COIT 11222 - Visual ProgrammingAuthor(s): Mike O

1
COIT 11222 Visual Programming

• Lecture Week 4
• References
• Java Programming - Complete
• Concepts and Techniques,
• 3rd Edition, Shelly Cashman et al.

2
Topics For This Week

• More GUI Development logos, drawString,
  colours, checkboxes, radio buttons, etc
• Pseudo Code and Flowcharts
• UML and UML Class Diagrams
• Math class, random numbers, etc
• Exception handling
• User Defined Methods
• Public Vs Private
• Lots of topics this week (again).
• We've had to set a cracking pace to cover enough
  to allow you to do Assignment 1.
• Dont Panic next week the pace is easier (for a
  while).
• Assignment 1 90

3
Applets Adding a Logo

• Adding a logo to our Applet's window is very
  easy.
• First, we need to declare an Image object with
  our other GUI Components
• Image Logo
• Next, in our init () method, we need to load the
  logo with the getImage() method
• Logo getImage (getDocumentBase(),
  "cqulogo.gif")
• The getDocumentBase() tells Java to look for our
  logo "cqulogo.gif" in the directory where our
  source code is located.

4
Applets Adding a Logo (cont)

• Finally, we need a paint method to draw the logo.
• The paint method has a Graphics object passed to
  it (called g below)
• public void paint (Graphics g)
• g.drawImage (Logo, 120, 160, this)
• The drawImage () method draws the logo 120 pixels
  from the left and 160 pixels from the top of the
  window.

5
The paint() Method

• Accepts a Graphics object as a parameter
• The Graphics object is commonly referred to by
  the variable name g (we could call it anything we
  wanted, but we will assume it is called g).
• The variable g is automatically created and
  initialized in the init() method
• The variable g is a reference variable (a
  specific instance of an object).
• The return type is void (it returns nothing).

6
Applets Adding a Logo (cont)

• If we put the logo "cqulogo.gif" in the same
  directory as our Java source code, and re-compile
  and re-run our Applet, we will see

7
Applets Adding a Logo (cont)

• Some examples in some text books load the logo
  from file and draw it in the paint method.
• For example
• public void paint (Graphics g)
• Logo getImage (getDocumentBase(),
• "cqulogo.gif")
• g.drawImage (Logo, 120, 160, this)
• However, this isnt ideal because the logo is
  re-loaded every time paint is executed.

8
Paint and drawString

• The paint method can also use the drawString
  method to draw text directly onto the user
  interface.
• For example
• g.drawString ("My Text", 100, 200)
• Draws My Text 100 pixels from the left and 200
  pixels from the top of the window.
• If it is drawn in the same place as any labels,
  buttons, or other GUI components, then the drawn
  text goes behind these GUI components.
• If it is drawn in the same place as the logo,
  then it will go on top of the logo or behind the
  logo depending on the order in which they are
  drawn.

9
Paint and drawString (cont)

• Example
• public void paint (Graphics g)
• g.drawImage (Logo, 120, 200, this)
• g.drawString ("Hi there _at_ 220, 160", 220,
  130)
• g.drawString ("Hi there _at_ 220, 220", 220,
  220)
• g.drawString ("Hi there _at_ 220, 300", 220,
  250)

10
Paint and drawString (cont)

• The first drawString outputs some of the text
  behind the Calculate button. The next, on top of
  the logo, and the third below the logo.

11
Colour Background and Foreground

• The setForeground and setBackground enable you to
  set the foreground and background colours.
• Examples
• setForeground (Color.blue)
• setBackground (Color.green)
• The predefined colours are
• black blue cyan darkGray
• gray green lightGray magenta
• orange pink red white
• yellow

12
Colour Background and Foreground (cont)

• Or, you can specify millions of other colours
  using red, green, and blue (RGB) values between 0
  and 255.
• Example, set the colour directly
• setBackground (new Color(44,55,200))
• Or, define a colour and then use this
• Color Back_Col new Color(44,55,200)
• setBackground (Back_Col)

13
Colour Background and Foreground (cont)

• For the Applets background, and for the
  foreground or background of any text written to
  the interface with drawString, the colours are
  set / changed / updated whenever the Applets
  background and foreground colours are changed.
  Example
• setForeground (Color.blue)
• setBackground (Color.green)
• This affects the Applets colours and the colours
  of all text written to the interface with
  drawString.
• This does NOT affect any GUI components (labels,
  buttons, etc) that have already been added to the
  user interface.

14
Colour Background and Foreground (cont)

• For GUI components (labels, buttons, etc) you can
  either
• Set the interface's colours before adding the
  components to the interface (they will then be
  added with the interface's colour), OR,
• Set the component colours at any time by invoking
  the setForeground and setBackground specifically
  for each required GUI component
• My_Label.setBackground (new Color(44,55,200))
• My_Label.setForeground (Color.blue)

15
Checkboxes

• A checkbox is a graphical component that can be
  in either an "on" (true) or "off" (false) state.
• Clicking on a check box automatically changes its
  state from "on" to "off" or from "off" to "on ".
• To use Checkboxes, you need the following import
• import java.awt.Checkbox
• Checkboxes can be created much like other GUI
  Components. Example (Customer Check-in System)
• Checkbox Deluxe_Checkbox new Checkbox ("Deluxe
  Room")
• Checkboxes are added to the user interface in
  exactly the same way as any other GUI Component
  via the add command in the init method
• add (Deluxe_Checkbox)

16
Checkboxes (cont)

• To programmatically check a checkbox, use the
  setState() method. Example
• Deluxe_Checkbox.setState (true)
• And, to uncheck the checkbox
• Deluxe_Checkbox.setState (false)
• To determine if a checkbox is checked, use the
  getState() method. Example
• if (Deluxe_Checkbox.getState () true)
• // Customer wants a Deluxe Room.

17
Radio buttons (Checkbox Group)

• Checkboxes in a group are called Radio Buttons,
  and only one Radio Button in the group can be
  checked (selected) at any time.
• To create checkboxes in a group, you need to
  import the required packages
• import java.awt.Checkbox
• import java.awt.CheckboxGroup
• And then create a checkbox group. Example
• CheckboxGroup Credit_Cards new CheckboxGroup ()

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 17
18
Radio buttons (Checkbox Group) (cont)

• And then add checkboxes to this group. Example
• Checkbox Visa_Card
• new Checkbox ("Visa Card", true,
  Credit_Cards)
• Checkbox Master_Card
• new Checkbox ("Master Card", false,
  Credit_Cards)
• Checkbox AMEX_Card
• new Checkbox ("AMEX Card", false,
  Credit_Cards)
• The Visa card will be selected by default,
  because it has true specified.
• As before, the checkboxes are added to the user
  interface via the add() method in the init()
  method
• add (Visa_Card)
• add (Master_Card)
• add (AMEX_Card)

19
Radio buttons (Checkbox Group) (cont)

• Screen shot (Smoking / Non Smoking discussed on
  following slides)

20
Radio buttons (Checkbox Group) (cont)

• And, you use getState and setState methods (as
  before with single checkboxes) to get and set the
  state of these radio buttons.
• Example of getting the state
• if (Visa_Card.getState () true)
• Paying_Str "Visa Card"
• Example of setting the state
• Master_Card.setState (true)

21
Hidden Checkbox

• For some applications, a program needs to provide
  a default selection for one of the radio buttons.
  That is, one of the radio buttons will be
  checked / selected by default.
• However, for other situations, your program may
  need to force the user to select one of the radio
  buttons before continuing.Example a motel /
  hotel Customer Check-in System may require the
  customer to actively specify a smoking or
  non-smoking room. Why ?
• If a smoker is placed in a non-smoking room, and
  they smoke, then this can cause a great deal of
  cleanup, and expense for the hotel.
• e.g. Many hotels in Brisbane now charge a cleanup
  fee of 150 for smoking in a non-smoking room.
• Similarly, if a non-smoker is checked into a
  smoking room, then they may feel quite ill
  because of the lingering smoke and residue from
  years of smoking in the room.

22
Hidden Checkbox (cont)

• If your program needs to force the user to select
  a checkbox, then one way to achieve this is the
  hidden checkbox method.
• This involves creating an extra checkbox in the
  group, but not adding it to the user interface.
• This checkbox will be checked by default, and the
  application will make sure that it is NOT checked
  before proceeding with processing.
• That is, it forces the user to check one of the
  visible checkboxes.

23
Hidden Checkbox (cont)

• First, we need to create the Checkbox group
  (including the Hidden checkbox)
• CheckboxGroup Smoking_Group new
  CheckboxGroup ()
• Checkbox Smoking_Checkbox
• new Checkbox ("Smoking", false,
  Smoking_Group)
• Checkbox Non_Smoking_Checkbox
• new Checkbox ("Non-Smoking", false,
  Smoking_Group)
• Checkbox Hidden_Checkbox
• new Checkbox ("Hidden", true,
  Smoking_Group)
• Then, add all checkboxes in the group to the user
  interface, except the Hidden Checkbox
• add (Smoking_Checkbox)
• add (Non_Smoking_Checkbox)
• So, only these two checkboxes will be visible to
  the user and checkable by the user.

24
Hidden Checkbox (cont)

• Then, when we are processing the customer check
  in, we need to check if the Hidden checkbox is
  still selected / checked, and, if it is, display
  an error and prevent the check in from
  proceeding
• if (Hidden_Checkbox.getState () true)
• JOptionPane.showMessageDialog (null,
• "Please select SMOKING or NON-SMOKING
  !!",
• "Smoking Status", JOptionPane.ERROR_MESSAG
  E)
• else
• // Customer check in can proceed

25
Hidden Checkbox (cont)

• As you can see, this technique can be very useful
  when your application wants to force the user to
  make a selection, rather than just accept the
  default selection.

26
Activating Checkboxes

• In Week 3 we saw
• To make a button react to mouse clicks, we need
  to assign an ActionListener to the button.
  Example
• My_Button.addActionListener (this)
• We can make checkboxes and radio buttons (groups
  of checkboxes) react to mouse clicks by assigning
  an ItemListener to them in the init () method
• My_Checkbox.addItemListener (this)
• Note Add a listener to each Checkbox in the
  group. You dont add the listener to the
  CheckboxGroup object.

27
Activating Checkboxes (cont)

• We also need to tell Java that we want the Applet
  to be able to deal with active checkboxes.
• To do this, we need to add "implements
  ItemListener" to the class statement.
• For example, if you need just ActionListener
• public class ... extends ...
• implements ActionListener
• But, if you need both ActionListener and
  ItemListener
• public class ... extends ...
• implements ActionListener, ItemListener

28
Activating Checkboxes (cont)

• Finally, we need to implement an itemStateChanged
  method
• public void itemStateChanged (ItemEvent choice)
• // Enter code below that needs to run
• // when checkbox is clicked on by the user.
• // public void itemStateChanged

29
Activating Checkboxes (cont)

• In most cases, where a checkbox is active, all
  the itemStateChanged method will do is update
  labels, perform simple calculations, and so on.
• However, in some cases, the itemStateChanged
  method may do a lot more, especially if your
  application has no buttons on the user interface.

30
Activating Checkboxes (cont)

• Example
• public void itemStateChanged (ItemEvent choice)
• // The user has clicked on a checkbox, let's
  find out
• // which one and update a label's text and
  colour.
• if (Smoking_Checkbox.getState () true)
• Output_Label.setText ("You are a smoker.")
• Output_Label.setForeground (Color.blue)
• else if (Non_Smoking_Checkbox.getState ()
  true)
• Output_Label.setText ("You are a
  NON-smoker.")
• Output_Label.setForeground (Color.red)
• // public void itemStateChanged

31
Activating Checkboxes (cont)

• Ref W04_01_Customer_Check_In_System.java

32
Swing Vs AWT

• All of the graphical user interface (GUI)
  components we use in this course are AWT
  Components.
• AWT stands for Abstract Windows Toolkit,
• AWT provides a set of basic GUI components
  (labels, buttons, textfields, menus, etc) that
  are easy for beginner programmers to understand
  and use.
• In Java, there are Swing Dialogs, which we have
  also been using a lot so far.
• However, there are also Swing Components, which
  you can use instead of the AWT Components that we
  have been using so far.
• We do not cover Swing Components in this course.

33
Swing Vs AWT (cont )

• With SWING, you get improved versions of all of
  the AWT components as well as a whole range of
  new components.
• Swing Components look a lot nicer than AWT
  components, and also provide more functionality
  and give the programmer a lot more control over
  the look and feel of the application.
• Swing Components allow you to develop very sexy
  and very complex interfaces.

34
Swing Vs AWT (cont )

• For example
• Swing buttons and labels can display images and
  text. AWT buttons can only display text.
• You can put borders around most Swing components.
• For our purposes, in this course, radio buttons
  are multiple checkboxes in an AWT checkbox group.
• However, Swing supports a proper Radio button
  component.
• With Swing its a case of more, more, more more
  functionality, more control, better look and
  feel, etc.
• However, all of this comes at a price Swing
  Components are more complex to use.

35
Swing Vs AWT (cont )

• Java text books often introduce AWT first and
  then move onto Swing because AWT is easier to
  work with, the AWT components are simpler and
  have fewer options, and there are less components
  to deal with.
• However, AWT isn't really that much simpler.
• In fact, introducing AWT first can lead to
  confusion and frustration when you are going to
  move to Swing later.
• The problem is that (from a programming point of
  view), changing an application over from AWT to
  Swing is a bit of a pain - you need to change
  components (Label -gt JLabel, etc), change the
  code behind the scenes, and do quite a bit of
  messing about.

36
Swing Vs AWT (cont )

• Prior to 2008-T1, we covered Swing and AWT
  components in this course. However, the decision
  was made to only cover AWT from 2008-T1 onwards.
• Note Well we do not cover Swing Components in
  this course.
• Swing Components are covered in the follow on
  Java courses.
• Please Note Radio Buttons Vs Checkbox Group.
  In follow-on Java courses, if they say radio
  buttons, then they mean proper Swing radio
  buttons, not an AWT checkbox group !!
• Please keep this in mind for the future !

37
UML

• UML Unified Modelling Language
• UML is used specifically for OOD Object
  Oriented Design
• UML is a notation used to describe object
  behaviours and interaction through a series of
  documentation standards and diagrams such as
  Class Diagrams, Event Diagrams, Use Cases, etc
• Gaining wide popularity and use in industry.

38
UML Class Diagram

• A UML Class Diagram shows for each class
• the class name
• class attributes, the data type for each
  attribute, and whether it is public or private
  and
• class methods, the data type for each method, and
  whether it is public or private.
• Usually is used to indicate public, and for
  private.
• Note In this course, we are only dealing with
  single class projects, so our UML Class
  Diagrams will be very simple and only show a
  single class.

39
UML Class Diagram (cont ...)

• Example
• So, the Employee class has 5 data attributes (all
  are private), and 3 public methods.

Employee ----------------------------------
---------------------- - Employee_ID int - Name
String - Address String - Date-of-Birth Date -
Salary float ------------------------------------
-------------------- calculateTax() float
calculateSuperAnnuation() float
deleteEmployee() bool
40
Pseudo Code

• Pseudo Code, also called Structured English, is a
  compact, informal, and high-level description of
  a process or algorithm.
• Pseudo Code allows the designer to focus on the
  logic of the algorithm without being distracted
  by details of a programming languages syntax.
• Pseudo Code is great for specifying, documenting,
  and clarifying user requirements, business rules,
  and processing strategies and rules, and so on.

41
Pseudo Code (cont)

• Pseudo Code should describe the entire logic of a
  process or algorithm so that implementation
  (programming) becomes the simple mechanical task
  of translating the pseudo code line by line into
  a programming language source code.
• Recommendation Before jumping straight into
  coding, ensure you explore and understand what is
  required, and plan out what you are going to do
  and how you are going to do it with pseudo code.

42
Pseudo Code (cont)

• Example 1 Get Ready for Work
• Get out of bed
• Shower
• Brush teeth
• Comb hair
• Get Dressed
• etc

43
Pseudo Code (cont)

• Example 2 Calculate Area of Rectangle
• Get height of rectangle
• Get width of rectangle
• Calculate area height width
• Example 3 Allocate an Airline Seat
• Before we could begin coding a Allocate an
  Airline Seat function in Java (or any other
  programming language) we would need to understand
  exactly what is required to allocate an airline
  seat.

44
Pseudo Code (cont)

• For a first pass of the Allocate an Airline Seat
  we might come up with the following
• Get customers flight details and seat
  preferences
• Check each available seat on the flight against
  the customers preferences.
• Lock best seats (while customer chooses seat)
• Display list of best seats to customer.
• Get customers seat preference.
• Book airline seat for customer.
• However, we still could NOT turn this algorithm
  into a real program.
• Before we can do this, we will need to explore
  and expand each of these steps.

45
Pseudo Code (cont)

• For example, for Step 2 Check each available
  seat on the flight against the customers
  preferences, the pseudo code could be
• Initialise Best Seats list
• For each seat on the flight
• If seat is available (not occupied / booked)
• If seat matches class preference (business,
  economy, etc) ?
• Add 10 to Seat Score
• If seat matches customers leg room preference ?
• Add 1 to Seat Score
• If seat matches customers window / aisle seat
  preference ?
• Add 2 to Seat Score
• And so on for other criteria e.g. Non /
  Smoking ? Wing seat ? Proximity to Toilet ?
  Front or rear of plane ?
• If seat qualifies for Best Seats list, add it to
  the list.

46
Pseudo Code (cont)

• We have now expanded Step 2 slightly.
• But, we still are NOT at the stage where we can
  turn this algorithm into a real program.
• Before we can do this, we still need to explore
  and expand various steps.
• For example
• How do you add a seat to the Best Seats List ?
  How are these seats stored ? Are they sorted ?
  How ? When ?
• How do you initialise the Best Seats List ?
• Where are seat details and seat allocations held
  ? How is this data accessed ?
• Etc.
• Until everything is thoroughly explored and
  documented, we cannot hope to develop a program
  which performs the correct functionality.

47
Pseudo Code (cont)

• There are many styles of Pseudo Code, such as
  styles which are closer to programming languages,
  like Java.
• For example, we could also write the pseudo code
  for step 2,as follows
• Initialise Best Seats list
• While Still seats to check
• Load next seat
• If Seat.booked false
• If Seat.class Customer.class
• score score 10
• If Seat.leg room Customer.leg room
• score score 1
• If Seat.window / aisle Customer.window / aisle
• score score 2
• Etc for other criteria and steps ...
• End If
• End While

48
Flow Charts

• Flowcharts are diagrams that can show exactly the
  same information as pseudo code, but in
  diagrammatic form.
• Benefits over Pseudo Code
• Diagrams tend to be more attractive to look at
  than pseudo code.
• May appear more user friendly than pseudo code.
• People who are frightened by anything that looks
  like a programming language may be more
  comfortable with a diagram.

49
Flow Charts (cont)

• Disadvantages compared to Pseudo Code
• It is usually more work to document something
  with a flow chart than with pseudo code.
• If requirements or processing rules change, then
  it is usually much quicker and easier to update
  pseudo code than flow charts.
• An algorithm documented in a flow chart tends to
  take up more space (more pages when printed out)
  than the equivalent pseudo code. So, many pages
  may be needed for a flow chart, when 1 page of
  pseudo code may do the same job.
• Flow charts tend to have a bad name amongst
  programmers.
• They went out of fashion in the 1970s - and many
  claim that they still are ! ?

50
Flow Charts (cont)

• The main symbols Examples
• (There are many others !!)

Process
Get Customers Leg Room Preference
Decision
Is Seat.Class Customer.Class ?
Process flow
Start / End Terminator
Data Store
Flight Seats
51
Flow Charts (cont)
START
Is Seat.class Customer.class ?
Yes
Initialise Best Seats list
score score 10
No
No
Is Seat booked?
Yes
Is Seat.leg room Customer.leg room ?
Yes
score score 1
Still Seats to Check ?
Yes
Load next seat
No
Is Seat.window/aisle Customer.window/aisle ?
No
Yes
score score 2
Etc for other criteria and steps ... Need to
expand this
No
END
52
Essential Skills

• All programmers (and all students in this course)
  need to be able to convert between the following
• Pseudo Code
• Flow Chart
• Java Program code
• This is an essential skill !!
• Ensure you get practice with doing this.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 52
53
Math Class
54
Math Class (cont)

• Example 1 To set a variable of type double to a
  random number between 0 and 1
• double d1 Math.random ()
• Example 2 To set a variable of type double to a
  random number between 0 and 80
• double d1 80 Math.random ()

55
Math Class (cont)

• Exercise How could you set a variable of type
  double to a random number between 10.0 and 90.0 ?
• Solution below.
• Answer
• double d2 10.0 80.0 Math.random ()

56
Math Class (cont)

• Exercise How could you set a variable of type
  integer to a random integer between 10 and 90 ?
• Solution below.
• Answer
• int i2 (int)(10.0 80.0 Math.random ())

57
Exception Handling

• An exception is an event resulting from an
  erroneous situation which disrupts normal program
  flow.
• Exception Handling is the technique of planning
  for possible exceptions by directing the program
  to deal with them gracefully, without
  terminating, crashing, or producing ugly error
  messages that the user of the program should not
  be subjected to.
• e.g. internal Java error messages.

58
Exception Handling (cont)

• Three kinds of exceptions
• Input Output (I/O)
• Run-time
• Checked
• The compiler checks each method to ensure each
  method has a handler to deal with the exceptions.

59
Exception Handling (cont)

• There are 4 Java commands to deal with
  exceptions
• try
• throw
• catch
• finally
• The try statement identifies a block of
  statements that may potentially throw an
  exception.

60
Exception Handling (cont)

• The throw statement transfers execution from the
  method (or block of code) that caused the
  exception to the exception handler.
• The catch statement
• Contains the code to deal with the exception,
  providing the exception occurs within a try
  statement.
• Identifies the type of exception being caught and
  contains statements to describe, work around, or
  fix the error.

61
Exception Handling (cont)

• The finally statement is
• Optional and is always executed regardless of
  whether an exception has taken place.
• Placed after the catch statement.
• Note It all might sound complex, but it is
  actually quite simple.
• Examples on following slides

62
Exception Handling (cont)

• // Author Mike O'Malley
• // Source File W04_03_Exception_Handling.java
• // Description Exception handling examples
  (Console Application).
• import javax.swing.JOptionPane // For Swing
  Dialogs
• public class W04_03_Exception_Handling
• public static void main (String args)
• // Handle division by zero and display an
  error
• // to the command window.
• try
• int i 100 / 0 // Division by zero.
• catch(ArithmeticException e)
• System.out.println ("\nError division
  by zero.")

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 62
63
Exception Handling (cont)
// Handle conversion when a string contains
invalid float data // and display an error
using a Swing Dialog. String In_String
"12ab.0" double Input_Value 0.0
try Input_Value
Double.parseDouble (In_String)
catch(NumberFormatException e)
JOptionPane.showMessageDialog (null,
"Error '" In_String "' could not be
converted " "to a floating point
number.", "Error",
JOptionPane.ERROR_MESSAGE)
COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 63
64
Exception Handling (cont)
// Convert a string to float and range
check, so that the data // fails the range
check, and display any error to the command
// window and in a Swing Dialog. // Use
constants in range checking and error message so
that // the boundary values aren't
repeated in multiple places. String
In_String2 "12.0" double Input_Value2
0.0 final double MIN_VALUE 20.0
final double MAX_VALUE 100.0 try
Input_Value2 Double.parseDouble
(In_String2) if ((Input_Value2 lt
MIN_VALUE) (Input_Value2 gt MAX_VALUE))
throw new NumberFormatException()
COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 64
65
Exception Handling (cont)
catch(NumberFormatException e)
String Error_Message "Error
'" In_String "' could not be converted to "
"a floating point number, or
value is not between " MIN_VALUE
" and " MAX_VALUE "."
System.out.println ("\n" Error_Message)
JOptionPane.showMessageDialog
(null, Error_Message, "Error", JOptionPane.ERROR_M
ESSAGE)
COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 65
66
Exception Handling (cont)

• If our program was an Applet, then we could
  display the error message in a Label, a Swing
  Dialog or both, for example here we do both
• catch(NumberFormatException e)
• String Error_Message
• "Error '" In_String "' could not be
  converted to "
• "a floating point number, or value is not
  between "
• MIN_VALUE " and " MAX_VALUE "."
• Output_Label.setText (Error_Message)
• JOptionPane.showMessageDialog
• (null, Error_Message, "Error",
  JOptionPane.ERROR_MESSAGE)

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 66
67
User Defined Methods

• User Defined Methods are named blocks of code
  that the user of the programming language (the
  programmer) can call whenever they are needed.
• In console applications, the main is a mandatory
  method, but you can add as many User Defined
  Methods you like.
• In Applets, the init is a mandatory method, and
  you also need paint, actionPerformed, etc
  depending on what you are trying to achieve, but
  once again you can add as many User Defined
  Methods you like.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 67
68
User Defined Methods (cont)

• User Defined Methods can be passed data
  (integers, floats, Strings, etc) so that they can
  do whatever processing is desired.
• If a User Defined Method does not need to return
  any data, then it is a void method.
• If a User Defined Method need to return a value
  of a specific type (integer, float, String, etc)
  then it is declared to be of this type, and needs
  to contain a return statement to return the
  required value.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 68
69
User Defined Methods (cont)

• Example 1 we need a method to accept a
  products price or a service charge (excluding
  GST) and calculate and display the GST and the
  price including GST.
• Like any method, we want to be able to call this
  method whenever we need to, such as
• Calculate_And_Display_GST (100.0)
• Calculate_And_Display_GST (25.0)
• Calculate_And_Display_GST (2999.99)

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 69
70
User Defined Methods (cont)

• Here, the method is displaying the data to the
  Console Screen (we are running a Console
  Application)
• private static void Calculate_And_Display_GST
  (double In_Amount_Excl_GST)
• final double GST_RATE 10.0 / 100.0
• double GST_Amount In_Amount_Excl_GST
  GST_RATE
• double Amount_Incl_GST In_Amount_Excl_GST
  GST_Amount
• System.out.println ("\nAmount Excl GST "
  In_Amount_Excl_GST)
• System.out.println ("\n GST Amount "
  GST_Amount)
• System.out.println ("\n Amount Inc GST "
  Amount_Incl_GST)
• System.out.println ()
• Our method is returning no data so its type is
  void.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 70
71
User Defined Methods (cont)

• If we had an Applet and wanted to display the
  output in one or more Labels, we could change the
  method as follows
• private static void Calculate_And_Display_GST
  (double In_Amount_Excl_GST)
• final double GST_RATE 10.0 / 100.0
• double GST_Amount In_Amount_Excl_GST
  GST_RATE
• double Amount_Incl_GST In_Amount_Excl_GST
  GST_Amount
• Out1_Label.setText (Amount Excl GST "
  In_Amount_Excl_GST)
• Out2_Label.setText ( GST Amount "
  GST_Amount)
• Out3_Label.setText ( Amount Inc GST "
  Amount_Incl_GST)
• Our method is returning no data so its type is
  void.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 71
72
User Defined Methods (cont)

• Example 2 Lets create a new method which
  calculates and returns the GST without displaying
  it to the screen. The calling program can then
  do whatever it likes with the GST amount that is
  returned.
• Here, out Applet is displaying the value in a
  Label
• double GST_Amount 0.0
• GST_Amount Calculate_GST (100.0)
• Out_Label.setText ("GST Amount "
• GST_Amount)

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 72
73
User Defined Methods (cont)

• private double Calculate_GST (double
  In_Amount_Excl_GST)
• final double GST_RATE 10.0 / 100.0
• double GST_Amount In_Amount_Excl_GST
  GST_RATE
• return GST_Amount
• Our method is returning a double value so it is
  of type double.
• Our method returns the double value via the
  return statement.
• If our method is non-void, then it must contain a
  return statement.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 73
74
Public Vs Private - Methods

• Public methods are available for use by other
  methods even those which are not in our class.
• Some methods must be declared as public, such as
  main, init, paint, actionPerformed, etc.
• Private methods are only available for use other
  methods in our class.
• For example, if a method is not reusable, or
  should not be callable by a method outside of our
  class, then declare it as private.
• Recommendation Declare all of the User Defined
  Methods you create as private unless there is
  good reason for them to be public.
• See the User Defined Examples in the prior slides.

75
Public Vs Private - Data

• Public class data is data that is accessible and
  modifiable by other methods, even those which are
  not in our class.
• Private class data is data that is accessible and
  modifiable only by class methods that is,
  methods of the current class.
• Recommendation Declare all class data as private
  unless there is very good reason for it to be
  public.
• Examples on following slides

76
Public Vs Private (cont)
public class W03_02_Wages_Calc__Applet ...
// Declare class data. private String
Emp_ID private int Hours, Rate
private double Gross_Pay etc
public void init() // Code for this
method goes here ... public void
actionPerformed (ActionEvent e) //
Code for this method goes here ...
COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 76
77
Public Vs Private (cont)
public void paint (Graphics g) //
Code for this method goes here ...
private void Calculate_And_Display_GST (...)
// Code for this method goes here ...
private double Calculate_GST (...)
// Code for this method goes here ...
etc
COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 77
78
Assignment 1 90

• You can now complete 90 of Assignment 1.
• Before you proceed with Assignment 1, make sure
  you
• Thoroughly read, review, and explore all topics
  discussed in the lecture material for Weeks 1-4.
• Do all of the tutorial questions for Weeks 1-4.
• Follow the Development Hints and Tips in the
  assignment specification (Course Profile).
• Students who tried to avoid these activities in
  the past have found that the Assignment took them
  far longer to finish !! e.g. 70 hours instead of
  5 hours.

79
Summary of Topics Covered

• More GUI Development logos, drawString,
  colours, checkboxes, radio buttons, etc
• Pseudo Code and Flowcharts
• UML and UML Class Diagrams
• Math class, random numbers, etc
• Exception handling
• User Defined Methods
• Public Vs Private
• Lots of topics this week (again).
• We've had to set a cracking pace to cover enough
  to allow you to do Assignment 1.
• Dont Panic next week the pace is easier (for a
  while).
• Assignment 1 90

80
End of Lecture