©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 2 Getting Started with Java

©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapter 2

Getting Started with Java


Chapter 2 Objectives

After you have read and studied this chapter, you should be able to

• Identify the basic components of Java programs.

•Write simple Java programs.

•Describe the difference between object declaration and object creation.


Chapter 2 Objectives, cont.

After you have read and studied this chapter, you should be able to

•Describe the process of creating and running Java programs.

•Use the Date, SimpleDateFormat, String, and JOptionPane classes from the standard Java packages.

•Develop Java programs using the incremental development approach.


2.1 The First Java Program

This first program displays a window on the screen.

The size of the window is 300 x 200 pixels, and the default title is My First Java Program.

The fundamental OOP concept illustrated by the program:

•An object-oriented program uses objects.


2.1 The First Java Program/*Chapter 2 Sample Program: Displaying a WindowFile: Ch2Sample1.java

*/

import javax.swing.*;

class Ch2Sample1 {

public static void main(String[ ] args){

JFrame myWindow;

myWindow = new JFrame();

myWindow.setSize(300, 200); myWindow.setTitle(“My First Java Program”); myWindow.setVisible(true);}

}


Fig. 2.1

Result of running the Ch2Sample1 program.


Fig. 2.2

The program diagram for the Ch2Sample1 program.


Fig. 2.3

The program diagram for the Ch2Sample1 program that shows the dependency relationship.



To use an object in a program, first we declare and create and object. Then we send messages to it.



When we declare an object, we must give it a name. A Java identifier is a sequence of letters, digits, underscores, and dollar signs used to name a class, object, method, etc.

The first character in a Java identifier must be a letter.



Uppercase and lowercase letters are distinguished.

myWindow, mywindow, MYWINDOW

No spaces are allowed in an identifier.

Java naming conventions dictate the use of an uppercase letter for the first letter of class names, and a lowercase letter for the first letter of object names.•class Account•object account



No objects are created by the declaration. The declaration only assigns an identifier to the object.

We create an object by invoking the new operation:

<object name> = new <class name> (<arguments>);

myWindow = new JFrame ( );


Fig. 2.4

Distinction between object declaration and object creation.


Fig. 2.5

Relationship between the state-of-memory diagram and the program diagram notation.


Fig. 2.6

The state after two new commands are executed.



The object that receives a message must possess a corresponding method.

The expressions “sending a message” and “calling a method” are synonymous.


Fig. 2.7

Correspondence between message sending as represented in the program diagram and in the actual Java statement.


Fig. 2.8

How the beginning and ending comment markers are matched.


2.2 Program Components

Below are the program components for the program Ch2Sample1:•Comments•Import Statement•Class Declaration•Method Declaration



Programs contain comments in which we state the purpose of the program, explain the meaning of code, and provide other descriptions to help programmers use and understand the code.

A comment is any sequence of text that begins with the marker /* and ends with the marker */.



In Java, classes are grouped into packages, and the Java system comes with numerous packages.

To use a class from a package, we refer to the class in the program by using the following syntax:

<package name>.<class name>

This notation is called dot notation.



Using the fully qualified name of a class can be cumbersome. Using the import statement solves this problem.

import javax.swing.*;



A Java program is composed of one or more classes. The syntax for declaring a class is

class <class name> {

<class member declarations>

}

A class member is either a data value or a method.



One of the classes in the program must be designated as the main class.

If we designate a class as a main class, we must define a method called main, because when a Java program is executed, the main method of a main class is executed first.



A sample method declaration and its components.


Fig. 2.9

A program template for simple Java applications.


2.2 Program Components/*Chapter 2 Sample Program: Freehand Drawing

File: Ch2FunTime.java

The program will allow you to draw a picture by dragging a mouse (move the mouse while holding the left mouse button down; hold the button on Mac). To erase the picture and start over, click the right mouse button (command-click on Mac).

*/

import javabook.*;

class Ch2FunTime {

public static void main (String[ ] args) {

SketchPad doodleBoard;doodleBoard = new SketchPad( );doodleBoard.setVisible( true );

}}


Fig. 2.10

The window that appears on the screen when the program starts running.


Fig. 2.11

The same window after a picture is drawn.


Fig. 2.12

The program diagram for the FunTime program.


Fig. 2.13

A MiniBrowser object displaying a sample web page.


2.3 Edit-Compile-Run Cycle

Step One: Edit the program.•Type in the program, using a

text editor, and save the program to a file.

•Use the name of the main class and the suffix .java for the filename.

•This file is called a source file.



Step 2: Compile the source file.•The process of compiling the

source file creates the bytecode file.

•The name of the compiler-generated bytecode file will have the suffix .class while its prefix is the same as the source file’s.



A sample source file and its bytecode file.



Step 3: Execute the bytecode file.•A java interpreter will go

through the bytecode file and execute the instructions in it.

•If your program is error-free, a window will appear on the screen.

•If an error occurs while running the program, the interpreter will catch it and stop its execution.



The result after the interpreter executes the instructions in the bytecode file.


2.4 Sample Java Standard Classes

Using existing Java classes is an important step in becoming a successful and proficient programmer.

We will introduce four standard classes here: •JOptionPane •String•Date•SimpleDateFormat.



Using the JOptionPane class is a simple way to display a result of a computation to the user.

JOptionPane.showMessageDialog(null, “I Love Java”);


Fig. 2.14

A simple “message” dialog created by the showMessageDialog method of the JOptionPane class.


Fig. 2.15

A dialog with multiple lines of text.



The textual values passed to the showMessageDialog method are instances of the String class.

A sequence of characters separated by double quotes is String constants.

String text;

text = “Espresso”;

JOptionPane.showMessageDialog(text.substring(2, 7));


Fig. 2.16

A dialog showing the substring of “espresso” from index position 2 to 6.


Fig. 2.17

Individual characters in a string are numbered from 0.


Fig. 2.18

The effect of the substring method is shown. The original string remains intact.



Date and SimpleDateFormat

The Date class is used to represent a time instance to a millisecond. This class is in the java.util package.

SimpleDateFormat can be used to provide an alternative format to the Date class. This class is in the java.text package.


Fig. 2.19

JOptionPane can also be used for input: Below is the result of calling the showInputDialog method of the JOptionPane class with “What is your name?” as the method’s second argument.


2.5 Sample Development: Printing the Initials

Problem statement:Write an application that asks for the user’s first, middle, and last names and replies with their initials.

Overall plan:•Get the user’s first, middle, and

last names.•Extract the initials to form the

monogram.•Output the monogram.


Fig. 2.20

The program diagram for Ch2Monogram.



/*Chapter 2 Sample Program: Displays the Monogram

File: Step1/Ch2Monogram.java

*/

import javax.swing.*;class Ch2Monogram {public static void main (String [ ] args) {

String name;name = JOptionPane.showInputDialog(null,

"Enter your full name (first, middle, last):");

JOptionPane.showMessageDialog(null, name);}

}



/*Chapter 2 Sample Program: Displays the Monogram

File: Step 2/Ch2MonogramStep2.java

*/import javax.swing.*;

class Ch2Monogram {public static void main (String [ ] args) {

String name, first, middle, last, space, monogram;

space = " ";

//Input the full namename = JOptionPane.showInputDialog(null, "Enter

your full name (first, middle, last):");



//Extract first, middle, and last namesfirst = name.substring(0, name.indexOf(space));name = name.substring(name.indexOf(space)+1, name.length());

middle = name.substring(0, name.indexOf(space));last = name.substring(name.indexOf(space)+1, name.length());

//Compute the monogrammonogram = first.substring(0, 1) + middle.substring(0, 1) + last.substring(0,1);

//Output the resultJOptionPane.showMessageDialog(null, "Your monogram is " + monogram);}

}


Fig. 2.21

Apply the two sequences of indexOf and substring methods to extract three substrings from a given string.

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©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 2 Getting Started with Java