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Applets Java API
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Applet Examples Java Programs:
Application programs Applet programs Applets are designed to run on Web pages. Here are a couple of applet, from JSDK examples. (These can be found in j2sdk1.4/demo/applets.) Clock Molecule 1 Molecule 2 Molecule 3 Tic-Tac-Toe
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Applet Restrictions Applets have restrictions for security reasons.
Cannot read or write files on the user's computer, to prevent damaging files and spreading viruses. Cannot run any program on the user's system. Otherwise, the local system could be destroyed or modified. Cannot connect the user's computer to any other computer, except to the server where the applets are hosted. Without this restriction, an applet may connect the user's computer to another computer without the user's knowledge.
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<applet> Tag in HTML Page
<html> <head> <title>Applet Demo</title> </head> <body> <h1>An Applet Demo</h1> <applet code="Sample.class“ codebase=“code/applets” width="200" height="200> </applet> </body> </html>
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Applet Class You create an applet by extending java.applet.Applet class. Applet has no main(), since the program is executed automatically by the browser. Applet contains several methods, which can be overridden by the programmer. public void init() public void stop() public void destroy() public void paint()
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Applet Class (cont.) public void init() -- called when the applet is loaded, as the enclosing Web page is opened. This method might include creating objects that the applet needs--e.g., creating an interface, setting fonts and loading images, and reading applet parameters from the web page. Like a constructor in application. public void stop()--called when the user leaves the page containing the applet. The method may be overridden by a code, for example, that would stop any processing to conserve resources. In such a case, a corresponding code must be included in the start() method to restart the processing.
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Applet Class (cont.) public void destroy()--called when the browser exits normally. The stop() method is always called before the this method in called. public void paint(Graphcs g)--called whenever the applet displays something on the screen, such as text, line, color, or image. The method is called automatically whenever the window containing an applet is redrawn, for example, after visiting a different page. Ordinarily, the methods init() and the paint() are overridden. Others can be left alone, in which case, nothing specific will be done when they are invoked by the browser.
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Example: Applet Welcome
Image of an applet interface
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Welcome.java Source Code
import java.applet.Applet; import java.awt.Graphics; import java.awt.Color; public class Welcome extends Applet { String msg = "Welcome to the world “ “of Applets."; public void paint(Graphics g) { setBackground(Color.cyan); g.drawOval(5, 50, 200, 40); g.drawString(msg, 15, 75); g.drawLine(5, 100, 205, 100); } }
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Welcome.java Source Code
Note: Only paint(Graphics g) method needs to be over-ridden. Graphics g is the handle to the graphics environment for drawing figures and texts in the applet. setBackground(Color.cyan) sets background color of the applet—think of it as a frame
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Welcome.html <html> <head>
<title>Welcome</title> </head> <body> <h1>Welcome Message</h1> <hr> <applet code="Welcome.class“ codebase=“subfolder/” //optional width=220 height=120> </applet> </body> </html> View Applet
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Interactive Applet Welcome2
Applet Welcome2 prompts the user to click a button, causing an input panel to pop up and to display a welcome message to a text field. It incorporates event detection and event handler in the same way that you have already seen with application programs. Run Welcome2 Code Welcome2.java
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Welcome2 (cont.) In the source code, note:
Interface has three objects, but on btnStart is associated with an event. In init() method: Objects are created They are arranged in a panel Button is associated with event listener Panel is inserted into the applet In actionPerformed() method: yourName = JOptionPane.showInputDialog pops
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Applet with Parameters
In Welcome example the background color (cyan) was hard-coded in the applet code. It would be more convenient if the color can be changed during its use in the Web page. The color information can be received by the applet as a parameter from the Web page.
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Applet with Parameters (cont.)
<html> <head> <title>Welcome</title> </head> <body> <h1>Applet with Parameter</h1> <applet code="Welcome3.class" width=300 height=100> <param name="bgcolor" value="green"> </applet> </body> </html>
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Applet with Parameters (cont.)
Using the same Welcome3 applet: Web Page 1 Web Page 2 Web page 3 Welcome3.java Source Code
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Applet with Paramers (cont.)
Note: String color = getParameter("bgcolor"); captures the value of parameter “bgcolor” from the HTML page. if (color.equals("green")) setBackground(Color.green); else if Not color == “green”, because String variable is a reference.
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Applet to Display Graph
Next example uses an applet (Bargraph) to display a bar graph. Values of bar heights are captured from the Web page.
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Applet to Display a Graph (cont.)
<html> <head> <title>Graph Test</title></head><body> <applet code="BarGraph.class" width=300 height=200> <param name="title" value="Quiz Scores"> <param name="num0" value="85"> <param name="num1" value="90"> <param name="num2" value="73"> <param name="num3" value="100"> <param name="num4" value="82"> <param name="num5" value="68"> </applet> </body> </html>
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Applet to Display a Graph (cont.)
barGraph.HTML BarGraph.java Source Code BAR_WIDTH, MAX_HEIGHT, & GAP are constants that specify the bars and the spacing between them. title = getParameter("title"); captures the value of the “title” parameter. value = getParameter("num" + count); In the HTML page, parameters are named “num0”, “num1”, etc., so that the applet code can use a loop to form those names.
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Applet to Display a Graph (cont.)
Values of parameters are stored in array numList[]. In the paint() method, g.setColor(Color.BLUE); Once a color is set, it remains the same for subsequent drawings until it is changed. Both Color.BLUE & Color.blue are OK. fillRectangle() uses the values in numList[] to draw rectangles.
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With Different Parameters
<html> <head> <title>Graph Test</title></head><body> <applet code="BarGraph.class" width=300 height=200> <param name="title" value="Quiz Scores"> <param name="num0" value=“20"> <param name="num1" value=“40"> <param name="num2" value=“60"> <param name="num3" value=“80"> <param name="num4" value=“100"> <param name="num5" value=“120"> </applet> </body> </html> Run barGraph2.html
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Converting Application to Applet
To show the relationship between an application and an applet, the next example shows how to convert a previous application—ConverDistance.java—to an applet. Application ConvertDistance.java Source Code
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Converting Application to Applet
Extend Applet, instead of Frame class DistanceConverter extends Applet implements ActionListener { instead of class DistanceConverter extends JFrame implements ActionListener { Applet has no constructor. Method init() takes its place. Most of code in constructor is appropriate in init(), exept for super(), setSize(), setDefaultCloseOperation() , and setVisitble(true) . Applet ConvertDistance.java Source Code
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