chapter 2 - part 2

Java Programming, 3eConcepts and Techniques

Chapter 2 - Part 2Creating a Java

Application and Applet

2Chapter 2: Creating a Java Application and Applet

Chapter Objectives

• Code output• Use the println() method• Compile a Java program• 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


Chapter 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


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 default display– println() is the method


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


Testing the Solution

• Compile the source code– Clean and Build

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

• If the compilation was successful, run the program


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


Debugging the Solution


Running the Application

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

• Use the Run command

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


Editing the Source Code


Java Code Packages

• The Java Software Development Toolkit (SDK) provides libraries of code which can be used in programs.

• System.out.println() is a method provided in the SDK.


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


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();


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

to move the output of data


Editing the Source Code - cont.

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

changes to the source code


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


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


Creating a Java Applet Class

• Right click on the Welcome package and select New– If “Applet” is not listed click on “Other” and

choose “Applet” and click Next

• Set the Class Name to “WelcomeApplet” and the package to “welcome”

• Click Finish


Adding the Java SDK packages

• Add:import java.util.Date;

import java.awt.*;


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 a reference variable, or a

specific instance of an object

• The return type is void


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


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


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


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

the applet

• If you run the file, NetBeans will build a HTML file for you in the “build” folder.– Copy the image.gif file into the folder and open the

HMTL


Final Result…


Rest of Today

• Create the Java Applet for the Welcome Program– Add your name to Applet– Pick a different color

• Show me the result when your done

chapter 2 - part 2

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