This chapter is from the book
This chapter is from the book
This chapter is from the book
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
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.4
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 Fortetm for Javatm, 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
To create a source file, you have two options. You can save the file HelloWorldApp.java5 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.
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!");
}
}
NOTE
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.
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.
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
command—you must reenter the drive letter, followed by a colon.
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.
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.
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.
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 HotJavatm 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.
Create a Source File
You have two options to create a source file. You can save the files HelloWorld.java and Hello.html6 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.
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:
<HTML>
<HEAD>
<TITLE>A Simple Program</TITLE>
</HEAD>
<BODY>
Here is the output of my program:
<APPLET CODE="HelloWorld.class" WIDTH=150 HEIGHT=25>
</APPLET>
</BODY>
</HTML>
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.
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:\jdk1.3\bin 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:\jdk1.3\bin or the directory where you saved the Java 2 SDK, followed by \bin. 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).