Decision Making and Repetition with Reusable Objects

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Chapter 4

• Decision Making and Repetition with Reusable
  Objects

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Objectives

• Design a program using methods
• Code a selection structure to make decisions in
  code
• Describe the use of the logical AND, OR, and NOT
  operators
• Define exceptions and exception handling

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Objectives

• Code a try statement and a catch statement to
  handle exceptions
• Create a user-defined method
• Code a repetition structure using the while
  statement
• Write a switch statement to test for multiple
  values in data

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Objectives

• Format numbers using a pattern and the format()
  method
• Construct a Color object
• Use a Checkbox and a CheckboxGroup in the user
  interface

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Introduction

• Control structures alter the sequential execution
  of code
• Selection structure
• Repetition structure
• User-defined methods break tasks into reusable
  sections of code
• Exception handling in Java allows for the testing
  of valid and accurate input

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The Sales Commission Program

• Calculate sales commission for agents in a travel
  agency
• Coded as a console application and an applet
• Input
• The user chooses from three types of commission
  codes
• The commission code identifies the type of sale
  and the commission rate
• The user enters a sales amount
• Calculate commission and display output

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Program Development

• Problem analysis
• Each user decision corresponds to a program task
• Develop and test each task before adding it to
  the program
• Design the solution
• Design storyboards for the two user interfaces
• Program design
• Design a flowchart consisting of required methods
• Write related pseudocode for each method
• Validate Design

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Coding the Program

• Code the program using program stubs
• Stubs are incomplete portions of code that serve
  as a template or placeholder for later code
• Stubs allow ease in debugging through incremental
  compilation and testing
• Import java.swing.JOptionPane
• Import java.text.DecimalFormat
• Formats decimal output into Strings
• Declare variables
• Compile and test the program stub

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Coding the Program

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Writing Methods

• A program with modules allows for clarity,
  reusability, and refinement
• The code for each module can be separated into
  programmer-defined methods
• The main() method transfers execution to the
  methods through a call statement
• The called method returns control to its caller
  with the return statement or the ending brace
• The return statement returns any required data
  back to the calling method

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The ifelse Statement

• Single line 29, line 30
• Block lines 15-26, lines 19-20, lines 23-24
• Nested lines 17-25, lines 29-30

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Testing with an if statement

• Testing a single condition
• if (answer null)
• if (!done)
• Testing multiple conditions
• if ((gender male) (age gt 18))
• if ((age lt 13) (age gt 65))
• AND and OR expressions evaluate the right operand
  only if the left operand is not sufficient to
  decide the condition

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Exception Handling

• An exception is an event resulting from an
  erroneous situation which disrupts normal program
  flow
• Exception handling is the concept of planning for
  possible exceptions by directing the program to
  deal with them gracefully, without terminating
• Two kinds of exceptions
• Run-time
• Checked
• The compiler checks each method to ensure each
  method has a handler

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Handling Exceptions

• The try statement identifies a block of
  statements that may potentially throw an
  exception
• The throw statement transfers execution from the
  method that caused the exception to the handler
• Transfers execution to the catch statement if the
  throw is placed within a try statement
• The catch statement identifies the type of
  exception being caught and statements to describe
  or fix the error
• The finally statement is optional and is always
  executed regardless of whether an exception has
  taken place
• Placed after the catch statement

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Catch an exception
Throw an exception
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Testing methods

• Compile the program after coding each method and
  call statement
• Run the program with correct input
• Run the program to test the exception handling
  with invalid input
• Alphabetic data
• Negative values
• Null or zero values
• Verify that the user is allowed to reenter data
• Verify the program closes correctly

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Repetition Structure
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The getSales() method

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The getCode() method

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The Case Structure

• A type of selection structure that allows for
  more than two choices when the condition is
  evaluated
• Used when there are many possible, valid choices
  for user input
• The code evaluates the user choice with a switch
  statement and looks for a match in each case
  statement
• Each case statement contains a ending break
  statement which forces exit of the structure

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Arguments and Parameters

• When a method is called, the calling method sends
  arguments the called method accepts the values
  as parameters
• Different but related identifier names for the
  arguments and the parameters should be used for
  good program design
• The variables are only visible in their
  respective methods
• Arguments and parameters for a called method and
  the calling statement must be of the same number,
  order, and data type

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The getComm() Method

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Formatting Numeric Output

• The DecimalFormat class formats decimal numbers
  into Strings for output
• Supports different locales, leading and trailing
  zeros, prefixes/suffixes, and separators
• The argument is a pattern, which determines how
  the formatted number should be displayed

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The output() method

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The finish() method

• Exits system when program completes successfully

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Moving to the Web

• Create the host document to execute the applet

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Coding an Applet Stub

• Enter general block comments
• Import java.awt., java.applet.,
  java.awt.event., and java.text.DecimalFormat
• Implement the ItemListener interface to listen
  for the user choice on a Checkbox
• Code the method headers for the init() and the
  itemStateChanged() method
• itemStateChanged() is an ItemListener method to
  process user choices
• Declare variables and construct a Color object

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Making Decisions in Applets

• Use a CheckboxGroup to allow user choices

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Constructing Components

• Construct Labels for input and output
• Construct a CheckboxGroup for user options

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Adding Components

• Add Labels and CheckboxGroup to the applet
• Add an ItemListener to each Checkbox component
  with the addItemListener() method
• Add color with the setForeground() and the
  setBackground() methods
• Set the insertion point with the requestFocus()
  method

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The init() Method

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Handling Exceptions

• Check for valid data when the itemStateChanged()
  method is triggered, which happens when the user
  clicks an option button

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The getSales() Method

• Parse the data from the TextField and return a
  valid sales amount or throw an exception to the
  init() method

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The getCode() Method

• Initialize the code to 0
• Use nested if statements to assess the boolean
  state of the Checkboxes and return the code to
  init()

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The getComm() Method

• Identical to the application
• Return the commission value to init()

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The getOutput() Method

• Send output to the Label using setText()
• Construct and use DecimalFormat, using the
  special character to make a leading zero absent
  

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The paint() Method

• Display a graphic

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Testing the Applet

• Compile the applet
• Execute the applet using AppletViewer
• Test exception handling by clicking an option
  button and then typing invalid data into the text
  box
• Verify the error message and the data clearing
  mechanisms
• Test all options with valid data
• Test in a browser window
• Document the applet interface and source code

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Summary

• Repetition Structures
• while statement
• Selection Structure
• ifelse statement
• switch and case statements
• Exception handling
• Try and catch statements
• Modularity
• Smaller segments of reusable code through methods
• Program clarity and refinement

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Summary

• Program stubs
• Incremental testing
• CheckboxGroup
• User options for user interfaces
• Responding to user options
• ItemListener, addItemListener(),
  itemStateChanged()
• Data validation and error messages
• Testing invalid data

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Chapter 4 Complete