CSE 1341 Principles of Computer Science I

Spring 2008

Mark Fontenotmfonten@engr.smu.edu

CSE 1341Principles of Computer Science I

Note Set 2

Note Set 2 OverviewATM Case Study IntroductionUse Case DiagramsWhat is an object?Declaring classLearn/refresh basic object terminologyInstance Variables and Instance Methods – Different from

1340?Implementing the Class vs. Using the class

Car AnalogyTo 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 systemIn a software system, and under OOP, objects interactMust 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 appClass GradeBookTest – Will be used to test the Grade-book

class

GradeBook Class EvolutionExample 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 - 1Iteration 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 byother classes

class – what we’redeclaring/creating

identifier – name of the classthat 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 LineWhat is going on “behind the scenes” when you compile and run

from inside NetBeans?Assuming that you’ve created the 2 source files

GradeBook.javaGradeBookTest.java

Compile the files with javac GradeBook.java GradeBookTest.java

Run Program with java GradeBookTeststarts 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 DiagramUse a class diagram to graphically represent the

class/object

plus sign (+) means public – can be accessed outside the classex: 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 multipleparameters by including each in the parameter listseparated 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 ReviewPrimitive Data Types –

boolean, byte, char, short, int, long, float, doublewhen declared, memory reserved for a piece of data of that

typee.g. int mySpecialIntVariable;

ReferenceAny other data typewhen 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 somewhereGradeBook gb = new GradeBook();

Reference Variables

GradeBook gb = new GradeBook();

Executed first – reserves place in memory for a GradeBook object.

GradeBookObject

then, reference is stored in reference variable

gb

GradeBook – 3 – Instance VariablesInstance Variable

attribute of an object that is being modelede.g. Car object – color might be an attributee.g. Bank Account Object – account number would be attribute

declared inside the class but outside the body of any methodsBook 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 VariablesInstance Variables

Always given a default valueObject references are initialized to null meaning they don’t

reference an object yetbyte, 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 becauseit doesn’t reference to any particular String object in memory yet.

GradeBook – 3 – Instance VariablesClass provide interface to instance variables through

methods2 main categories for access to data members

accessor methods – return the value stored in a fieldusually start with get e.g. getCourseName

mutator methods – set the value of a fieldusually 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 Versionpublic 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 typeof what the methodis returning

GradeBook – 3 – Full Versionpublic 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

checkpointDefine

public?

private?

accessor?

mutator?

instantiate?

GradeBook – 4 – The ConstructorWhen 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 1341gb1

courseName = CSE 2341gb2

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

GradeBook – 4 – Class Diagram

constructor added to class diagram

notice that it is preceded by <<constructor>>

no return type indicated

CheckpointWhat is a constructor?

When are constructors called?

Can constructors have return types? parameters?

From ScratchCreate 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