Sams Teach Yourself Visual J++ 6 in 21 Days
(Publisher: Macmillan Computer Publishing)
Author(s): Rick Leinecker
ISBN: 0672313510
Publication Date: 11/01/98

Listing 1.1 The Java Source Code for the Hello1 Applet

1   // Hello1.java
2
3   import java.awt.*;
4   import java.applet.*;
5
6   /**
7    * This class reads PARAM tags from its HTML host page and sets
8    * the color and label properties of the applet. Program execution
9    * begins with the init() method.
10   */
11  public class Hello1 extends Applet
12  {
13      /**
14       * The entry point for the applet.
15       */
16      public void init()
17      {
18          initForm();
19
20          usePageParams();
21
22          // TODO: Add any constructor code after initForm call.
23      }
24
25      private    final String labelParam = "label";
26      private    final String backgroundParam = "background";
27      private    final String foregroundParam = "foreground";
28
29      /**
30       * Reads parameters from the applet's HTML host and sets applet
31       * properties.
32       */
33      private void usePageParams()
34      {
35          final String defaultLabel = "Default label";
36          final String defaultBackground = "C0C0C0";
37          final String defaultForeground = "000000";
38          String labelValue;
39          String backgroundValue;
40          String foregroundValue;
41
42          /**
43           * Read the <PARAM NAME="label" VALUE="some string">,
44           * <PARAM NAME="background" VALUE="rrggbb">,
45           * and <PARAM NAME="foreground" VALUE="rrggbb"> tags from
46           * the applet's HTML host.
47           */
48          labelValue = getParameter(labelParam);
49          backgroundValue = getParameter(backgroundParam);
50          foregroundValue = getParameter(foregroundParam);
51
52          if ((labelValue == null) || (backgroundValue == null) ||
53              (foregroundValue == null))
54          {
55              /**
56               * There was something wrong with the HTML host tags.
57               * Generate default values.
58               */
59              labelValue = defaultLabel;
60              backgroundValue = defaultBackground;
61              foregroundValue = defaultForeground;
62          }
63
64          /**
65           * Set the applet's string label, background color, and
66           * foreground colors.
67           */
68          label1.setText(labelValue);
69          label1.setBackground(stringToColor(backgroundValue));
70          label1.setForeground(stringToColor(foregroundValue));
71          this.setBackground(stringToColor(backgroundValue));
72          this.setForeground(stringToColor(foregroundValue));
73      }
74
75      /**
76       * Converts a string formatted as "rrggbb" to an awt.Color object
77       */
78      private Color stringToColor(String paramValue)
79      {
80          int red;
81          int green;
82          int blue;
83
84          red = (Integer.decode("0x" +
            ⇒paramValue.substring(0,2))).intValue();
85          green = (Integer.decode("0x" +
            ⇒paramValue.substring(2,4))).intValue();
86          blue = (Integer.decode("0x" +
            ⇒paramValue.substring(4,6))).intValue();
87
88          return new Color(red,green,blue);
89      }
90
91      /**
92       * External interface used by design tools to show properties of
         ⇒an applet.
93       */
94      public String[][] getParameterInfo()
95      {
96          String[][] info =
97          {
98              { labelParam, "String", "Label string to be displayed" },
99              { backgroundParam, "String", "Background color, format
                 ⇒\"rrggbb\"" },
100             { foregroundParam, "String", "Foreground color, format
                 ⇒\"rrggbb\"" },
101         };
102         return info;
103     }
104
105     Label label1 = new Label();
106
107     /**
108      * Initializes values for the applet and its components
109      */
110     void initForm()
111     {
112         this.setBackground(Color.lightGray);
113         this.setForeground(Color.black);
114         label1.setText("label1");
115         this.setLayout(new BorderLayout());
116         this.add("North",label1);
117     }
118 }

Visual J++ creates HTML code for you whenever an applet project is created. You can see the HTML code for our program in Listing 1.2.

Listing 1.2 The HTML Code for the Hello1 Project

1   <HTML>
2   <HEAD>
3   <TITLE>Hello1 Program</TITLE>
4   </HEAD>
5   <BODY>
6
7   <!-- Insert HTML here -->
8       <applet
9           code=Hello1.class
10          name=Hello1
11          width=320
12          height=200 >
13          <param name=label value="Hello Visual J++ World.">
14          <param name=background value="008080">
15          <param name=foreground value="FFFFFF">
16      </applet>
17
18  </BODY>
19  </HTML>

What Comes First

One of the first things you’ll need to learn is in which order the common Java methods are executed. These common methods are ones that can be overridden in your applet classes. It’s important because you might be expecting things to be initialized when you hit the constructor code, and they’re blowing up your applet with millions of divide-by-zero exceptions. Knowing the order will allow you to do everything in the proper sequence and avoid initialization and setup pitfalls.

Overriding methods is a way in which classes that extend other classes can use their own method rather than the method that’s part of the class from which they’re extended. This is explained in more detail in Day 5’s chapter, “Frame Windows.”

Member Variable Declarations

The first thing most applet classes have are variable and class declarations. When variable declarations come first, this makes it easier for others to understand the program. This is because they don’t have to scan through the source code to find the variables. For this reason, it’s recommended that you place variables at the beginning of the applet class. Local variables are the obvious exception. They still have to be in the method that declares and uses them.

Variable initialization can be done at the same time as the declaration. You can set integer values or allocate a class. Following are some examples of legal initializations:

int x = 0;
int y = 0;
int horizontal = 0, vertical = 0;
String str1 = new String();
String str2 = new String( "This is my string" );

If you’re declaring variables at the beginning of the class, you can’t then assign them with regular code. In fact, you can’t just stick code in the applet—code must be within a method. Here are some examples of declarations with legal and illegal initialization code:

public class ThreadedHelloWorld extends Applet implements Runnable
{
  int x, y;

  x = 2;     // ILLEGAL-not in a method
  y = x * 4; // ILLEGAL-not in a method

  public void InitXY()
  {
    x = 2;     // LEGAL-in a method
    y = x * 4; // LEGAL-in a method
  }

}