Answers to Questions and Exercises: Java Applets - Java Tutorial 5.0 英文版

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Trail: Deployment
Lesson: Applets

Answers to Questions and Exercises: Java Applets

Questions

Question: Which classes can an applet extend?
Answer: 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.

Question: How do you cause the applet GUI in the browser to be refreshed when data in it may have changed?
Answer: You invoke the repaint() method. This method causes the paint method, which is required in any applet, to be invoked again, updating the display.

Question: For what do you use the start() method?
Answer: 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.

Question: True or false: An applet can make network connections to any host on the Internet.
Answer: False: An applet can only connect to the host that it came from.

Question: How do you get the value of a parameter specified in the APPLET tag from within the applet's code?
Answer: You use the getParameter("Parameter name") method, which returns the String value of the parameter.

Question: True of false: An applet can run multiple threads.
Answer: True. The paint and update methods are always called from the AWT drawing and event handling thread. You can have your applet create additional threads, which is recommended for performing time-consuming tasks.

Question: Match the following tag names with the descriptions in the following lists:

EMBED tag
APPLET tag
OBJECT tag

Use to deploy applets to a multi-browser environment.
Use to deploy applets that are to be used only with the Mozilla family of browsers
Use to deploy applets that are to be used only with Internet Explorer

Answer:

EMBED tag: b
APPLET tag: a
OBJECT tag: c

Exercises

Exercise: For an applet using the Exercise class, write the Applet tag that sets the ButtonName parameter to Save.
Answer:
<APPLET CODE=AppletButton.class> 
  <PARAM NAME=ButtonName VALUE=Save>
</APPLET> 
Exercise: Write the method to display an applet in the browser, so that the contents are contained in a rectangle around the phrase "Exercise Applet". Have the applet be one pixal less than the size specified ont he web page, and the phrase starting at the coordinates 5, 15.
Answer:
public void paint(Graphics g) {
	//Draw a Rectangle around the applet's display area.
        g.drawRect(0, 0, 
		   getWidth() - 1,
		   getHeight() - 1);

	//Draw the current string inside the rectangle.
        g.drawString("Exercise Applet", 5, 15);
    }

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