Java Programming, 2EIntroductory Concepts

and Techniques

Chapter 2Creating a Java

Application and Applet

2Chapter 2: Creating a Java Application and Applet

Objectives

• Write a simple Java application

• Use TextPad

• Understand the different types and uses of comments

• Use proper naming conventions for classes and files

3Chapter 2: Creating a Java Application and Applet

Objectives

• Identify the parts of a class header and method header

• Code output

• Use the println() method

• Compile a Java program

4Chapter 2: Creating a Java Application and Applet

Objectives

• Understand the common types of errors

• Run a Java Program

• Edit Java source code to insert escape characters and a system date

• Print source code

5Chapter 2: Creating a Java Application and Applet

Objectives

• Differentiate between an application and an applet

• Create an applet from Java source code

• Write code to display a graphic, text, color, and the date in an applet

• Create an HTML host document

• Run a Java applet

6Chapter 2: Creating a Java Application and Applet

Introduction

• Users enter data and instructions into a computer and receive feedback from the computer through a user interface

• Programmers can create many types of user interfaces in Java

• We will create a program with two types of user interfaces– Console application

• Command line interface

– Applet• Graphical user interface displayed in a browser

7Chapter 2: Creating a Java Application and Applet

The Welcome to My Day Program

• This program will display a splash screen– A splash screen is a screen that is displayed

before the main program starts

• The screen will contain a welcome message, user’s name, and system date – The console application will display text only– The applet will contain text, color, and a

graphic

8Chapter 2: Creating a Java Application and Applet

9Chapter 2: Creating a Java Application and Applet

Program Develpment

10Chapter 2: Creating a Java Application and Applet

Analysis and Design

• Verify that the requirements are specific enough

• Design the user interface using a storyboard

• Design the program logic using a flowchart and event diagram

11Chapter 2: Creating a Java Application and Applet

12Chapter 2: Creating a Java Application and Applet

13Chapter 2: Creating a Java Application and Applet

14Chapter 2: Creating a Java Application and Applet

Using TextPad

• TextPad has several window areas– Coding window – Selector window– Clip Library window

• TextPad can display line numbers– Helpful for finding compiler errors

• TextPad has color-coding capabilities– Save a document before entering code to

enable Java related color-coding

15Chapter 2: Creating a Java Application and Applet

Coding the Program -Comments as Documentation • Purpose of comments

– Provides clear description when reviewing code– Helps programmer think clearly when coding

• Placement of comments– Use a comment header to identify a file and its

purpose– Place a comment at the beginning of code for each

event and method– Place comments near portions of code that need

clarification

16Chapter 2: Creating a Java Application and Applet

Coding the Program -Comments as Documentation

17Chapter 2: Creating a Java Application and Applet

Coding the Program - The Class Header • Identify how the code can be accessed with an

access modifier– public indicates that the code can be accessed by all

objects in the program and can be extended for a subclass

• Specify a unique name for the class– The class name at the beginning of the program must

match the file name exactly– Java is case-sensitive – By convention, uppercase letters are used for class

names and to distinguish words in class names

18Chapter 2: Creating a Java Application and Applet

Coding the Program - The Class Header

• Use braces {} after the class header to enclose the class body

19Chapter 2: Creating a Java Application and Applet

Coding the Program -The Method Header • The method header contains modifiers, return value,

method name, and parameters along with their data type• Every stand-alone Java application must contain a

main() method, which is the starting point during execution

20Chapter 2: Creating a Java Application and Applet

Coding the Program -The Method Header

21Chapter 2: Creating a Java Application and Applet

Coding the Program -The Method Header • Modifiers set properties for a method

– public allows other programs to invoke this method– static means this method is unique and can be

invoked with creating an instance

• Parameters are pieces of data received by the method to help the method perform its operation– Identifiers are used to name the variable sent to the

method

• Return type is the data type of the data returned by the method– If no data is returned, the keyword void is used

22Chapter 2: Creating a Java Application and Applet

Coding Output

• Call the System.out.println() method in the SDK to display output to the monitor– System is the class– out is the object representing the output device– println() is the method

23Chapter 2: Creating a Java Application and Applet

Coding Output

• When calling a method, arguments are placed in parentheses– String literals are placed in quotation marks– Numeric literals and variables do not need quotation

marks

• Period delimiters separate the class, object, and method

• Semicolons must be placed after every statement except headers and braces

• Braces {} enclose the body of a method

24Chapter 2: Creating a Java Application and Applet

Testing the Solution

• Compile the source code– javac.exe command

• In TextPad, use the Compile Java command• At the command prompt, type javac filename.java

– A new bytecode file for each class is created with a .class extension

• If the compiler detects errors, fix the errors and compile again

• If the compilation was successful, run the program

25Chapter 2: Creating a Java Application and Applet

Debugging the Solution

• System Errors– System command is not set properly– Software is installed incorrectly– Location of stored files is not accessible

• Syntax Errors– One or more violations of the syntax rules of Java

• Semantic Errors– The code meaning is unrecognizable to the compiler

• Logic and Run-Time Errors– Unexpected conditions during execution of a program

26Chapter 2: Creating a Java Application and Applet

Debugging the Solution

27Chapter 2: Creating a Java Application and Applet

Running the Application

• After compilation is successful, run the program to test for logic and run-time errors

• Use the Run Java Application command in TextPad– TextPad automatically finds the class file with the

same name

• Use the java command from the command prompt – Syntax: java classname (no extension)

28Chapter 2: Creating a Java Application and Applet

Editing the Source Code

29Chapter 2: Creating a Java Application and Applet

Import Packages

• Use the import statement to access classes in the SDK– The java.lang package is automatically

imported– Place the import statement before the class

header– Use an asterisk (*) after the package name

and period delimiter to import all necessary classes in the package

30Chapter 2: Creating a Java Application and Applet

31Chapter 2: Creating a Java Application and Applet

Call a System Date Constructor

• Use the Date class in the java.util package to access the system date

• Store the Date in an object variable• Declare the object variable by calling the Date

constructor– The constructor is a method denoted by the new

keyword followed by the object type and parentheses– Declaration syntax:

objectType variableName = new objectType();

32Chapter 2: Creating a Java Application and Applet

Format Output Using Escape Characters• Use escape characters inside String arguments

to move the output of data

33Chapter 2: Creating a Java Application and Applet

Editing the source code - cont.

• Recompile and run the application– The bytecode should be updated after any

changes to the source code

• Print a hard copy of the source code– The final step of the program development

cycle is to document the solution

• Quit TextPad by clicking on the Close button

34Chapter 2: Creating a Java Application and Applet

Moving to the Web

• Characteristics of an applet – Applets run within a browser/viewer and are usually

delivered to the client machine via the Web– Applets cannot use system resources or files on the

client machine

• Convert the application into an applet– Import two packages– Change the class name and extend the Applet class– Include a paint method to draw text and display color

and a graphic

35Chapter 2: Creating a Java Application and Applet

Import Applet Packages

• Applet package (java.applet.*)– Allows applets to inherit attributes and

methods

• AWT package (java.awt.*)– Provides access to color, draw methods, and

GUI elements

36Chapter 2: Creating a Java Application and Applet

37Chapter 2: Creating a Java Application and Applet

Change the Class Name and Extend the Applet Class• Change the class name and file name to

create a new applet file

• Edit the comment header in the applet file

• Add “extends Applet” in the class header to inherit from the superclass, Applet– Provides the init() method to load the applet in

the browser window

38Chapter 2: Creating a Java Application and Applet

The paint() Method

• Accepts a Graphics object as a parameter

• The Graphics object is commonly referred to by the variable name g– The variable g is created and initialized in the

init() method– The variable g is a reference variable, or a

specific instance of an object

• The return type is void

39Chapter 2: Creating a Java Application and Applet

The drawString() Method

• Displays text in the applet window

• Accepts three arguments– The String data

• If the data is not a String object, convert it to a String object using the toString() method

– The horizontal and vertical coordinates of the String

• The coordinates are measured in pixels

• Called by the Graphics object, g

40Chapter 2: Creating a Java Application and Applet

Draw an Image

• Declare an Image object• Use the getImage() method to load the image

– The getImage() method calls the getDocumentBase() method to pull the image from the current folder

• Use the drawImage() method to set the coordinates of the image

41Chapter 2: Creating a Java Application and Applet

Set the Background Color

• Use the setBackground() method to change the background color of the applet window– The setBackground() method does not need to be

called from a reference variable

42Chapter 2: Creating a Java Application and Applet

Creating an HTML Host Document• A host program, such as a Web page executes

the applet

43Chapter 2: Creating a Java Application and Applet

Creating an HTML Host Document• The Web page contains HTML tags to

define a section or format– A tag consists of a start tag, denoted by <>

and an end tag, denoted by </>

• The tag, <APPLET>…</APPLET>, informs the browser of the applet– The applet tag encloses the name of the

bytecode applet file and the width and height of the applet window

44Chapter 2: Creating a Java Application and Applet

Running an Applet

• An applet is run by opening the HTML host document

• In TextPad, use the Run Java Applet command• At the command prompt, type appletviewer

followed by the name of the host document• Use Applet Viewer to test the applet

– Ignores irrelevant HTML code– Uses less memory than a browser– Does not have to be Java-enabled

45Chapter 2: Creating a Java Application and Applet

Chapter Summary

• Use TextPad to write, compile, and run code• Learn the basic form of an application and an

applet • Insert comments as documentation• Code Class and Method headers• Create a console application

– Use the println() method– Format output using escape characters– Import the java.util package– Call a Date constructor

46Chapter 2: Creating a Java Application and Applet

Chapter Summary

• Compile a Java program• Debug a Java program

– Differentiate between types of errors

• Execute a Java program• Edit the application to create an applet

– Import applet packages– Extend the Applet class– Use the paint(), drawString(), and getImage methods

• Create a HTML Host Document

Java Programming, 2EIntroductory Concepts

and Techniques

Chapter 2 Complete