Red Hat Linux 9 Professional Secrets [Electronic resources] نسخه متنی

Writing Your First Java Program

In the grand old tradition of C programming books, I'll begin by showing you how to display

"Hello, World!" from a Java program. However, before I proceed, I should point out the difference between three types of Java programs:

• 
  

  Standalone Java application-This is a Java program that can be executed by a Java interpreter. A Java standalone application is just like the standalone C or C++ programs that you might know. The application has a main method and the java interpreter can run the application. General-purpose Java applications take the form of standalone applications.
  



• 
  

  Java applet-This is a Java class that can be loaded and executed by the appletviewer program or by a Java-capable Web browser. You have to first embed the applet inside an HTML document using the <

  applet > tag and then load that HTML document to activate the applet. As a Webmaster, you'll mostly write Java applets.



• 
  

  Java servlet-This is a Java class loaded and executed by a Web server (or a server that helps the Web server such as the Apache Tomcat server). To develop servlets, you need the Java 2 Enterprise Edition.

Whether you write a standalone application or an applet or a servlet, the basic steps are similar except for the last step when you execute the program. You use the java interpreter to run standalone programs and appletviewer to test applets. For servlets, you simply place them in a specific directory and insert HTML code to refer to the servlets. These development steps are as follows:

1. 
   

   Use a text editor to create the Java source file. A Java application is a class (a type of object-collection of data and methods). The source file's name must be the class name with a

   .java extension.



2. 
   

   Process the source file with javac (the Java compiler) to generate the class file with a

   .class extension.



3. 
   

   If it's a standalone application, run the class with the java interpreter with the following command:

   java classname
   

   
   


4. 
   

   If the program is an applet, create an HTML document and embed the applet using the <

   applet > tag (see 'The <applet> Tag' section for the syntax). Then, load that HTML document using

   appletviewer or a Java-capable Web browser. When the browser loads the HTML document, it also activates all embedded Java applets.

Writing a Standalone 'Hello, World!' Program

The classic C-style

"Hello, World!" application is easy to write in Java. That's because many of Java's syntactical details are similar to that of C and C++. The following code implements a simple

"Hello, World!" program in Java (you have seen this in an earlier section, but it's so short that I'll just show you again):

public class HelloWorld
{
public static void main(String[] args)
{
System.out.println("Hello, World!");
}
}

Even this simple Java program illustrates some key features of Java:

• 
  

  Every Java program is a public class definition.



• 
  

  A standalone application contains a

  main method that must be declared

  public static void . The interpreter starts execution at the

  main method.

Save this program in a file named

HelloWorld.java . Then, compile it with the following command:

javac HelloWorld.java

PATH=$PATH:/usr/java/j2sdk1.4.1_01/bin

The

javac compiler creates a class file named

HelloWorld.class . To run the application, use the Java interpreter and specify the class name as a parameter, like this:

java HelloWorld

The program prints the following output:

Hello, World!

Writing a Simple Java Applet

The other model of a Java program is the applet that runs inside the

appletviewer program or a Java-capable Web browser. Specifically, a Java applet is a class in the Java Abstract Windowing Toolkit (AWT). In an applet, you do not have to provide a

main method. Instead, you provide a

paint method where you place code to draw in an area of a window. You can use the applet model to implement GUIs and other graphical programs.

For a

"Hello, World!" applet, I'll do the following:

• 
  

  Instead of displaying the message in a default font, pick a specific font to display the message.



• 
  

  Use the information about the font sizes to center the message within the area where the applet is required to display its output.



• 
  

  Draw the text in red instead of the default black color.

Listing 26-1 shows the Java code that implements the

"Hello, World!" applet.

Listing 26-1: The HelloWorld Java Applet

//---------------------------------------------------------------
// File: HelloWorld.java
//
// Displays "Hello, World!" in Helvetica font and in red color.
//---------------------------------------------------------------
import java.applet.*;
import java.awt.*;
//---------------------------------------------------------------
// H e l l o W o r l d
//
// Applet class to display "Hello, World!"
public class HelloWorld extends java.applet.Applet
{
String hellomsg = "Hello, World!";
//---------------------------------------------------------------
// p a i n t
//
// Method that paints the output
public void paint(java.awt.Graphics gc)
{
// Draw a rectangle around the applet's bounding box
// so we can see the box.
gc.drawRect(0, 0, getSize().width-1, 
getSize().height-1);
// Create the font to be used for the message.
Font helv = new Font("Helvetica", Font.BOLD, 24);
// Select the font into the Graphics object.
gc.setFont(helv);
// Get the font metrics (details of the font size).
FontMetrics fm = gc.getFontMetrics();
int mwidth = fm.stringWidth(hellomsg);
int ascent = fm.getAscent();
int descent = fm.getDescent();
// Compute starting (x.y) position to center string
// The size() method returns size of the applet's
// bounding box.
int xstart = getSize().width/2 - mwidth/2;
int ystart = getSize().height/2 + ascent/2 - descent/2;
// Set the color to red.
gc.setColor(Color.red);
// Now draw the string.
gc.drawString(hellomsg, xstart, ystart);
}
}

By browsing through this code, you can learn a lot about how to display graphics output in Java. Here are the key points to note:

• 
  

  The

  import statement lists external classes that this program uses. The name that follows the import statement can be the name of a class or can be a name with a wildcard (*), which tells the Java compiler to import all the classes in a package. This example uses the

  Applet class as well as a number of graphics classes that are in the

  java.awt package.



• 
  

  The

  HelloWorld applet is defined as an extension of the

  Applet class. That's what the statement

  public class HelloWorld extends java.applet.Applet means.
  



• 
  

  An applet's

  paint method contains the code that draws the output. The

  paint method receives a

  Graphics object as argument. You have to call methods of the

  Graphics object to display output.



• 
  

  The

  getSize method returns the size of the applet's drawing area.



• 
  

  To use a font, you have to first create a

  Font object and then call the

  setFont method of the

  Graphics object.



• 
  

  To draw text in a specific color, invoke the

  setColor method of the

  Graphics object with an appropriate

  Color object as argument.



• 
  

  If you know C++, you'll notice that Java's method invocation is similar to the way you call the member function of a C++ object. Indeed, there are many similarities between C++ and Java.

Save the listing in a file named

HelloWorld.java . Then, compile it with the command:

javac HelloWorld.java

This step creates the applet class file:

HelloWorld.class . To test the applet, you have to create an HTML document and embed the applet in that document, as shown in the following example:

<html>
<head>
<title>Hello, World! from Java</title>
</head>
<body>
<h3>"Hello, World!" from Java</h3>
A Java applet is given an area where it displays
its output. In this example, the applet draws a
border around its assigned area and then displays
the "Hello, World!" message centered in that box.
<br>
<applet code=HelloWorld width=200 height=60>
If you see this message, then you do not have a
Java-capable browser.
</applet>
Here is the applet!
</body>
</html>

As this HTML source shows, you have to use the <

applet > tag to insert an applet in an HTML document. In the 'Using the <applet> Tag' section, you'll learn the detailed syntax of the <

applet > tag.

You can use two tools to test the applet. The first one is

appletviewer , which comes with JDK. To view the

HelloWorld applet, you have to run the

appletviewer program and provide the name of the HTML document that includes the applet. Suppose that the HTML document is in the file named

hellol . Then, you'd run

appletviewer with the following command:

appletviewer hellol

Figure 26-1 shows how the

appletviewer displays the applet.

Figure 26-1: Running the 'Hello, World!' Java Applet in

appletviewer .

Notice that

appletviewer displays only the applet; the rest of the text in the HTML document is ignored. However, the appearance is quite different in a Java-capable Web browser.

To view the applet in a Web browser, start the Mozilla Web browser and select File>Open File from the menus. From the Open File dialog, go to the directory where you have the

hellol file and the

HelloWorld.class file (for the applet). Then, select the

hellol file, and click Open. Mozilla then renders the HTML document containing the

HelloWorld applet.

Unlike the

appletviewer , Mozilla should display the entire HTML document and the Java applet appears embedded in the document just like an image. The applet draws inside the rectangle assigned to it through the width and height attributes of the <

applet > tag.

If Mozilla fails to display the applet when you load an HTML document with an embedded applet, the most likely cause is that you have not yet installed Sun's Java plug-in for Netscape (Mozilla was derived from Netscape). The Java plug-in comes with the Java 2 SDK that you have already downloaded. To install the plug-in, you have to copy a file from the Java 2 SDK directory to the Mozilla plug-in directory. Here are the commands I type (after logging in as

root ) to install the Java plug-in file for J2EE version 1.4.1:

cd /usr/java/j2sdk1.4.1_01/jre/plugin/i386/ns610
cp libjavaplugin_oji.so /usr/lib/mozilla-1.2.1/plugins

To check that the plug-ins are loaded, start Mozilla and select Help>About Plug-ins from the menus. Mozilla then displays information about the installed plug-ins in its window. In particular, you should see a plug-in that can handle the MIME data type

application/x-java-vm because that's the data type for a Java applet.

For the latest information about the Java plug-in, point your Mozilla browser to

http://java.sun.com/products/plugin/ .