The basic applet the framework methods

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Libraries are often grouped according to their functionality. Some libraries, for example, are used as is, off the shelf. The standard Java library String and Vector classes are examples of these. Other libraries are designed specifically as building blocks to build other classes. A certain class of library is the application framework , whose goal is to help you build applications by providing a class or set of classes that produces the basic behavior that you need in every application of a particular type. Then, to customize the behavior to your own needs you inherit from the application class and override the methods of interest. The application framework’s default control mechanism will call your overridden methods at the appropriate time. An application framework is a good example of “separating the things that change from the things that stay the same,” since it attempts to localize all the unique parts of a program in the overridden methods.



init( )

Called when the applet is first created to perform first-time initialization of the applet

start( )

Called every time the applet moves into sight on the Web browser to allow the applet to start up its normal operations (especially those that are shut off by stop( )). Also called after init( ).

paint( )

Part of the base class Component (three levels of inheritance up). Called as part of an update( ) to perform special painting on the canvas of an applet.

stop( )

Called every time the applet moves out of sight on the Web browser to allow the applet to shut off expensive operations. Also called right before destroy( ).

destroy( )

Called when the applet is being unloaded from the page to perform final release of resources when the applet is no longer used

// Very simple applet
package c13;
import java.awt.*;
import java.applet.*;
public class Applet1 extends Applet {
  public void paint(Graphics g) {
    g.drawString("First applet", 10, 10);
} ///:~ 

Note that applets are not required to have a main( ). That’s all wired in to the application framework; you put any startup code in init( ).

To run this program you must place it inside a Web page and view that page inside your Java-enabled Web browser. To place an applet inside a Web page you put a special tag inside the HTML source for that Web page [54] to tell the page how to load and run the applet. This is the applet tag, and it looks like this for Applet1:


Testing applets

You can perform a simple test without any network connection by starting up your Web browser and opening the HTML file containing the applet tag. (Sun’s JDK also contains a tool called the appletviewer that picks the <APPLET> tags out of the HTML file and runs the applets without displaying the surrounding HTML text. [55]) As the HTML file is loaded, the browser will discover the applet tag and go hunt for the .class file specified by the code value. Of course, it looks at the CLASSPATH to find out where to hunt, and if your .class file isn’t in the CLASSPATH then it will give an error message on the status line of the browser to the effect that it couldn’t find that .class file.

A more graphical example

The example above isn’t too thrilling, so let’s try adding a slightly more interesting graphic component:

// Easy graphics
import java.awt.*;
import java.applet.*;
public class Applet2 extends Applet {
  public void paint(Graphics g) {
    g.drawString("Second applet", 10, 15);
    g.draw3DRect(0, 0, 100, 20, true);
} ///:~ 

This puts a box around the string. Of course, all the numbers are hard-coded and are based on pixels, so on some machines the box will fit nicely around the string and on others it will probably be off, because fonts will be different on different machines.

There are other interesting things you can find in the documentation for the Graphic class. Any sort of graphics activity is usually entertaining, so further experiments of this sort are left to the reader.


the framework methods

// Shows init(), start() and stop() activities
import java.awt.*;
import java.applet.*;
public class Applet3 extends Applet {
  String s;
  int inits = 0;
  int starts = 0;
  int stops = 0;
  public void init() { inits++; }
  public void start() { starts++; }
  public void stop() { stops++; }
  public void paint(Graphics g) {
    s = "inits: " + inits + 
      ", starts: " + starts +
      ", stops: " + stops;
    g.drawString(s, 10, 10);
} ///:~ 

Normally when you override a method you’ll want to look to see whether you need to call the base-class version of that method, in case it does something important. For example, with init( ) you might need to call super.init( ). However, the Applet documentation specifically states that the init( ), start( ), and stop( ) methods in Applet do nothing, so it’s not necessary to call them here.

When you experiment with this applet you’ll discover that if you minimize the Web browser or cover it up with another window you might not get calls to stop( ) and start( ). (This behavior seems to vary among implementations; you might wish to contrast the behavior of Web browsers with that of applet viewers.) The only time the calls will occur is when you move to a different Web page and then come back to the one containing the applet.

[54] It is assumed that the reader is familiar with the basics of HTML. It’s not too hard to figure out, and there are lots of books and resources.

[55] Because the appletviewer ignores everything but APPLET tags, you can put those tags in the Java source file as comments:

// <applet code=MyApplet.class width=200 height=100></applet>

This way, you can run " appletviewer " and you don’t need to create tiny HTML files to run tests.


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