Understanding How Java Programs Work

Size: px

Start display at page:

Download "Understanding How Java Programs Work"

Helena Thompson
5 years ago
Views:

1 HOUR 4 Understanding How Java Programs Work An important distinction to make in Java programming is where your program is supposed to be running. Some programs are intended to work on your computer; you type in a command or click an icon to start them up. Other programs are intended to run as part of a World Wide Web page. You encountered several examples of this type of program during the previous hour s whirlwind vacation. Java programs that run locally on your own computer are called applications. Programs that run on web pages are called applets. During this hour, you ll learn why that distinction is important, and the following topics will be covered:. How applications work. Organizing an application. Sending arguments to an application. How applets work. The required parts of an applet. Sending parameters to an applet. Using HTML tags to put an applet on a page Creating an Application Although Java has become well-known because it can be used in conjunction with World Wide Web pages, you can also use it to write any type of computer program. The Saluton program you wrote during Hour 2, Writing Your First Program, is an example of a Java application. To try out another program, use your word processor to open up a new file and enter everything from Listing 4.1. Remember not to enter the line numbers and colons along the left side of the listing; these items are used to make parts of programs easier to describe in the book. When you re done, save the file as Root.java, making sure to save it in textonly or plain ASCII text format.

2 52 HOUR 4: Understanding How Java Programs Work LISTING 4.1 The Full Text of Root.java 1: class Root { 2: public static void main(string[] arguments) { 3: int number = 225; 4: System.out.println( The square root of 5: + number 6: + is 7: + Math.sqrt(number) ); 8: } 9: } The Root application accomplishes the following tasks:. Line 3: An integer value of 225 is stored in a variable named number.. Lines 4 7: This integer and its square root are displayed. The Math.sqrt (number) statement in Line 7 displays the square root. Before you can test out this application, you need to compile it using the Java Development Kit s javac compiler or the compiler included with another Java development environment. If you re using the JDK, go to a command line, open the folder that contains the Root.java file, and then compile Root.java by entering the following at a command line: javac Root.java If you have entered Listing 4.1 without any typos, including all punctuation and every word capitalized exactly as shown, it should compile without any errors. The compiler responds to a successful compilation by not responding with any message at all. Java applications are compiled into a class file that can be run by a Java interpreter. If you re using the JDK, you can run the compiled Root.class file by typing this command: java Root The output should be the following: The square root of 225 is 15.0 When you run a Java application, the interpreter looks for a main() block and starts handling Java statements within that block. If your program does not have a main() block, the interpreter will respond with an error.

3 Sending Arguments to Applications 53 Sending Arguments to Applications Because Java applications are run from a command line, you can send information to applications at the same time you run them. The following hypothetical example uses the java interpreter to run an application called TextDisplayer, and it sends two extra items of information to the application, readme.txt and /p: java TextDisplayer readme.txt /p Extra information you can send to a program is called an argument. The first argument, if there is one, is provided one space after the name of the application. Each additional argument is also separated by a space. If you want to include a space inside an argument, you must put quotation marks around the argument, as in the following: java TextDisplayer readme.txt /p Page Title This example runs the TextDisplayer program with three arguments: readme.txt, /p, and Page Title. The quote marks prevent Page and Title from being treated as separate arguments. You can send as many arguments as you want to a Java application. In order to do something with them, however, you have to write some statements in the application to handle them. To see how arguments work in an application, create a new file in your word processor called Blanks.java. Enter the text of Listing 4.2 into the file and save it when you re done. Compile the program, correcting any errors that are caused by typos. LISTING 4.2 The Full Text of Blanks.java 1: class Blanks { 2: public static void main(string[] arguments) { 3: System.out.println( The + arguments[0] 4: + + arguments[1] + fox 5: + jumped over the 6: + arguments[2] + dog. ); 7: } 8: } To try out the Blanks application, run it with a Java interpreter such as the JDK s java tool. Give it three adjectives of your own choosing as arguments, as in the following example: java Blanks retromingent purple lactose-intolerant

4 54 HOUR 4: Understanding How Java Programs Work The application uses the adjectives to fill out a sentence. Here s the one produced by the preceding three arguments: The retromingent purple fox jumped over the lactose-intolerant dog. Try it with some of your own adjectives, making sure to always include at least three of them. Arguments are a useful way to customize the performance of a program. They often are used to configure a program so it runs a specific way. Java stores arguments in arrays, groups of related variables that all hold the same information. You ll learn about arrays during Hour 9, Storing Information with Arrays. Watch Out! When you run the Blanks application, you must include at least three words as command-line arguments. If not, you ll see an intimidating error message mentioning an ArrayOutOfBoundsException, something that will be complete gobbledygook for another five hours. Applet Basics When the Java language was introduced in 1995, the language feature that got the most attention was applets, Java programs that run on a World Wide Web page. Before Java, web pages were a combination of text, images, and forms that used Common Gateway Interface programs running on the computer that hosted the pages. These gateway programs required special access to the web server presenting the page. Applets require no special access and can be written by programmers of all skill levels. You ll write several during the span of these 24 hours. You can test them with any web browser that handles Java programs. Most of the Java programs you toured during the previous hour were all applets. Their structure differs from applications in several important ways, and they are designed specifically for presentation on the Web. By the Way Because the applets you ll write in this book use Java 2, these applets must be run with a browser that supports the most current version of the language. During Hour 17, Creating Interactive Web Programs, you ll learn how to set up a browser to use the Java plug-in to run Java 2 applets. All applets can also be tested with the appletviewer tool that is included with the Java Development Kit. Unlike applications, applets do not have a main() block. Instead, they have several different sections that are handled depending on what is happening in the applet, as detailed fully during Hour 17. Two of the sections are the init() block statement and the paint() block. init() is short for initialization, and it is used to take care

5 Applet Basics 55 of anything that needs to be set up as an applet first runs. The paint() block is used to display anything that should be displayed. To see an applet version of the Root application, create a new file in your word processor and call it RootApplet.java. Enter the code in Listing 4.3 and save it when you re done. LISTING 4.3 The Full Text of RootApplet.java 1: import java.awt.*; 2: 3: public class RootApplet extends javax.swing.japplet { 4: int number; 5: 6: public void init() { 7: number = 225; 8: } 9: 10: public void paint(graphics screen) { 11: Graphics2D screen2d = (Graphics2D) screen; 12: screen2d.drawstring( The square root of + 13: number + 14: is + 15: Math.sqrt(number), 5, 50); 16: } 17: } Compile this file using your Java development software. If you are using the javac compiler tool in the JDK, type the following at a command line: javac RootApplet.java This program contains a lot of the same statements as the Root application that did the same thing. The primary difference is in how it is organized the main() block has been replaced with an init() block and a paint() block. The sample programs in this hour are provided primarily to introduce you to the way Java programs are structured. Some aspects of these programs will be introduced fully later, so don t feel like you re falling behind. The main purpose of this hour is to get the programs to compile and see how they function when you run them. By the Way Unlike applications, compiled Java applets cannot be tested using a Java interpreter. You have to put them on a web page and view that page in one of two ways:. Use a web browser that can handle Java 2 applets by using Sun s Java plug-in, which requires special configuration of the web page.. Use the appletviewer tool that comes with the Java Development Kit.

56 HOUR 4: Understanding How Java Programs Work To create a web page that can display the RootApplet program, return to your word processor and create a new file. Enter Listing 4.

6 56 HOUR 4: Understanding How Java Programs Work To create a web page that can display the RootApplet program, return to your word processor and create a new file. Enter Listing 4.4 in that file and save it as RootApplet.html. LISTING 4.4 The Full Text of RootApplet.html 1: <applet code= RootApplet.class height= 100 width= 300 > 2: </applet> This web page contains the bare minimum needed to display a Java applet on a web page. The applet tag is used to specify that a Java program is being put on the page, the code attribute provides the name of the applet, and the height and width attributes describe the size of the applet s display area. These items will be described in detail during Hour 17. To see this applet using the appletviewer tool included with Java Development Kit, type the following at a command line: appletviewer RootApplet.html To see it using your computer s default web browser, type the following command instead: RootApplet.html You also can load this page with your browser: Choose File, Open, then navigate to the folder that contains RootApplet.html and select the file. Figure 4.1 shows what the applet looks like when loaded with Mozilla Firefox. FIGURE 4.1 The RootApplet applet displayed with a web browser.

7 Sending Parameters to Applets 57 If you try to load this page with your web browser and get an error, the most likely cause is that your browser is not yet equipped with the Java plug-in. To correct this, visit the web page to download and install the free Java Runtime Environment, which includes the Java plug-in. Watch Out! Sending Parameters to Applets Java applets are never run from the command line, so you can t specify arguments the way you can with applications. Applets use a different way to receive information at the time the program is run. This configuration information is called parameters, and you can send parameters through the HTML page that runs the applet. You have to use a special HTML tag for parameters called param. To see how the Blanks application would be rewritten as an applet, open your word processor, enter the text of Listing 4.5, then save the file as BlanksApplet.java. LISTING 4.5 The Full Text of BlanksApplet.java 1: import java.awt.*; 2: 3: public class BlanksApplet extends javax.swing.japplet { 4: String parameter1; 5: String parameter2; 6: String parameter3; 7: 8: public void init() { 9: parameter1 = getparameter( adjective1 ); 10: parameter2 = getparameter( adjective2 ); 11: parameter3 = getparameter( adjective3 ); 12: } 13: 14: public void paint(graphics screen) { 15: screen.drawstring( The + parameter1 16: + + parameter2 + fox 17: + jumped over the 18: + parameter3 + dog., 5, 50); 19: } 20: } Save the file and then compile it. Using the JDK, you can accomplish this by typing the following at the command line: javac BlanksApplet.java Before you can try out this applet, you need to create a web page that displays the BlanksApplet applet after sending it three adjectives as parameters. Open your word processor and enter Listing 4.6, saving it as BlanksApplet.html.

8 58 HOUR 4: Understanding How Java Programs Work LISTING 4.6 The Full Text of BlanksApplet.html 1: <applet code= BlanksApplet.class height=80 width=500> 2: <param name= adjective1 value= lachrymose > 3: <param name= adjective2 value= magenta > 4: <param name= adjective3 value= codependent > 5: </applet> Save the file when you re done, and load the page using Internet Explorer, the JDK s appletviewer, or another browser equipped with the Java plug-in. The page when loaded in your browser should resemble Figure 4.2. Change the values of the value attribute in Lines 2 4 of the BlanksApplet.html file, replacing lachrymose, magenta, and codependent with your own adjectives, then save the file and run the program again. Try this several times, and you ll see that your program is now flexible enough to handle any adjectives, no matter how well or how poorly they describe the strange relationship between the fox and the dog. FIGURE 4.2 The BlanksApplet program displayed with a web browser. You can use as many parameters as needed to customize the operation of an applet, as long as each has a different name attribute specified along with the param tag. Workshop: Viewing the Code Used to Run Applets As a short workshop to better familiarize yourself with the applet tag and how it can be used to alter the performance of an applet, visit this book s World Wide Web site at Load this site by using one of the web browsers that can run Java applets, such as Microsoft Internet Explorer, Mozilla Firefox, or Apple Safari. Go to the section of the site labeled Hour 4 Showcase, and you ll be given a guided tour through several working examples of applets.

9 Q&A 59 On each of the pages in the Hour 4 Showcase, you can use a pull-down menu command to view the HTML tags that were used to create the page:. In Internet Explorer or Opera, choose View, Source.. In Navigator or Firefox, choose View, Page Source. Compare the parameters that are used with each applet to the way the applet runs. Appendix E, This Book s Website, describes other things you can find on the book s site. The website is intended as a complement to the material covered in this book, and a way to find out about corrections, revisions, or other information that make these 24 hours more productive. Summary During this hour, you had a chance to create both a Java application and an applet. These two types of programs have several important differences in the way they function and the way they are created. Both kinds of Java programs can be easily customized at the time they are run. Applications use arguments, which are specified on the command line at the time the program is run. Applets use parameters, which are included on the web page that displays the applet by using HTML tags. The next several hours will continue to focus on applications as you become more experienced as a Java programmer. Applications are quicker to test because they don t require you to create a web page to view them; they can be easier to create and more powerful as well. Q&A Q Can a single Java program be both an applet and an application? A It is possible to make a program serve as both applet and application, but it s often an unwieldy solution unless the program is simple. An applet could be set up to run as an application also by including a main() block in the applet, but you would not be able to use the init() block or paint() block in the automatic fashion they are used in an applet. Most programs are written as either an application or as an applet, rather than attempting to do both. Q Do all arguments sent to a Java application have to be strings? A Java makes all arguments into strings for storage when an application runs. When you want to use one of these arguments as an integer or some other

10 60 HOUR 4: Understanding How Java Programs Work non-string type, you have to convert the value. You ll learn how to do this during Hour 11, Describing What Your Object Is Like. Q I get errors when I try to load RootApplet.html into either Mozilla Firefox or Microsoft Internet Explorer. What s the problem? A In most cases, the problem is that the browser isn t equipped to run Java 2 applets. Internet Explorer and Firefox lack support for Java 2 applets, which are run by the Java plug-in offered by Sun. When you re in doubt about why an applet won t work in a browser, try loading it with the appletviewer tool included with the Java Development Kit. If it works in appletviewer, the problem is with the browser rather than your Java applet. Q Why don t Java applets require the same kind of special access as Common Gateway Interface programs? A Java applets don t have the same access requirements because they don t pose the same risk to a website provider. A Common Gateway Interface (CGI) program is run by the provider s web server. A web server administrator must trust the people writing gateway programs, because there s no security to prevent the program from harming the server presenting the web page, whether intentionally or by mistake. Java applets, on the other hand, have strict restrictions to prevent them from being used to write harmful programs. Also, Java programs do not run on a website s server they run on the computer of the person viewing the page. This also means that the server will not slow down due to numerous people running a Java applet on a page. Q In the movie, It s a Wonderful Life, Sam Wainwright cabled Bedford Falls from London authorizing his local office to advance George Bailey up to $25,000 to help him. How much would this money be worth in today s dollars? A Assuming that the scene took place on Christmas Eve 1945, Wainwright s offer was equivalent to $263,000 in current dollars. More inflation-adjusted figures from the 1946 film: The $8,000 Bailey Building and Loan deposit stolen by Henry Potter is equal to $84,000 today and the $5,000 loan that William Bailey owed Potter in 1919 is like $61,000. Also in 1919, George Bailey clicked a drugstore lighter and wished for a million dollars. Children today must wish for $12.3 million to be equally enriched.

11 Quiz 61 S. Morgan Friedman offers an inflation calculator on the Web at He uses Consumer Price Index statistics from the Historical Statistics of the United States and the Statistical Abstract of the United States. Quiz Test your knowledge of the material covered in this chapter by answering the following questions. Questions 1. Which type of Java program can be run by a Java interpreter? a. Applets b. Applications c. None 2. What special HTML tag is used to put a Java program onto a web page? a. <applet> b. <program> c. <run> 3. If you get into a fight with someone over the way to send information to a Java application, what are you doing? a. Struggling over strings b. Arguing about arguments c. Feudin for functionality

12 62 HOUR 4: Understanding How Java Programs Work Answers 1. b. Applications are run with the interpreter tool, and web pages containing applets can be run with the appletviewer tool as well as Java-capable World Wide Web browsers. 2. a. The <applet> tag is used along with the <param> tag to send parameters to the applet. You ll learn about a second tag that can be used to present applets during Hour b. Applications receive information in the form of arguments. Can t we all just get along? Activities If you would like to apply your acumen of applets and applications, the following activities are suggested:. Check out EarthWeb s Java Applet Ratings Service website at and use the search term headline to see links and descriptions to all of the applets that have been written to display news headlines. Each of these applets uses parameters to modify the text that is displayed.. Write a Java applet that can handle a parameter named X and a parameter named Y. Display the two parameters in a drawstring() statement like the one in the BlanksApplet program. To see a Java program that implements the second activity, visit the book s website at

Road Map. Introduction to Java Applets Review applets that ship with JDK Make our own simple applets

Road Map. Introduction to Java Applets Review applets that ship with JDK Make our own simple applets Java Applets Road Map Introduction to Java Applets Review applets that ship with JDK Make our own simple applets Introduce inheritance Introduce the applet environment html needed for applets Reading: