COIT 11222 - Visual ProgrammingAuthor(s): Mike O

1
COIT 11222 Visual Programming

• Lecture Week 5
• References
• Java Programming - Complete
• Concepts and Techniques,
• 3rd Edition, Shelly Cashman et al.

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Topics For This Week

• Review and Revision
• Review of Java Applets.
• Validation Strategies A Deeper Look
• Exception Handling A Deeper Look
• Unary and Assignment Operators
• Loops - for and while
• Tracing Program Code and Pseudo Code
• Batch Programs Vs Event Driven Programs
• Arguments Vs Parameters
• Assignment 1 100

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 2
3
Review of all Java Applets

• We will now review what we have learnt so far
  about
• Java Applets
• In addition, some probing questions will be
  asked, and these should hopefully help improve
  your understanding of everything covered so far.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 3
4
GUI Applet - General Form

• public class My_Applet extends Applet
  implements ..
• // Declare variables
• // Construct GUI Components
• public void init()
• // Add GUI components, Setup / Add
  ActionListner, Load Logo(s)
• public void actionPerformed (ActionEvent e)
• // Act on button click. e.g. Get inputs,
  process, display output.
• public void paint (Graphics g)
• // Draw Logo on screen

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 4
5
GUI Applet (cont )

• import java.applet. // For Applet support
• import java.awt. // For GUI components
• import java.awt.event. // For event handling
• public class W03_Wages_Calc__Applet extends
  Applet implements ActionListener
• // Declare variables
• Image Logo
• String Emp_ID
• int Hours, Rate
• double Gross_Pay
• // Construct GUI Components
• Label Window_Title_Label new Label ("Gross Pay
  Calculator")
• Label Emp_ID_Label new Label ("Enter Employee
  ID")
• Label Hours_Label new Label ("Enter Hours
  Worked")
• Label Rate_Label new Label ("Enter Hourly
  Rate")
• Label Output_Label new Label ("Please click
  the 'Calculate' button !!")
• TextField Emp_ID_TextField new TextField
  (20)

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 5
6
GUI Applet (cont )

• public void init()
• setForeground (Color.blue)
• add (Window_Title_Label)
• add (Emp_ID_Label)
• add (Emp_ID_TextField)
• add (Hours_Label)
• add (Hours_TextField)
• add (Rate_Label)
• add (Rate_TextField)
• add (Calc_Button)
• add (Output_Label)
• Calc_Button.addActionListener (this)
• Logo getImage (getDocumentBase(),
  "cqulogo.gif")

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 6
7
GUI Applet (cont )

• public void actionPerformed (ActionEvent e)
• // Get Text Input and Convert to Integers if
  required.
• Emp_ID Emp_ID_TextField.getText()
• Hours Integer.parseInt(Hours_TextField.getTe
  xt())
• Rate Integer.parseInt(Rate_TextField.getTex
  t())
• // Calculations
• Gross_Pay Hours Rate
• Output_Label.setText ("Gross Pay "
  Gross_Pay)
• public void paint (Graphics g)
• g.drawImage (Logo, 120, 160, this)

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 7
8
GUI Applet (cont )

• In the prior Applet example
• Why is the logo drawn on screen in the "paint"
  method ?
• Answer Only the paint method has access to the
  Graphics canvas for the Applet's window.
• Could we draw the logo in the "init" method or
  the "actionPerformed" method ?
• Answer No, only the paint method has access to
  the Applet's graphics canvas.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 8
9
GUI Applet (cont )

• In the prior Applet example
• Could we do without the "init" method ? Could we
  place all the code they contain in the
  "actionPerformed" method ?
• Answer No and No.
• If there was no "init" method, no GUI components
  would be created or added to the user interface.
• actionPerformed runs when a button is clicked.
  With no "init" method, how could the user click a
  button that does NOT exist and has NOT been
  assigned a Listener ?

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 9
10
GUI Applet (cont )

• In the prior Applet example
• Could we add and use Swing Dialogs in the Applet
  to display information and error messages, prompt
  user for input, etc ?
• Answer Yes, see tutorial questions.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 10
11
GUI Applet (cont )

• In the prior Applet example
• Can the "paint" method be invoked multiple times
  ? e.g. Is it invoked when another program's
  window covers your Applet's window and then focus
  returns to your Applet ?
• Answer Yes and Yes - see tutorial questions.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 11
12
Unary Operators

• and -- are operators that increment or
  decrement an integer by 1 they are called Unary
  Operators.
• Example, each of these lines of code increment
  the value of k by 1
• k
• k
• Each of these lines of code decrement the value
  of k by 1
• k--
• --k

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Unary Operators (cont)

• Note Well
• When the operation is applied depends on whether
  the Unary Operator is before the variable, a
  pre-increment or pre-decrement, or after the
  variable, a post-increment or post-decrement.
• If the operator is before the variable, then the
  operator is applied when it is encountered, and
  the updated value used immediately.
• If the operator is after the variable, then the
  current value of the variable is used, and the
  operator is then applied.
• Java works from left to right applying these
  operators and substituting the latest value of
  each variable as it goes !
• Keep the above in mind ! It is very easy to
  make errors in complex calculations.

14
Unary Operators (cont)

• Exercise what would be output by the following
  code
• int k 5
• System.out.println (k)
• System.out.println (k)
• Solution below.
• Answer We would see the original value of k, a
  5, for the first println, and the
  post-incremented value of k, a 6, for the second
  println.

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Unary Operators (cont)

• Exercise what would be output by the following
  code
• int k 5
• System.out.println (k)
• System.out.println (--k)
• System.out.println (k--)
• System.out.println (k)
• Solution below.
• Answer We would see the pre-incremented value of
  k, a 6, for the first println, the
  pre-decremented value, a 5, for the second
  println, the current value, a 5, for the third
  println, and the post-decremented value, a 4, for
  the final println.

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Unary Operators (cont)

• Exercise what values would j and m have after
  the following code
• int m 5
• int j ((m 2) / (--m - 2)) - --m
• Remember the values are updated and substituted
  as they are encountered from left to right.
• Solution below.
• Answer After the first m, 5 is substituted
  because we have a post-increment, and m is then
  immediately updated to 6. Then, for the first
  --m, the pre-decrement operator is applied on the
  latest value of m, so 6-1 5 is substituted.
  For the final --m, the operator is applied and
  the value 5-1 4 is substituted. So, the
  equation translates to this
• j ((5 2) / (5 - 2)) - 4 10 / 3 - 4
  3 - 4 -1

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Unary Operators (cont)

• Exercise what values would j and m have after
  the following code
• int m 5
• int j (--m (((m 2)) / (--m - 2)))
• Answer j8, m4. Students to verify this now -
  fill in the ___ blanks below.
• j (--m (((m 2)) / (--m - 2)))
• (___ (((___ 2)) / (___ - 2)))
• (___ (___ / ___ ))
• ___ ___ _____
• Final value of m _____

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Unary Operators (cont)

• Recommendations
• Never write code like this !!
• It is way too easy to make mistakes and create a
  mess of code that is difficult to debug, test,
  and understand.
• A much better way to write this code and make it
  easy to debug, test, and understand is to create
  additional variables and then use these in the
  calculation(s).
• For example, if we need various incremented and
  decremented values of m, we could create
• int m_Plus_1 m 1
• int m_Minus_1 m - 1
• int m_Minus_2 m - 2
• // etc ...
• and then use these when needed in calculations.

19
Assignment Operators

• As well as the standard Assignment Operator ()
  there are other Assignment Operators , -, ,
  /, and .
• Original Code Using an Assignment Operator
• k k 10 k 10
• k k - 10 k - 10
• k k 10 k 10
• k k / 10 k / 10
• k k 10 k 10 // Modulo Arithmetic

20
Loops

• Loops are blocks of code that are repeated
  multiple times until some sort of condition is
  satisfied.
• We cover two types of Java loops
• for
• while
• for loops are normally counter controlled loops -
  the code within them is executed a specific
  number of times. Assignment and unary operators
  are often used to update the counter.
• while loops are conditional loops - the code
  within them is executed until a specific
  condition (or group of conditions) is satisfied.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 20
21
Loops (cont)

• The code in the body of the loop - between the
  braces - is executed for every iteration of
  the loop.
• If there are no braces , then the body of loop
  includes the code upto and including the next
  semi-colon.

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for Loops

• A for loop consists of
• Three Java commands separated by semi-colons
  inside round brackets
• The initialiser is executed before anything else.
• The condition is checked before the body of the
  loop is executed.
• If the condition is satisfied, the body of the
  loop is executed - that is, an iteration of the
  loop occurs.
• If the condition is not satisfied, the loop is
  exited and program execution continues at the
  first line of code after the loop.
• The update is executed after every iteration of
  the loop.
• Followed by the body of the loop.

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for Loops (cont)

• For example
• for (int k 0 k lt 10 k)
• System.out.println (k)
• Declares an integer k and initialises it to 0.
• Checks if k is less than 10, and if so, executes
  the code within the body of the loop, which is
  just a single println statement in this simple
  example.
• After all code within the body of the loop is
  executed, the value of k is incremented by 1 via
  the k command.
• Steps 2 and 3 are repeated until k is no longer
  less than 10.

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for Loops (cont)

• Recall what happens if there are no braces.
• For example
• for (int k 0 k lt 10 k)
• System.out.println (k) // In the loop.
• System.out.println (k) // NOT in the loop!

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for Loops (cont)

• Exercise What code is contained in the body of
  this loop ?
• for (int k 0 k lt 10 k)
• System.out.println (k)
• System.out.println ("hi")
• Solution below.
• Answer There is a semi-colon at the end of the
  for statement, so no lines of code and none of
  the println commands are included in the loop.

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for Loops (cont)

• Exercise what output would the following code
  produce
• for (int k 0 k lt 10 k)
• System.out.println (k)
• k 11
• Solution below.
• Answer the first value of k, 0, would be output,
  and then k is set to 11, so the loop would then
  exit.

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for Loops (cont)

• Exercise what output would the following code
  produce
• for (int k 5 k lt 14 k k 3)
• System.out.println (k)
• Solution below.
• Answer the first value of k, 5, would be output,
  and then k would be incremented by 3, the
  condition checked, the loop body executed, and so
  on until k is not lt 14.
• The values 5, 8, and 11 would be output and then
  the loop would exit.

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for Loops (cont)

• Exercise what output would the following code
  produce
• for (int k 11 k lt 10 k k 3)
• System.out.println (k)
• System.out.println ("Hi")
• Solution below.
• Answer k is initialised to 11, so the loop
  condition is not satisfied, so the loop exits and
  "Hi" is output.

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for Loops (cont)

• Exercise which of the following loops produce
  the same results
• for (int k 1 k lt 10 k)
• for (int k 1 k lt 10 k)
• for (int k 1 k lt 10 k k 1)
• for (int k 1 k lt 10 k 1)
• Solution below.
• Answer all loops produce exactly the same
  results.

30
while Loops

• while loops are conditional loops - the code
  within them is executed until a specific
  condition (or group of conditions) is satisfied.
• while loops can do everything a for loop can do.

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while Loops (cont)

• For example, these for and while loops both loop
  the same number of times
• for (int k 0 k lt 10 k)
• // Body of loop here ...
• int k 0
• while (k lt 10)
• // Body of loop here ...
• k

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while Loops (cont)

• Exercise The prior slides for and while loops
  are not quite identical though why ?
• Solutions below
• Answer the integer k only exists for the life of
  the for loop because it is declared in the for
  loop.
• However, for the while loop the integer k is
  declared outside of the loop, so it exists and
  can be used after the end of the while loop.

33
while Loops (cont)

• Exercise How could both of these loops and the
  lives of associated variables be made identical ?
• Solutions below
• Answer simply declare the integer k before the
  for loop
• int k 0
• for (k 0 k lt 10 k)
• // Body of loop here ...

34
while Loops (cont)

• With while loops, you can specify multiple
  conditions, for example
• int Num_Retries 0
• boolean Data_Valid false
• while ((Data_Valid false) (Num_Retries lt
  10))
• // Body of loop here ...
• Num_Retries

35
Tracing Program Code and Pseudo Code

• To be sure that your algorithm or strategy for
  solving a problem is correct and complete, you
  need to be able to trace through your pseudo code
  and program code and keep track of the values of
  all relevant variables at key points in the code.
• This is an essential skill for all analysts,
  designers, and programmers.
• If you dont do this, then you may waste time
  developing a program which is wrong, or you may
  develop a program that produces wrong results and
  you cannot determine why.
• Tracing through pseudo code and program code to
  verify the processing is essential to checking
  everything is correct, checking that everything
  is understood, checking that there are no gaps or
  no missing steps, and that the desired and
  correct results are being produced.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 35
36
Tracing Program Code and Pseudo Code (cont)

• Exercise What would be output by this simple
  pseudo code (further explanations on next slide)?
• number 10
• OUTPUT number ", "
• WHILE number lt 25
• if (Remainder (number, 3) 0)
• number number 2
• else if (Remainder (number, 4) 0)
• number number 5
• else
• number number 1
• OUTPUT number ", "
• END WHILE

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 36
37
Tracing Program Code and Pseudo Code (cont)

• Further explanations
• Assume the program is a console application and
  that OUTPUT displays output to the console
  screen.
• In the above pseudo code, Remainder is a function
  that returns the remainder of the division of the
  two parameters passed to it.
• For example Remainder (20, 6) would return 2,
  because 20 / 6 3 with 2 remainder.
• Note Remainder is exactly the same as Javas
  operator - the modulo arithmetic operator (see
  Week 2s lecture slides).

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 37
38
Tracing Program Code and Pseudo Code (cont)

• The best way to trace through even the most
  complex pseudo code (or program code) is to use a
  multi-columned table to keep track of the values
  of relevant variables at particular points during
  the processing.
• e.g. at the start and end of loops, before and
  after if tests, etc.
• Example of the table that could be used for this
  exercise on the next screen.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 38
39
Tracing Program Code and Pseudo Code (cont)
Intera-tion Value of "number" at top of loop Num 3 Num 4 Value of "number" at end of loop Output So Far










COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 39
40
Tracing Program Code and Pseudo Code (cont)
Intera-tion Value of "number" at top of loop Num 3 Num 4 Value of "number" at end of loop Output So Far
Before 10,
1 10 1 2 11 10, 11,








COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 40
41
Tracing Program Code and Pseudo Code (cont)
Intera-tion Value of "number" at top of loop Num 3 Num 4 Value of "number" at end of loop Output So Far
Before 10,
1 10 1 2 11 10, 11,
2 11 2 3 12 10, 11, 12,







COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 41
42
Tracing Program Code and Pseudo Code (cont)
Intera-tion Value of "number" at top of loop Num 3 Num 4 Value of "number" at end of loop Output So Far
Before 10,
1 10 1 2 11 10, 11,
2 11 2 3 12 10, 11, 12,
3 12 0 n/a 14 10, 11, 12, 14,
4 14 2 2 15 10, 11, 12, 14, 15,
5 15 0 n/a 17 10, 11, 12, 14, 15, 17,
6 17 2 1 18 10, 11, 12, 14, 15, 17, 18,
7 18 0 n/a 20 10, 11, 12, 14, 15, 17, 18, 20,
8 20 2 0 25 10, 11, 12, 14, 15, 17, 18, 20, 25,
Num 25, so loop exits. Num 25, so loop exits. Num 25, so loop exits. Num 25, so loop exits. Num 25, so loop exits. Num 25, so loop exits.
COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 42
43
Tracing Program Code and Pseudo Code (cont)

• So, the output would be
• 10, 11, 12, 14, 15, 17, 18, 20, 25,
• And there would be a total of 8 iterations of
  loop. That is, the code in the body of the loop
  would be executed 8 times.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 43
44
Batch Programming

• In Batch Programming, the processing flow is
  determined by the programmer.
• All of the Console Applications we develop in
  this course are batch programs.
• When they are executed, Batch Programs step
  through the exact sequence of steps that the
  programmer coded.
• With a Batch Program, if the user enters invalid
  input, there are really only 2 options
• Display an error and exit.
• Display an error and ask the user to try again.
  That is, keep looping asking for input until
  valid input is entered.

45
Event Driven Programming

• Event Driven Programs sit there doing nothing
  until the user clicks on a button, a menu option,
  etc and then they leap into action and do their
  processing, such as validating inputs, performing
  calculations, generating output, etc.
• Most GUI programs are Event Driven Programs.
• All GUI Programs in this course are Event Driven
  Programs.
• That is all of the Java Applets and GUI
  Applications in this course are Event Driven
  Programs.

46
Event Driven Programming (cont)

• For example, in an Event Driven Program, the user
  could click the Save button, or they could
  click the Calculate menu option, or they could
  click on a check box and each of these actions
  could initiate different processing and provide
  different results.
• The Event Driven Program is responding to events
  and dealing with them as the events happen.

47
Event Driven Programming (cont)

• With an Event Driven Program, if the user enters
  invalid input, there is really only 1 option
• Display an error message and STOP.
• That is, the user can then fix their input data
  and click the button again, or they may decide to
  click on something else or do something else.
• With Event Driven Programs, there is no forcing
  the user down a single road. e.g. they dont
  loop until valid data is entered.

48
Event Driven Programming (cont)

• That is the beauty and simplicity of event driven
  programs
• They do nothing until the user clicks something.
• If user input is required, then they check the
  input and either continue if all is OK, or
  display an error message and stop if an error is
  found.
• This is completely different to what happens in a
  Batch Program.

49
Validation Strategies A Deeper Look

• We will now review what we have already covered
  with Data Validation and explore things more
  deeply.
• When validating a lot of input data, it is easy
  to end up with a mess of nested ifs.
• We will now look at simple techniques to help us
  perform the validation of any number of inputs
  and avoid having too many nested if statements or
  ending up with a mess of code.

50
Validation Strategies (cont)

• Remember (from Batch and Event Driven Programming
  slides earlier)
• If an invalid input is encountered by an Applet
  or a GUI Application, then we want to display an
  error message and then STOP.
• Note STOP does NOT mean exit the program, it
  means wait for the user to do something. e.g.
  wait for them to fix the input and click the
  button again.
• If an invalid input is encountered by a Console
  Application, then we want to display an error
  message and then ask the user to input the data
  again.

51
Validation Strategies Batch Programs

• // Validation Strategy Batch Programs
• // Author Mike O'Malley
• // Note This is all pseudo code ! This is NOT
  Java code.
• // If converted to Java, the resulting code could
  be running, for example,
• // in the main method for a Console Application.
• // The user cannot proceed until valid inputs are
  entered at each stage !!
• boolean Valid_Input false // Assume First
  Input is invalid.
• while (Valid_Input false) // Loop until FIRST
  input is valid
• Get the FIRST input item.
• Check the input is valid - data type check,
  range check, or whatever is needed.
• if data is not valid
• display an error
• else
• Valid_Input true
• end if
• end loop

52
Validation Strategies Batch Programs (cont)

• Valid_Input false // Assume Second Input is
  invalid.
• while (Valid_Input false) // Loop until
  SECOND input is valid
• Get the SECOND input item.
• Check the input is valid - data type check,
  range check, or whatever is needed.
• if data is not valid
• display an error
• else
• Valid_Input true
• end if
• end loop
• etc for all other inputs
• if (Valid_Input true) // Are all inputs valid
  ??
• Do whatever calculations and processing are
  needed.
• Display any results.
• etc.

53
Validation Strategies Batch Programs (cont)

• As you can see, the program will only proceed to
  the next step in the processing once the current
  input has been validated.
• Keep this strategy in mind for all Console
  Applications.

54
Validation Strategies Event Driven Programs

• // Validation Strategy Event Driven Programs by
  Mike O'Malley
• // Note This is all pseudo code ! This is NOT
  Java code.
• // If converted to Java, the resulting code could
  be running, for example,
• // in an Applet's actionPerformed method,
  responding to mouse clicks on a button.
• boolean Valid_Input true // Assume inputs are
  valid and try to disprove this.
• if (Valid_Input true)
• Get the FIRST input item.
• Check the input is valid - data type check,
  range check, or whatever is needed.
• if data is not valid
• display an error
• Valid_Input false
• end if
• end if
• if (Valid_Input true) // Are all inputs
  valid so far ??
• Get the SECOND input item.
• Check the input is valid - data type check,
  range check, or whatever is needed.

55
Validation Strategies Event Driven Programs
(cont)

• etc for all other inputs
• if (Valid_Input true) // Are all inputs valid
  ??
• Do whatever calculations and processing are
  needed.
• Display any results.
• etc.
• end if
• // end of method.

56
Validation Strategies Event Driven Programs
(cont)

• As you can see, once any inputs are invalid, no
  other if tests are entered so none of the
  processing they contain is done.
• The method being executed (such as
  actionPerformed) reaches the end, and the program
  sits there waiting for the user to do something
  else, such as fix inputs and click the button
  again.
• Keep this strategy in mind for all Applets and
  GUI Applications.

57
Exception Handling A deeper Look

• We will now review what we have already covered
  for Exception Handling and explore things more
  deeply.
• Anything that could cause your program to crash
  or produce ugly error messages (that the user
  should not be exposed to) should be placed in a
  try ... catch block.

58
Exception Handling A deeper Look (cont)

• For example, code for
• Converting strings to floats, ints, etc
• Performing calculations (where there could be
  division by zero, etc),
• Or any other situation where the code could cause
  the program to crash or display ugly errors to
  the user.
• should go in try ... catch blocks.
• Also, if we were writing more advanced programs,
  reading and writing files, connecting to and
  querying databases, etc then all of this code
  would go inside try ... catch blocks.

59
Exception Handling A deeper Look (cont)

• However, if you put all of the conversions in a
  single try ... catch block, then you end up
  with "generic error messages which are not
  ideal.
• For example
• try
• Weight Double.parseDouble (weightField.getText
  ())
• Height Double.parseInt (heightField.getText()
  )
• // etc for other data items
• catch (NumberFormatException e)
• // Generate a nasty "generic" error message !
• outputLabel1.setText("You must enter a valid
  amount.")

60
Exception Handling A deeper Look (cont)

• Splitting this into separate try ... catch ...
  blocks would be slightly better
• try
• Weight Double.parseDouble (weightField.getText
  ())
• catch (NumberFormatException e)
• outputLabel1.setText("Please enter a valid
  Weight.")
• try
• Height Double.parseInt (heightField.getText()
  )
• catch (NumberFormatException e)
• outputLabel1.setText("Please enter a valid
  Height.")

61
Exception Handling A deeper Look (cont)

• However, the error messages are still very
  uninformative, and there is not range checking.
• A better solution is to also do the range
  checking inside try block and manually throw an
  exception (to execute all of the code in the
  catch block) if needed.
• For example, lets assume the Weight must between
  10 and 200 see below.

62
Exception Handling A deeper Look (cont)

• try
• Weight Double.parseDouble (weightField.getText
  ())
• if ((Weight lt 10) (Weight gt 200))
• throw new NumberFormatException()
• catch (NumberFormatException e)
• outputLabel1.setText("You must enter Weight in
  KG between 10 and 200.")

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Exception Handling A deeper Look (cont)

• So, the error message will appear if they enter 1
  or 270 or abcd and so on.
• We could (and should !) improve this code further
  by using Global Constants to hold the Min and Max
  values for each input value as we have done in
  prior lectures.
• See last weeks lecture material.

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Exception Handling A deeper Look (cont)

• One problem with the above code is that you end
  up with a try catch blocks for each input
  item.
• If you have a dozen inputs, then you might need a
  dozen (or so) try catch blocks.
• So, a better way is to create a User Defined
  Method for each type of input data and call this
  for each input of that type.
• For example, for double checking, the method
  could be called something like Get_Valid_Double.

65
Exception Handling A deeper Look (cont)

• We would need to pass such a method the data to
  allow it to do its job, which might include
• Min value allowed
• Max value allowed
• The name we want to be displayed in prompts,
  error messages, etc (if required).
• And anything else that might be required.
• We could call this method like this
• Weight Get_Valid_Double ("Weight (Kg)", 10.0,
  200.0)
• Height Get_Valid_Double ("Height (m)", 1.0,
  2.2)

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Exception Handling A deeper Look (cont)

• Your Get_Valid_Double method needs to be able to
  signal to a calling method when invalid input was
  entered, so why not return a value less than the
  minimum allowed ?
• So, for example, you might do this
• Weight Get_Valid_Double ("Weight (Kg)", 10.0,
  200.0)
• if (Weight lt 10.0)
• // Oops, the input wasn't valid ...

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Exception Handling A deeper Look (cont)

• What might the implementation of this method look
  like ? Heres one possibility (no Java code
  supplied just comments and hints)
• private double Get_Valid_Double (String
  Data_Name,
• double Min_Value, double
  Max Value)
• // Get the data (e.g. from the user)
• // Check it isnt blank
• // Type check it. e.g. convert input string
  to a double.
• // Range check the data against Min and Max
  values.
• // Hint A try ... catch ... block is ideal
  for this !!
• // If input data is invalid, display an
  informative
• // error message and then return a value which
  is less
• // than the Minimum allowed to signal to the
  calling
• // program that an error occurred.

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Exception Handling A deeper Look (cont)

• If you want to pass the string data that was
  entered by the user to the method, how could you
  do this ? Heres one possibility
• First, we add another String parameter
• private double Get_Valid_Double (String
  Data_Name,
• String Input_String,
• double Min_Value, double
  Max Value)
• And then we need to pass the extra string in each
  function call, for example
• Weight Get_Valid_Double ("Weight (Kg)",
• Weight_TextField.getText (), 10.0,
  200.0)
• See this weeks tutorials !!

69
Arguments Vs Parameters

• Arguments are variables and values that are
  provided in the calling method and are used
  within a method call. Examples
• My_Method (33, "Fred", 3.2)
• My_Method (In_Age, In_Name, In_BSL)
• GST_Val Calculate_GST (125.0)
• GST_Val Calculate_GST (Total_Excl_GST)
• Red text above indicates the Arguments.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 69
70
Arguments Vs Parameters (cont)

• Parameters are variables defined in the methods
  header that receive the Argument values when the
  method is called.
• private void My_Method (int Age, String Name,
  double BSL)
• // Body of method
• private double Calculate_GST (double Tot_Excl)
• // Body of method
• Red text above indicates the Parameters.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 70
71
Assignment 1 100

• You can now complete 100 of Assignment 1.
• Before you attempt to finish Assignment 1, make
  sure you
• Thoroughly read, review, and explore all topics
  discussed in the lecture material for Weeks 1-5.
• Do all of the tutorial questions for Weeks 1-5.
• 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.

COIT 11222 - Visual Programming Author(s) Mike
OMalley Slide 71
72
Summary of Topics Covered

• Review and Revision
• Review of Java Applets.
• Validation Strategies A Deeper Look
• Exception Handling A Deeper Look
• Unary and Assignment Operators
• Loops - for and while
• Tracing Program Code and Pseudo Code
• Batch Programs Vs Event Driven Programs
• Arguments Vs Parameters
• Assignment 1 100

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End of Lecture