some background activities around classes the last day!!!! we discussed some classes and programs...

Some Background ActivitiesAround Classes

• The last day!!!! We discussed some classes and programs that called them.

• For example we had the GradeBook and Grade Book Test Classes

Another Example

• // Class declaration with one method.

• public class GradeBook• {• // display a welcome message to the GradeBook user• public void displayMessage()• {• System.out.println( "Welcome to the Grade Book!" );• } // end method displayMessage

• } // end class GradeBook

This is called from this class• // Create a GradeBook object and call its displayMessage method.

• public class GradeBookTest• {

• GradeBookTest()• {• // create a GradeBook object and assign it to myGradeBook• // call the constructor for the class GradeBook• GradeBook myGradeBook = new GradeBook();

• // call myGradeBook's displayMessage method• myGradeBook.displayMessage();• } // end main

• public static void main (String[] args) {• // Start the program running from its constructor• new GradeBookTest();}

• } // end class GradeBookTest

Consider what is happening

• We are creating an instance of GradeBook from this class GradeBookTest

• See the line• GradeBook myGradeBook = new GradeBook();• We then call on the line

• myGradeBook.displayMessage();• This calls the displayMessage method from the

GradeBook class

Next Lecture

• How do we implement all this• We need to do this by linking all the classes

together.• One way of doing this is with the –

classpath qualifier at compilation time using the javac command

• We have already seen this at run time with the java command. But we can use the same idea at compilation time

• This will tell the compiler where to find classes.• See the following dialogue

Directory mnemonics

• In Dos the directory hierarchy uses some short cuts

• \ refers to the root directory• So if we type cd \• We will be returned to the root directory• The parent directory of any directory is indicated

by ..• So if we were in C:\johns\java\progs and we

typed cd ..• We will be returned to C:\johns\java

Directory mnemonics . notation

• The current directory is indicated by .

• So when we use . We are referring to the current directory

• So javac –classpath . GradeBook will look for classes in the current directory. So the java classes used must be there.

Consider the following two Classes

• Book

• And Bookstore1 due to June Barrett

• Their Code is

Book.java• public class Book {

• // Declare instance fields• String name;• int price;

• // The constructor initialises the instance fields• Book (String n, int p) {• name = n;• price = p;• }

• // a method to output the object details• void write() {• System.out.println(name + " for £" +price);• }• }

Book.java• public class Book1{

• // Declare instance fields• String name;• int price

String booktype;

• // The constructor initialises the instance fields• Book1 (String n, int p,String bt) {• name = n;• price = p;• Booktype = bt;• }

• // a method to output the object details• void write() {• System.out.println(name + " for £" +price);• }• }

Actual price method

• void writeactualprice() {

float tot;

If (booktype == “hardback”)

tot = price * 2;

else if (booktype ==“paperback”)

tot = price * 1.5;

else if (booktype ==“booklet”)

tot = price * 0.5;

else

tot = price * 3;• System.out.println(name + " for £" +tot);• }

BookStore1.java

• public class BookStore1 {

• /* Illustrating the basic structure of an object oriented program */

• // Declare three object variables representing type of

• // goods sold in the store• Book TextBook, Novel,ShortStory;

BookStore1.java continued• // The constructor for the program is• // where the initial work gets done• BookStore1 () {

• // Create three objects with different initial values• TextBook = new Book("Java Programming", 6);• Novel = new Book("The Unsung Hero", 30);• ShortStory = new Book("Stories for 5 year olds", 80);

• // Print out a header• System.out.println("The Book Store sells\n");

• // Print the values contained in each of the objects• TextBook.write();• Novel.write();• ShortStory.write();• }

And Finally

• // The program control class must have a main method

• public static void main (String[] args) {• // Start the program running

from its constructor• new BookStore1 ();• }• }

The Constructor BookStore1

• We see from the code that the constructor BookStore1() make three instance of the class Book, namely TextBook,Novel and ShortStory

• It then calls on the Book write method to display details about the book.

Next How do we compile these

• Firstly Book.java and BookStore1.java must be in the same directory

From a Directory Listing

• We see that they are both in the c:\ directory

• Next we compile and run the classes using the

• Javac and Java commands with

• –classpath .

From this screen we see

• That the class correctly executes the specific objects.

Some Dos issues

• The javac command is in the jdk bin subdirectory and I am fed up typing out

• C:\program files\java\jdk1.5.0_16\bin\javac

• Or whatever in order to use the command

Solution set a dos path

• set path=c:\program files\java\jdk1.5.0_16\bin• Then the O/S will try this path when you invoke javac

Packages

• An alternative way of accessing classes

• We have already seen

• import java.util.Scanner;

• For example in our Leapyear program

Leapyear Class• // Second Program HelloWorld• import java.util.Scanner; • public class Leapyear {• public static void main(String args []){• Scanner input = new Scanner( System.in );• String theName = input.nextLine(); • int num1 = Integer.parseInt(theName);• if(num1 % 4 ==0 && (num1 % 100 != 0 ||num1 % 400 == 0))• System.out.println("leap");• else• System.out.println("not leap");• }}

Java Packages

• java.util.Scanner is a set of classes held in java listing device known as a package

• This allows us keep java classes in directories and subdirectories and import them into our user defined classes

How do we create Packages

• Consider our HelloWorld.java class, • Say we want to create a package world which

will contain the HelloWorld class. • All classes in the package must contain the package Keyword at the start

• First thing we have to do is to specify the keyword package with the name of the package we want to use (world in our case) on top of our source file, before the code that defines the real classes in the package, as shown in our HelloWorld class below:

HelloWorld.java

• package world;

• public class HelloWorld {

• public static void main(String[] args) { System.out.println("Hello World");

• }

• }

Create a world directory

• One thing you must do after creating a package for the class is to create nested subdirectories to represent package hierachy of the class. In our case, we have the world package, which requires only one directory. So, we create a directory world and put our HelloWorld.java into it.

• Then to use it we can use the import statement at the start of another class

• Consider

• The following variations of our Book class

Bookjg1.java• package jgpack;• public class Bookjg1 {

• // Declare instance fields• public String name;• public int price;

• // The constructor initialises the instance fields• public Bookjg1 (String n, int p) {• name = n;• price = p;• }

• // a method to output the object details• public void write() {• System.out.println(name + " for £nookjg1" +price);• }• }

Bookjg2.java• package jgpack;• public class Bookjg2 {

• // Declare instance fields• String name;• int price;

• // The constructor initialises the instance fields• public Bookjg2 (String n, int p) {• name = n;• price = p;• }

• // a method to output the object details• public void write() {• System.out.println(name + " for £nookjg2" +price);• }• }

Using a mkdir command

• We create a directory jgpack;

• We put these files into a directory jgpack;

Going back to c:\

• We have a modified version of BookStore1

BookStorejg1.java

• import jgpack.*;• public class BookStorejg1 {


• // Declare three object variables representing type of

• // goods sold in the store• Bookjg1 TextBook;• Bookjg2 Novel,ShortStory;

•

Part 2• // The constructor for the program is• // where the initial work gets done• BookStorejg1 () {

• // Create three objects with different initial values• TextBook = new Bookjg1("Java Programming", 6);• Novel = new Bookjg2("The Unsung Hero", 30);• ShortStory = new Bookjg2("Stories for 5 year olds", 80);



•

Part 3



from its constructor• new BookStorejg1 ();• }• }

BookStorejg1.java

• Note at the start we use the statement• import jgpack.*;• The wildcard * gets all files in the package jgpack• Then we use these classes for declaring our book

objects• // goods sold in the store• Bookjg1 TextBook;• Bookjg2 Novel,ShortStory;

•


• // Create three objects with different initial values• TextBook = new Bookjg1("Java Programming", 6);• Novel = new Bookjg2("The Unsung Hero", 30);• ShortStory = new Bookjg2("Stories for 5 year olds", 80);



•

Here

• We use the constructors to create new instances of these objects and procede as before

We can also use subdirectories

• Say we create a subdirectory for jgpack called subjgpack then we can create a subpackage for jgpack

• We include another variation of book in here

Our variation bookstore is now

Part 1• import jgpack.*;• import jgpack.subjgpack.*;• public class BookStorejg2 {


• // Declare three object variables representing type of• // goods sold in the store• Bookjg1 TextBook;• Bookjg2 Novel;• SubBookjg3 ShortStory;

• // The constructor for the program is• // where the initial work gets done• BookStorejg2 () {

•

Part 2• // Create three objects with different initial values• TextBook = new Bookjg1("Java Programming", 6);• Novel = new Bookjg2("The Unsung Hero", 30);• ShortStory = new SubBookjg3("Stories for 5 year olds", 80);



• // The program control class must have a main method• public static void main (String[] args) {• // Start the program running from its constructor• new BookStorejg2 ();• }• }

• However another way to do this is

Part 1• import jgpack.*;• //import jgpack.subjgpack.*;• public class BookStorejg3 {


• // Declare three object variables representing type of• // goods sold in the store• Bookjg1 TextBook;• Bookjg2 Novel;• jgpack.subjgpack.SubBookjg3 ShortStory;

•

Exercise 1

• Design A Person Class which models the listing in a mobile phone address book.

• Name, Mobile Number ,E-mail• The methods in this class should• 1: List the phone number of the person given

their name• 2: List the Name of the person given their Phone

number• 3: Display a persons email address;• Write a second class which uses the Person

Class

Exercise 2

• Construct a Class to represent a Grocery Item

• The following methods should be included• Method to display Stocklevel• Method to display Price• Method to sell quantity of item• Method to buy in quantity of item• Design another class which uses this class

Exercise 3

• Write a class which represents a date

• Provide methods which get the day, the month and the year

• Provide a method which displays the date

• Provide a method which calculates the successor of a given date

• Design a class which uses the date class

Exercise 4

• Write a class which represents a video in a video store

• Provide methods which display the Rental cost, the title and the status of the video i.e. whether it is in or out of the store

• A method to rent the video• A method to return the video• A method to change the price of renting the

video• A class which uses this video class


• // Create three objects with different initial values• TextBook = new Bookjg1("Java Programming", 6);• Novel = new Bookjg2("The Unsung Hero", 30);• ShortStory = new jgpack.subjgpack.SubBookjg3("Stories for 5 year olds",

80);



•

Part 3



from its constructor• new BookStorejg3 ();• }• }

some background activities around classes the last day!!!! we discussed some classes and programs...

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