Java Programming, 3e Concepts and Techniques Chapter 2 - Part 2 Creating a Java Application and Applet

Chapter 2: Creating a Java Application and Applet 2 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 2: Creating a Java Application and Applet 3 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

Chapter 2: Creating a Java Application and Applet 4 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

Chapter 2: Creating a Java Application and Applet 5 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

Chapter 2: Creating a Java Application and Applet 6 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

Chapter 2: Creating a Java Application and Applet 7 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

Chapter 2: Creating a Java Application and Applet 8 Debugging the Solution

Chapter 2: Creating a Java Application and Applet 9 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)

Chapter 2: Creating a Java Application and Applet 10 Editing the Source Code

Chapter 2: Creating a Java Application and Applet 11 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.

Chapter 2: Creating a Java Application and Applet 12 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

Chapter 2: Creating a Java Application and Applet 13

Chapter 2: Creating a Java Application and Applet 14 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();

Chapter 2: Creating a Java Application and Applet 15 Format Output Using Escape Characters Use escape characters inside String arguments to move the output of data

Chapter 2: Creating a Java Application and Applet 16 Editing the Source Code - cont. Recompile and run the application –The bytecode should be updated after any changes to the source code

Chapter 2: Creating a Java Application and Applet 17 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

Chapter 2: Creating a Java Application and Applet 18 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

Chapter 2: Creating a Java Application and Applet 19

Chapter 2: Creating a Java Application and Applet 20 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

Chapter 2: Creating a Java Application and Applet 21 Adding the Java SDK packages Add: import java.util.Date; import java.awt.*;

Chapter 2: Creating a Java Application and Applet 22 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

Chapter 2: Creating a Java Application and Applet 23 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

Chapter 2: Creating a Java Application and Applet 24 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

Chapter 2: Creating a Java Application and Applet 25 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

Chapter 2: Creating a Java Application and Applet 26 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

Chapter 2: Creating a Java Application and Applet 27 Final Result…

Chapter 2: Creating a Java Application and Applet 28 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