An applet is called from within an HTML script with the APPLET tag, such as: <applet code="test.class" width=200 height=300></applet>

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1 6 Java Applets 6.1 Introduction As has been previously discussed a Java program can either be run as an applet within a WWW browser (such as Microsoft Internet Explorer or Netscape Communicator) or can be interpreted as a standalone program. The basic code within each program is almost the same and they can be easily converted from one to the other (typically a Java program will be run through an interpreter to test its results and then converted to run as an applet). 6.2 Applet tag An applet is called from within an HTML script with the APPLET tag, such as: <applet code="test.class" width=200 height=300></applet> which loads an applet called Test.class and sets the applet size to 200 pixels wide and 300 pixels high. Table 6.1 discusses some optional parameters. Table 6.1 Other applet HTML parameters Applet parameters CODEBASE=codebaseURL CODE=appletFile ALT=alternateText NAME=appletInstanceName Description Specifies the directory (codebaseurl) that contains the applet s code. Specifies the name of the file (appletfile) of the compiled applet. Specifies the alternative text that is displayed if the browser cannot run the Java applet. Specifies a name for the applet instance (appletinstancename). This makes it possible for applets on the same page to find each other. 67

2 Table 6.1 Other applet HTML parameters (cont.) Applet parameters WIDTH=pixels HEIGHT=pixels ALIGN=alignment VSPACE=pixels HSPACE=pixels Description Specifies the initial width and height (in pixels) of the applet. Specifies the alignment of the applet. Possible values are: left, right, top, texttop, middle, absmiddle, baseline, bottom and absbottom. Specifies the number of pixels above and below the applet (VSPACE) and on each side of the applet (HSPACE) Applet viewer A useful part of the JDK tools is an applet viewer which is used to test applets before they are run within the browser. The applet viewer on the PC version is AppletViewer.exe and the supplied argument is the HTML file that contains the applet tag(s). It then runs all the associated applets in separate windows. 6.3 Creating an applet Java applet 6.1 shows a simple Java applet which displays two lines of text and HTML script 6.1 shows how the applet integrates into an HTML script. First the Java applet (j1.java) is created. In this case the edit program is used. The directory listing below shows that the files created are chap6_01. java and chap6_01.html (note that Windows NT/95 displays the 8.3 filename format on the left-hand side of the directory listing and the long filename on the right-hand side). Java applet 6.1 (chap6_01.java) import java.awt.*; import java.applet.*; public class chap6_01 extends Applet g.drawstring("this is my first Java",5,25); g.drawstring("applet...",5,45); HTML script 6.1 (chap6_01.html) <HTML><TITLE>First Applet</TITLE> <APPLET CODE=chap6_01.class WIDTH=200 HEIGHT=200></APPLET></HTML> 68 Mastering Java

3 Sample run 6.1 C:\java\src\chap6> edit chap6_01.java C:\java\src\chap6> edit chap6_01.html C:\java\src\chap6> dir CHAP6_~1 HTM /05/98 18:40 chap6_01.html CHAP6_~1 JAV /05/98 22:35 chap6_01.java Next the Java applet is compiled using the javac.exe program. It can be seen from the listing that, if there are no errors, the compiled file is named chap6_01.class. This can then be used, with the HTML file, to run as an applet. Sample run 6.2 C:\java\src\chap6> javac chap6_01.java C:\java\src\chap6> dir CHAP6_~1 HTM /05/98 18:40 chap6_01.html CHAP6_~1 JAV /05/98 18:43 chap6_01.java CHAP6_~1 CLA /05/98 18:43 chap6_01.class C:\java\src\chap6> appletviewer chap6_01.html 6.4 Applet basics Java applet 6.1 recaps the previous Java applet. This section analyses the main parts of this Java applet. Java applet 6.1 (chap6_01.java) import java.awt.*; import java.applet.*; public class chap6_01 extends Applet g.drawstring("this is my first Java",5,25); g.drawstring("applet...",5,45); Applet class The start of the applet code is defined in the form: public class chap6_01 extends Applet which informs the Java compiler to create an applet named chap6_01 that extends the existing Applet class. The public keyword at the start of the statement allows the Java browser to run the applet, while if it is omitted the browser cannot access the applet. Java applets 69

4 The class keyword is used to creating a class object named chap6_01 that extends the applet class. After this the applet is defined between the left and right braces (grouping symbols) Applet methods Methods allow Java applets to be split into smaller sub-tasks (just as C uses functions). These methods have the advantage that: They allow code to be reused. They allow for top-level design. They make applet debugging easier as each method can be tested in isolation to the rest of the applet. A method has the public keyword, followed by the return value (if any) and the name of the method. After this the parameters passed to the method are defined within rounded brackets. Recapping from the previous example: g.drawstring("this is my first Java",5,25); g.drawstring("applet...",5,45); This method has the public keyword which allows any user to execute the method. The void type defines that there is nothing returned from this method and the name of the method is paint(). The parameter passed into the method is g which has the data type of Graphics. Within the paint() method the drawstring() method is called. This method is defined in java. awt.graphics class library (this library has been included with the import java.awt.* statement. The definition for this method is: public abstract void drawstring(string str, int x, int y) which draws a string of characters using the current font and colour. The x,y position is the starting point of the baseline of the string (str). It should be noted that Java is case sensitive and the names given must be referred to in the case that they are defined as. 6.5 The paint() object The paint() object is the object that is called whenever the applet is redrawn. It will thus be called whenever the applet is run and then it is called whenever the applet is redisplayed. 70 Mastering Java

drawstring("value Square Cube",5,10); for (i=0;i<10;i++) g.drawstring(""+ i,5,20+10*i); g.drawstring(""+ i*i,45,20+10*i); g.drawstring(""+ i*i*i,85,20+10*i); HTML script 6.2 (chap6_02.

5 Java applet 6.2 shows how a for() loop can be used to display the square and cube of the values from 0 to 9. Notice that the final value of i within the for() loop is 9 because the end condition is i<10 (while i is less than 10). Java applet 6.2 (chap6_02.java) import java.awt.*; import java.applet.*; public class chap6_02 extends Applet int i; g.drawstring("value Square Cube",5,10); for (i=0;i<10;i++) g.drawstring(""+ i,5,20+10*i); g.drawstring(""+ i*i,45,20+10*i); g.drawstring(""+ i*i*i,85,20+10*i); HTML script 6.2 (chap6_02.html) <HTML> <TITLE>First Applet</TITLE> <APPLET CODE=chap6_02.class WIDTH=200 HEIGHT=200></APPLET></HTML> Java applet 6.3 uses a for() loop and the if() statement to test if a value is less than, equal to or greater than 5. The loop is used to repeat the test 10 times. Java applet 6.3 (chap6_03.java) import java.awt.*; import java.applet.*; import java.lang.math; public class chap6_03 extends Applet int i,x; double val; for (i=0;i<10;i++) val=math.random(); x=(int)(val*10.0); // Convert value between 0 and 10 if (x<5) g.drawstring("less than 5",5,20+i*10); else if (x==5) g.drawstring("equal to 5",5,20+i*10); else g.drawstring("greater than 5",5,20+i*10); Java applets 71

6 HTML script 6.3 (chap6_03.html) <HTML> <TITLE>First Applet</TITLE> <APPLET CODE=chap6_03.class WIDTH=200 HEIGHT=200> </APPLET></HTML> The random() method is used to generate a value between 0 and 1, the returned value is then multiplied by 10 so as to convert into a value between 0 and 10. Then it is converted to an integer using the data type modifier (int). The if() statement is then used to test the value. 6.6 Exercises Write a Java applet which displays the following table of powers. Value Square Cube Fourth power Write a Java applet which displays the following table of square root values from 1 to 15. Value Square root Write a Java applet which display 20 random numbers from between 0 and Mastering Java

7 6.6.4 Convert the following exercises so that they run within an applet: (i) Exercise (ii) Exercise (iii) Exercise (iv) Project at the end of Chapter 5. (v) Exercise Write a Java applet that simulates the rolling of two dice. A sample output is: Dice 1: 3 Dice 2: 5 Total: Using a Java applet determine the results of the following: (i) (ii) tan 1 ( 0.5) (iii) sin( 2.1) (iv) (v) (vi) e 2.5 (Hint: use atan()) 6.7 Project Write a Java applet which contains all the cards in a deck. It will then give the deck a shuffle and display the cards in the deck. Java program 6.4 shows an outline of a program but does not contain all of the cards in the deck. Sample run 6.3 shows a sample run of the program. Implement the following: (i) Convert the program to an applet. (ii) Add the rest of the card to the deck. (iii) Increase the number of shuffles. (iv) Add suits to the deck. Java program 6.4 (chap6_04.java) import java.lang.math; public class chap6_04 Java applets 73

8 public static void main(string[] args) String names[]="ace","king", "Queen","Jack", "10"; Cards card = new Cards(); card.name=names; card.nshuffle=20; // 20 shuffles of cards System.out.println("Before shuffle"); card.show(); card.shuffle(); System.out.println("After shuffle"); card.show(); class Cards String name[]; int nshuffle; public void show() int i; for (i=0;i<name.length;i++) System.out.println(name[i]); public void shuffle() int i, pos1, pos2; String tempstr; for (i=0;i<nshuffle;i++) pos1=(int) ((name.length)*math.random()); pos2=(int) ((name.length)*math.random()); tempstr=name[pos1]; name[pos1]=name[pos2]; name[pos2]=tempstr; Sample run 6.3 Before shuffle Ace King Queen Jack 10 After shuffle King 10 Jack Queen Ace 74 Mastering Java

SIMPLE APPLET PROGRAM

SIMPLE APPLET PROGRAM APPLETS Applets are small applications that are accessed on Internet Server, transported over Internet, automatically installed and run as a part of web- browser Applet Basics : - All applets are subclasses