Chapter 2

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Chapter 2

Getting Started with Java

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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.

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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.

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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.

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2.1 The First Java Program

/*

Chapter 2 Sample Program: Displaying a Window File: 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);

} }

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Fig. 2.1

Result of running the Ch2Sample1 program.

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Fig. 2.2

The program diagram for the Ch2Sample1 program.

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Fig. 2.3

The program diagram for the

Ch2Sample1 program that shows the dependency relationship.

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2.1 The First Java Program

To use an object in a program, first we declare and create and object.

Then we send messages to it.

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2.1 The First Java Program

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.

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2.1 The First Java Program

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

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2.1 The First Java Program

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:

myWindow = new JFrame ( );

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Fig. 2.4

Distinction between object

declaration and object creation.

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Fig. 2.5

Relationship between the state-of- memory diagram and the

program diagram notation.

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Fig. 2.6

The state after two new commands are executed.

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2.1 The First Java Program

The object that receives a message must possess a corresponding

method.

The expressions “sending a

message” and “calling a method”

are synonymous.

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Fig. 2.7

Correspondence between message sending as represented in the

program diagram and in the actual Java statement.

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Fig. 2.8

How the beginning and ending comment markers are matched.

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2.2 Program Components

Below are the program

components for the program Ch2Sample1:

•Comments

•Import Statement

•Class Declaration

•Method Declaration

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2.2 Program Components

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 */.

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2.2 Program Components

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:

This notation is called dot notation.

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2.2 Program Components

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

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2.2 Program Components

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

declaring a class is

class <class name> {

}

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

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2.2 Program Components

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.

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2.2 Program Components

A sample method declaration and its components.

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Fig. 2.9

A program template for simple Java applications.

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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 );

} }

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Fig. 2.10

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

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Fig. 2.11

The same window after a picture is drawn.

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Fig. 2.12

The program diagram for the FunTime program.

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Fig. 2.13

A MiniBrowser object displaying a sample web page.

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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.

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2.3 Edit-Compile-Run Cycle

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.

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2.3 Edit-Compile-Run Cycle

A sample source file and its bytecode file.

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2.3 Edit-Compile-Run Cycle

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.

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2.3 Edit-Compile-Run Cycle

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

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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.

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2.4 Sample Java Standard Classes

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

JOptionPane.showMessageDialog(null,

“I Love Java”);

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Fig. 2.14

A simple “message” dialog created by the showMessageDialog

method of the JOptionPane class.

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Fig. 2.15

A dialog with multiple lines of text.

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2.4 Sample Java Standard Classes

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));

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Fig. 2.16

A dialog showing the substring of

“espresso” from index position 2 to 6.

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Fig. 2.17

Individual characters in a string are numbered from 0.

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Fig. 2.18

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

remains intact.

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2.4 Sample Java Standard Classes

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.

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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.

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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.

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Fig. 2.20

The program diagram for Ch2Monogram.

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2.5 Sample Development:

Printing the Initials

/*

Chapter 2 Sample Program: Displays the Monogram File: Step1/Ch2Monogram.java

*/

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);

} }

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2.5 Sample Development:

Printing the Initials

/*

Chapter 2 Sample Program: Displays the Monogram File: Step 2/Ch2MonogramStep2.java

*/

class Ch2Monogram {

public static void main (String [ ] args) { String name, first, middle, last,

space, monogram;

space = " ";

//Input the full name

name = JOptionPane.showInputDialog(null, "Enter your full name (first, middle, last):");

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2.5 Sample Development:

Printing the Initials

//Extract first, middle, and last names

first = 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 monogram

monogram = first.substring(0, 1) +

middle.substring(0, 1) + last.substring(0,1);

//Output the result

JOptionPane.showMessageDialog(null, "Your monogram is " + monogram);

} }

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Fig. 2.21

Apply the two sequences of

indexOf and substring methods to extract three substrings from a given string.