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

Chapter Objectives Write a simple Java application Use TextPad Understand the different types and uses of comments Use proper naming conventions for classes and files

Chapter Objectives Identify the parts of a class header and method header Code output Use the println() method Compile a Java program

Chapter 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

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

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

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

Program Development

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

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

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

Coding the Program - Comments as Documentation

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

Coding the Program - The Class Header Use braces { } after the class header to enclose the class body

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

Coding the Program - The Method Header

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 without creating a subclass or instance Return value is the data type of the data returned by the method If no data is returned, the keyword void is used 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

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 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

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 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)

Editing the Source Code

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 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

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

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

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

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

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

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

Chapter Summary Write a simple Java application Use TextPad Understand the different types and uses of comments Use proper naming conventions for classes and files

Chapter Summary Identify the parts of a class header and method header Code output Use the println() method Compile a Java program

Chapter Summary 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 Summary 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 Complete