { return courseName; } public void displayMessage () { System.out.printf(“Welcome to %s!”, getCourseName()); } import java.util.*; public class GradeBookTest{ public static void main (String [] args) { Scanner kb = new Scanner(System.in); String name; GradeBook gb = new GradeBook (); System.out.printf(“Name before Init: %s\n”, gb.getCourseName()); System.out.println(“Enter course name: “); name = kb.nextLine(); gb.setCourseName( name ); System.out.println(); gb.displayMessage(); }

GradeBook – 3 – Class Diagram Put Attributes/Instance variables in the middle section of the class diagram indicates method is returning a String value

checkpoint Define public? private? accessor? mutator? instantiate?

GradeBook – 4 – The Constructor When we use new to create an object, that objects constructor is called. GradeBook gb = new GradeBook (); constructor – special method of a class that is called automatically when an object of that type is created default constructor – supplied by the compiler if the class does not explicitly supply one. Constructor is usually used to set up the initial state of the object. provide initial values to data members other than null, 0 and false. automatically calls constructor

public class GradeBook { private String courseName; //a constructor for class GradeBook public GradeBook (String name) { courseName = name; } public void setCourseName(String newName) { //set parameter value to instance variable courseName = newName; } public String getCourseName () { return courseName; } public void displayMessage () { System.out.printf(“Welcome to %s!”, getCourseName()); } GradeBook – 4 Constructor has: -> Same name as class -> No return type May or may not accept a parameter

GradeBookTest – 4 public class GradeBookTest { public static void main (String [] args) { GradeBook gb1 = new GradeBook(“CSE 1341”); GradeBook gb2 = new GradeBook(“CSE 3353”); } courseName = CSE 1341 gb1 courseName = CSE 2341 gb2 Each object instance of GradeBook that is created has its own copy of courseName stored inside it. This is true even though it looks like that variable is declared only once.

GradeBookTest – 4 public class GradeBookTest { public static void main (String [] args) { GradeBook gb1 = new GradeBook(“CSE 1341”); GradeBook gb2 = new GradeBook(“CSE 3353”); System.out.printf (“Course 1 Name: %s\n”, gb1.getCourseName()); System.out.printf (“Course 2 Name: %s\n”, gb2.getCourseName()); } Course 1 Name: CSE 1341 Course 2 Name: CSE 3353 Course 1 Name: CSE 1341 Course 2 Name: CSE 3353

GradeBook – 4 – Class Diagram constructor added to class diagram notice that it is preceded by > no return type indicated

Checkpoint What is a constructor? When are constructors called? Can constructors have return types? parameters?

From Scratch Create an class that represents a bank account. Each bank account object should maintain the account balance. It should also be able to return the current balance and credit an amount to the account. Class Name: ______________________ Attributes: _______________________ Methods: _______________________ Constructor: ____________________

Class Diagram for Bank Account Object In Class Exercise: Code the Account Class