First Steps (Win32)

The following detailed instructions will help you write your first program. These instructions are for users on Win32 platforms, which include Windows 95/98 and Windows NT/ 2000. (UNIX and Linux instructions are on page 16. Users on MacOS platforms can find instructions on page 24.) We start with a checklist of what you need to write your first program. Next, we cover the steps to creating an application, steps to creating an applet, and explanations of error messages you may encounter.

A Checklist

To write your first program, you need

  1. The Java 2 SDK, Standard Edition: The Java 2 SDK software is included on the CD that accompanies this book. You can download this platform to your PC or check http://java.sun.com/products/ for the latest version. [1]

    [1] Before version 1.2, the software development kit (SDK) provided by Sun Microsystems was called the "JDK," or Java Development Kit.

  2. A text editor: In this example, we'll use NotePad, the simple editor included with the Windows platforms. To find NotePad, go to the Start menu and select Programs > Accessories > NotePad. You can easily adapt these instructions if you use a different text editor.

Note

You may want to consider using an IDE to help you write your programs. Java 2 SDK, Standard Edition v. 1.3, is available bundled with an IDE, the Forte™ for Java™, Community Edition. This version is included on this book's CD.

 

Creating Your First Application

Your first program, HelloWorldApp, will simply display the greeting "Hello World!" To create this program, you complete each of the following steps.

  • Create a source file. A source file contains text, written in the Java programming language, that you and other programmers can understand. You can use any text editor to create and to edit source files.
  • Compile the source file into a bytecode file. The compiler takes your source file and translates the text into instructions that the Java VM can understand. The compiler converts these instructions into a bytecode file.
  • Run the program contained in the bytecode file. The Java interpreter installed on your computer implements the Java VM. This interpreter takes your bytecode file and carries out the instructions by translating them into instructions that your computer can understand.

Create a Source File

graphics/intfig04.gif

To create a source file, you have two options. You can save the file HelloWorldApp.java [1] on your computer and avoid a lot of typing. Then you can go straight to the second step of compiling the source file (page 10). Or, you can follow these longer instructions.

[1] Throughout this book, we use the CD icon to indicate that the code (in this case, HelloWorldApp.java) is available on the CD and online. See Code Samples (page 43) for the file location on the CD and online.

First, start NotePad. In a new document, type in the following code:

/** 
 * The HelloWorldApp class implements an application that 
 * displays "Hello World!" to the standard output. 
 */ 
public class HelloWorldApp { 
 public static void main(String[] args) { 
 // Display "Hello World!" 
 System.out.println("Hello World!"); 
 } 
} 

Be Careful When You Type

Type all code, commands, and file names exactly as shown. The compiler and interpreter are case sensitive, so you must capitalize consistently. In other words, HelloWorldApp is not equivalent to helloworldapp.

Second, save this code to a file. From the menu bar, select File > Save As. In the Save As dialog, do the following.

  • Using the Save in drop-down menu, specify the folder (directory) where you'll save your file. In this example, the folder is java on the C drive.
  • In the File name text box, type "HelloWorldApp.java", including the double quotation marks. The quotation marks force the file to be saved as a .java file rather than as a ".txt" text file.
  • From the Save as type drop-down menu, choose Text Document.

When you're finished, the dialog box should look like Figure 5. Now click Save and exit NotePad.

Figure 5. Saving the HelloWorldApp.java file with the correct .java extension.

graphics/01fig05.gif

Compile the Source File

From the Start menu, select the MS-DOS Prompt (Windows 95/98) or Command Prompt (Windows NT/2000) application. When the application launches, it should look like Figure 6.

Figure 6. The prompt in the MS-DOS Prompt application.

graphics/01fig06.gif

The prompt shows your current directory. When you bring up the prompt for Windows 95/ 98, your current directory is usually WINDOWS on your C drive (as shown in Figure 6) or WINNT for Windows NT. To compile your source code file, change your current directory to the one in which your file is located. For example, if your source directory is java on the C drive, you would type the following command at the prompt and press Enter:

cd c:java 

Now the prompt should change to C:java. To change to a directory on a different drive, you must type an extra command. As shown in Figure 7, to change to the java directory on the D drive, you must reenter the drive, d:.

Figure 7. Changing to a directory on another drive requires an extra commandyou must reenter the drive letter, followed by a colon.

graphics/01fig07.gif

In our example, the java directory is on the C drive. The dir command lists the files in your current directory. If your current directory is C:java and you enter dir at the prompt, you should see your file (Figure 8).

Figure 8. HelloWorldApp.java listed in the current directory.

graphics/01fig08.gif

Now you can compile. At the prompt, type the following command and press Enter:

javac HelloWorldApp.java 

If your prompt reappears without error messages, congratulations. You have successfully compiled your program. If you encounter errors, see Error Explanations (Win32) (page 14) to help you fix the problems.

The compiler has generated a Java bytecode file, HelloWorldApp.class. At the prompt, type dir to see the new file that was generated (Figure 9). Now that you have a .class file, you can run your program.

Figure 9. After you compile HelloWorld.java, the bytecode file, HelloWorldApp.class, is created in the same directory.

graphics/01fig09.gif

Run the Program

In the same directory, enter the following command at the prompt:

java HelloWorldApp 

Figure 10 shows what you should see.

Figure 10. When you run your HelloWorldApp program, you should see this result.

graphics/01fig10.gif

Creating Your First Applet

HelloWorldApp is an example of an application, a standalone program. Now you will create an applet called HelloWorld, which also displays the greeting "Hello World!" Unlike HelloWorldApp, however, the applet runs in a Java-enabled Web browser, such as the HotJava™ browser, Netscape Navigator, or Microsoft Internet Explorer.

To create this applet, you'll perform the basic steps as before: create a source file, compile the source file, and run the program. However, unlike for an application, you must also create an HTML file.

Creat a Source File

graphics/intfig04.gif

You have two options to create a source file. You can save the files HelloWorld.java and Hello.html [1] on your computer and avoid a lot of typing. Then you can go straight to the second step of compiling the source file (page 14). Or, you can follow these instructions.

[1] HelloWorld.java and Hello.html are available on this book's CD and online. See Code Samples (page 43).

First, start NotePad. In a new document, type the following code:

import java.applet.Applet; 
import java.awt.Graphics; 

public class HelloWorld extends Applet { 
 public void paint(Graphics g) { 
 // Display "Hello world!" 
 g.drawString("Hello world!", 50, 25); 
 } 
} 

Save this code to a file called HelloWorld.java.

Second, you also need an HTML file to accompany your applet. Type the following code into a new NotePad document:


 

A Simple Program

Here is the output of my program:

Save this code to a file called Hello.html.

Compile the Source File

At the prompt, type the following command and press Return:

javac HelloWorld.java 

The compiler should generate a Java bytecode file, HelloWorld.class.

Run the Program

Although you can use a Web browser to view your applets, you may find it easier to test your applets by using the simple appletviewer application that comes with the Java platform. To view the HelloWorld applet using appletviewer, enter at the prompt:

appletviewer Hello.html 

Figure 11 shows what you should see.

Figure 11. The successful execution of the HelloWorld applet.

graphics/01fig11.gif

Congratulations! Your applet works. If you encounter errors, see Common Problems and Their Solutions (page 391) to help you fix the problems.

Error Explanations (Win32)

Here we list the most common errors users have when compiling and running their first application or applet using the Java 2 SDK or an earlier JDK. For more error explanations, consult the section Common Problems and Their Solutions (page 391).

Bad command or file name (Windows 95/98)

The name specified is not recognized as an internal or external command, operable program or batch file (Windows NT/2000)

If you receive this error, Windows cannot find the Java compiler, javac. Here's one way to tell Windows where to find javac. Suppose that you installed the Java 2 Software Development Kit in C:jdk1.3. At the prompt, you would type the following command and press Enter:

C:jdk1.3in javac HelloWorldApp.java 

Note

If you choose this option, each time you compile or run a program, you must precede your javac and java commands with c:jdk1.3in or the directory where you saved the Java 2 SDK, followed by in. The bin directory contains the compiler and interpreter. To avoid this extra typing, consult the section Update the PATH Variable (Win32) (page 541).

 

Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorldApp

If you receive this error, the interpreter cannot find your bytecode file, HelloWorldApp.class.

One of the places java tries to find your bytecode file is your current directory. So, if your bytecode file is in C, you should change your current directory to that. To change your directory, type the following command at the prompt and press Enter:

cd c: 

The prompt should change to C:. If you enter dir at the prompt, you should see your .java and .class files. Now enter java HelloWorldApp again.

If you still have problems, you might have to change your CLASSPATH variable. To see whether this is necessary, try clobbering the class path with the following command:

set CLASSPATH= 

Now enter java HelloWorldApp again. If the program works now, you'll have to change your CLASSPATH variable. For more information, consult the section Path Help (page 540).





The Java Tutorial(c) A Short Course on the Basics
The Java Tutorial: A Short Course on the Basics, 4th Edition
ISBN: 0321334205
EAN: 2147483647
Year: 2002
Pages: 125
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