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Java in a Nutshell

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1.2 A Simple Example

By now you should have a pretty good idea of why Java is such an interesting language. So we'll stop talking about abstract concepts and look at some concrete Java code. Before we look at an interesting applet, however, we are going to pay tribute to that ubiquitous favorite, "Hello World."

Hello World

Example 1.1 shows the simplest possible Java program: "Hello World."

Example 1.1: Hello World

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

This program, like every Java program, consists of a public class definition. The class contains a method named main(), which is the main entry point for all Java applications--that is, the point at which the interpreter starts executing the program. The body of main() consists of only a single line, which prints out the message:

Hello World!

This program must be saved in a file with the same name as the public class plus a .java extension. To compile it, you would use javac: [1]

[1] Assuming you're using Sun's Java Development Kit (JDK). If you're using a Java development environment from some other vendor, follow your vendor's instructions.

% javac HelloWorld.java

This command produces the HelloWorld.class file in the current directory. To run the program, you use the Java interpreter, java:

% java HelloWorld

Note that when you invoke the interpreter, you do not supply the .class extension for the file you want to run.

A Scribble Applet

Example 1.2 shows a less trivial Java program. This program is an applet, rather than a standalone Java application like the "Hello World" program above. Because this example is an applet, it has a different structure than a standalone application; notably, it does not have a main() method. Like all applets, this one runs inside an applet viewer or Web browser, and lets the user draw (or scribble) with the mouse, as illustrated in Figure 1.1.

One of the major changes between Java 1.0 and Java 1.1 is in the way that Java programs are notified of "events", such as mouse motion. Example 1.2 uses the Java 1.0 event model rather than the preferred Java 1.1 event model. This is because the current generation of Web browsers (as this is written) still use Java 1.0. In order for this applet to be widely usable, it is coded with the old, "deprecated" event model. [2]

[2] If you are interested in updating this program to use Java 1.1, see Chapter 7, Events for information on how to use the new 1.1 event model. In addition, you need to change the call to bounds() in the action() method to a call to getBounds(), if you want to avoid a compilation warning about using a deprecated method.

Example 1.2: A Java Applet

import java.applet.*;
import java.awt.*;
public class Scribble extends Applet {
  private int last_x, last_y;                // Store the last mouse position.
  private Color current_color = Color.black; // Store the current color.
  private Button clear_button;               // The clear button.
  private Choice color_choices;              // The color dropdown list.
  // This method is called to initialize the applet.
  // Applets don't have a main() method.
  public void init() {
    // Set the background color.
    this.setBackground(Color.white);
    // Create a button and add it to the applet.  Set the button's colors.
    clear_button = new Button("Clear");
    clear_button.setForeground(Color.black);
    clear_button.setBackground(Color.lightGray);
    this.add(clear_button);
    // Create a menu of colors and add it to the applet.
    // Also set the menu's colors and add a label.
    color_choices = new Choice();
    color_choices.addItem("black");
    color_choices.addItem("red");
    color_choices.addItem("yellow");
    color_choices.addItem("green");
    color_choices.setForeground(Color.black);
    color_choices.setBackground(Color.lightGray);
    this.add(new Label("Color: "));
    this.add(color_choices);
  }
  // This method is called when the user clicks the mouse to start a scribble.
  public boolean mouseDown(Event e, int x, int y)
  {
    last_x = x; last_y = y;
    return true;
  }
  // This method is called when the user drags the mouse.
  public boolean mouseDrag(Event e, int x, int y)
  {
    Graphics g = this.getGraphics();
    g.setColor(current_color);
    g.drawLine(last_x, last_y, x, y);
    last_x = x;
    last_y = y;
    return true;
  }
  // This method is called when the user clicks the button or chooses a color.
  public boolean action(Event event, Object arg) {
    // If the Clear button was clicked on, handle it.
    if (event.target == clear_button) {
      Graphics g = this.getGraphics();
      Rectangle r = this.bounds();
      g.setColor(this.getBackground());
      g.fillRect(r.x, r.y, r.width, r.height);
      return true;
    }
    // Otherwise if a color was chosen, handle that.
    else if (event.target == color_choices) {
      if (arg.equals("black")) current_color = Color.black;
      else if (arg.equals("red")) current_color = Color.red;
      else if (arg.equals("yellow")) current_color = Color.yellow;
      else if (arg.equals("green")) current_color = Color.green;
      return true;
    }
    // Otherwise, let the superclass handle it.
    else return super.action(event, arg);
  }
}

Don't expect to be able to understand the entire applet at this point. It is here to give you the flavor of the language. In Chapter 2, How Java Differs from C and Chapter 3, Classes and Objects in Java we'll explain the language constructs you need to understand the example. Then, in Chapter 6, Applets and Chapter 7, Events we'll explain the applet and event-handling concepts used in this example.

The first thing you should notice when browsing through the code is that it looks reassuringly like C and C++. The if and return statements are familiar. Assignment of values to variables uses the expected syntax. Procedures (called "methods" in Java) are recognizable as such.

The second thing to notice is the object-oriented nature of the code. As you can see at the top of the example, the program consists of the definition of a public class. The name of the class we are defining is Scribble; it is an extension, or subclass, of the Applet class. (The full name of the Applet class is java.applet.Applet. One of the import statements at the top of the example allows us to refer to Applet by this shorter name.)

Classes are said to "encapsulate" data and methods. As you can see, our Scribble class contains both variable and method declarations. The methods are actually defined inside of the class. The methods of a class are often invoked through an instance of the class. Thus you see lines like:

color_choices.addItem("black");

This line of code invokes the addItem() method of the object referred to by the color_choices variable. If you're a C programmer, but not a C++ programmer, this syntax may take a little getting used to. We'll see lots more of it in Chapters 2 and 3. Note that this is a keyword, not a variable name. It refers to the current object; in this example, it refers to the Scribble object.

The init() method of an applet is called by the Web browser or applet viewer when it is starting the applet up. In our example, this method creates a Clear button and a menu of color choices, and then adds these GUI components to the applet.

The mouseDown() and mouseDrag() methods are called when the user clicks and drags the mouse. These are the methods that are responsible for drawing lines as the user scribbles. The action() method is invoked when the user clicks on the Clear button or selects a color from the menu of colors. The body of the method determines which of these two "events" has occurred and handles the event appropriately. Recall that these methods are part of the Java 1.0 event model. Chapter 7, Events explains this model and also explains the Java 1.1 event model that replaces it.

To compile this example, you'd save it in a file named Scribble.java and use javac:

% javac Scribble.java

This example is an applet, not a standalone program like our "Hello World" example. It does not have a main() method, and therefore cannot be run directly by the Java interpreter. Instead, we must reference it in an HTML file and run the applet in an applet viewer or Web browser. It is the applet viewer or Web browser that loads the applet class into its running Java interpreter and invokes the various methods of the applet at the appropriate times. To include the applet in a Web page, we'd use an HTML fragment like the following:

<APPLET code="Scribble.class" width=500 height=300>
</APPLET>

Example 1.3 shows a complete HTML file that we might use to display the applet. Chapter 15, Java-Related HTML Tags explains the HTML syntax for applets in full detail.

Example 1.3: An HTML File that Contains an Applet

<HTML>
<HEAD>
<TITLE>The Scribble Applet</TITLE>
</HEAD>
<BODY>
Please scribble away in the applet below.
<P>
<APPLET code="Scribble.class" width=500 height=300>
Your browser does not support Java, or Java is not enabled. Sorry!
</APPLET>
</BODY>
</HTML>

Suppose we save this example HTML file as Scribble.html. Then to run this applet, you could use Sun's appletviewer command like this:

% appletviewer Scribble.html

You could also display the applet by viewing the Scribble.html file in your Web browser, if your browser supports Java applets. Figure 1.1 showed the Scribble applet running in Netscape Navigator.


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