CIS3931 Intro to JAVA

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
ReminderUpdate 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
Thursdays 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.

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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

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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

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textactionapplet.java

• If you try to compile the program at this point,
  you will get a few errors
• thornton_at_program3/CIS3931/21-July-05gtjavac
  textactionapplet.java
• textactionapplet.java20 cannot find symbol
• symbol constructor JApplet(java.lang.String)
• location class javax.swing.JApplet
• super ("TextAction Example")
• textaction.java81 cannot find symbol
• symbol method addWindowListener(ltanonymous
  java.awt.event.WindowAdaptergt)
• location class textaction
• addWindowListener(new WindowAdapter( )
• 2 errors

9
textactionapplet.java

• The errors are as follows
• The JApplet class doest 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 arent 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.

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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)

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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.

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Webpage for the textactionapplet

• ltHEADgt
• ltBODYgt
• ltAPPLET codetextactionapplet.class height300
  width500gt
• lt/APPLETgt
• lt/BODYgt
• lt/HEADgt

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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.

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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.

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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.

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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.

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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 gt 0)
• balance dollars
• //If the balance is not positive, make it 0
• else dollars 0

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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
  cant input invalid data.

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Example modified

• public class bank_balance
• public String owner
• private int balance
• public bank_balance( String name, int dollars
  )
• owner name
• if (dollars gt 0)
• balance dollars
• else dollars 0
• public int getBalance()
• return balance
• public void setBalance(int dollars)
• if (dollars gt 0)
• balance dollars

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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

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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

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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)

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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.