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CIS3931 - Intro to JAVA

Learn how to create home and reload buttons for a web browser in Java GUI programming. Includes applets, spot forms, and an update on the final exam.

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CIS3931 - Intro to JAVA

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  1. CIS3931 - Intro to JAVA Lecture Notes Set 14 21-July-05 GUI Programming – Home and reload buttons for the webbrowser, Applets

  2. Spot Forms …

  3. Reminder…Update on the FINAL • The final will be entirely multiple choice • The lowest grade you can receive on the final and still pass the class is a 65 ! • The final will be approximately 30 questions (about 22-25 repeated questions from the previous midterms and 5-8 new questions) • There will be 4 or 5 extra credit questions, for a total of 20 available extra credit points

  4. Thursday’s class… • Turning a JAVA program into a web-viewable applet • A few more notes on program 6 • Notes on the “protected”, “private”, “public”, “static”, and “final” modifiers.

  5. Applets • The Applet class is part of the AWT. • A newer class called JApplet is included in Swing, but is not as commonly used as the Applet class. • HOWEVER : The JApplet class makes turning an existing GUI program into an applet VERY easy …

  6. JApplet • Instead of extending JFrame, you want to extend JApplet in your program. • JFrame and JApplet are very similar … so, almost everything will work without modification • There are a few exceptions …

  7. Example : Changing textaction.java into an Applet • textaction.java was discussed in the last class. • The first step is to take the textaction.java program and rename it to something like textactionapplet.java • Then --- Modify it so your public class extends JApplet instead of JFrame

  8. textactionapplet.java • If you try to compile the program at this point, you will get a few errors… thornton@program3:~/CIS3931/21-July-05>javac textactionapplet.java textactionapplet.java:20: cannot find symbol symbol : constructor JApplet(java.lang.String) location: class javax.swing.JApplet super ("TextAction Example"); ^ textaction.java:81: cannot find symbol symbol : method addWindowListener(<anonymous java.awt.event.WindowAdapter>) location: class textaction addWindowListener(new WindowAdapter( ) ^ 2 errors

  9. textactionapplet.java • The errors are as follows : • The JApplet class does’t have a super method associated with it … so, you have to remove that. • There is no window listener available with applets …

  10. textactionapplet.java • Since you aren’t extending your entire program from the JFrame class, you will have to create one somewhere else. • You will be modifying the “main” section of your program to create and customize the main JFrame.

  11. textactionapplet.java public static void main (String[] args) { //Create a JFrame here instead .... JFrame frame = new JFrame("Making an applet"); //Add the textactionapplet information to this frame frame.getContentPane().add(new textactionapplet()); //Set the JFrame size frame.setSize(300,300); //Set the JFrame to be visible frame.setVisible(true); }

  12. textactionapplet.java • Now, the program should compile without any errors. • Next, you will have to move the class files to your webserver. Make sure to set the appropriate file permissions on them. • Finally, you will need to create a webpage to display the applet.

  13. Webpage for the textactionapplet <HEAD> <BODY> <APPLET code=textactionapplet.class height=300 width=500> </APPLET> </BODY> </HEAD>

  14. The Applet Webpage • See http://www.cs.fsu.edu/~cis3931/applet.html • The applet should function just as the application did. • The .class file can still be run as an application from a command line using the normal “java” run command. • This program is now both and applet and an application.

  15. Public, Private, and Protected • The information needed to fully understand the use of these modifiers is a little beyond the scope of this class; however, it is important to at least understand at the basic level what each of these actually do. • These modifiers have a bearing on access privileges in programs that have multiple classes associated with them.

  16. Public, Private, and Protected • When used in a class declaration : Keywords used to specify the access levels for member variables and functions (methods). • When used on a method : Used to specify the access levels for the method and its variables. • When used on a variable : Used to specify the access level for the single variable.

  17. Public, Private, and Protected • Public : visible to all classes • Private : Visible only to the class to which it belongs • Protected : Visisble to the class to which they belong and to all subclasses of that class.

  18. Deciding when to use the modifiers • Think about whether or not an external object (or program) actually needs direct access to the information. • Really only applies in complex object oriented programming with multiple classes and methods.

  19. Example public class bank_balance { public String owner; public int balance; public bank_balance( String name, int dollars ) { owner = name; //Check to be sure the balance is positive if (dollars >= 0) balance = dollars; //If the balance is not positive, make it 0 else dollars = 0; } }

  20. Example • In this example, both the string and integer are public. • Any object in the system can change the balance (even to a negative value). • This could cause the program to fail even though there are checks in the bank_balance class to prevent negative values.

  21. Example : modified • The better way to make this program would be to prove two methods, getBalance and setBalance. • Also – the balance should be either private or protected. • Other objects can still access the data, but the can’t input invalid data.

  22. Example : modified public class bank_balance { public String owner; private int balance; public bank_balance( String name, int dollars ) { owner = name; if (dollars >= 0) balance = dollars; else dollars =0; } public int getBalance() { return balance; } public void setBalance(int dollars) { if (dollars >= 0) balance = dollars; else dollars = 0; } }

  23. The reload button • Your reload button should attempt to reload the page that is currently being displayed • It will be hard to tell if this is actually working because pages sometimes load very quickly • We will be checking your code to be sure you performed the reload correctly

  24. The reload button • The reload button first needs to determine the current page location • Your textfield should be showing the current page location (it should be updated each time you click on a link!!) • You will then simply call a setPage (or the function that loads your URLS) on the location

  25. The home button • The home button is loads a static page • For assignment 6, home should load http://www.google.com • Just call a setPage on the address (or use your function that loads the URL • Remember to update the text in the textfield as well! (When you click home, your textfield should say http://www.google.com)

  26. Questions ? • Any other topics to cover before the end of class? • Only one more lecture class left, Assignment 7 will be assigned • Next Thursday will be the review for the final • The final is the Tuesday of the last week of class • The last Thursday of class is intended to be used for help with Assignment 7 as well as to finalize grades and go over the grades on the final exam.

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