问题和练习的答案：Applets

问题

1. 问题：Which classes can an applet extend?

   答案：An applet can extend the java.applet.Applet class or the java.swing.JApplet class.

   The java.applet.Applet class extends the java.awt.Panel class and enables you to use the GUI tools in the AWT package.

   The java.swing.JApplet class is a subclass of java.applet.Applet that also enables you to use the Swing GUI tools.

2. 问题：For what do you use the start() method?

   答案：You use the start() method when the applet must perform a task after it is initialized, before receiving user input. The start() method either performs the applet's work or (more likely) starts up one or more threads to perform the work.

3. 问题：对或错:An applet can make network connections to any host on the Internet.

   答案：False: An applet can only connect to the host that it came from.

4. 问题：How do you get the value of a parameter specified in the JNLP file from within the applet's code?

   答案：You use the getParameter("Parameter name") method, which returns the String value of the parameter.

5. 问题：Which class enables applets to interact with JavaScript code in the applet's web page?

   答案：The netscape.javascript.JSObject class enables applets to interact with JavaScript code in the applet's web page.

6. 问题：对或错:Applets can modify the contents of the parent web page.

   答案：True:Applets can modify the contents of the parent web page by using the getDocument method of the com.sun.java.browser.plugin2.DOM class and the Common DOM API.

练习

1. 练习：The Exercise applet's parent web page has a JavaScript variable called memberId. Write the code to set the value of the memberId equal to "123489" in the applet's start method.

   答案：

   import java.applet.Applet;
   import netscape.javascript.*; // add plugin.jar to
                                 // classpath during
                                 // compilation
   
   public class Exercise extends Applet {
       public void start() {
           try {
               JSObject window =
                   JSObject.getWindow(this);
               window.setMember("memberId", "123489");
           } catch (JSException jse) {
               jse.printStackTrace();
           }
       }
   }