Java Programming, 2E
Introductory Concepts
and Techniques
Chapter 4
Decision Making and Repetition
with Reusable Objects
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 if…else 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 >= 18))
– if ((age < 13) || (age > 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
– if…else 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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Java Programming, 2E
Introductory Concepts
and Techniques
Chapter 4 Complete