Java%20Programming,%20Second%20Edition

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Java Programming, Second Edition

• Chapter Five
• Input and Selection

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In this chapter, you will

• Accept keyboard input
• Use the JOptionPane class for GUI input and
  output
• Draw flowcharts
• Make decisions with the if and ifelse structures
• Use compound statements in an if or ifelse
  structure
• Nest if and ifelse statements
• Use AND and OR operators
• Use the switch statement
• Use the conditional and NOT operators
• Understand precedence

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Accepting Keyboard Input

• Run time - When the program is executing
• A program that accepts values at run time is
  interactive because it exchanges communications
  with the user
• Providing values during run time requires input,
  usually through the keyboard
• The in object has access to a method named read()
  that retrieves data from the keyboard

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Accepting Keyboard Input

• An exception is an error situation
• There are many different error situations
• For example
• Keyboard issues
• The user might enter the wrong data type

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Accepting Keyboard Input

• Let the compiler handle the problem by throwing
  the exception, or passing the error, to the
  operating system
• Use throws Exception after the main() method
  header

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Accepting Keyboard Input

• Prompt- Message requesting user input
• For example
• The string Please enter a character
• You are not required to supply a prompt but it is
  helpful for the user if you do and the user will
  be more likely to enter an appropriate response

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Using the JOptionPane Class for GUI Input and
Output

• Swing components- The classes found in the
  javax.swing package define GUI elements and
  provide alternatives to the System.in.read() and
  System.out.println() methods
• Swing classes are part of the Java Foundation
  Classes, or JFC
• To access the Swing components import the
  javax.swing package using javax.swing.

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Using the JOptionPane Class for GUI Input and
Output

• JOptionPane - Used to create standard dialog
  boxes
• Three standard dialog boxes
• InputDialog - prompts the user for text input
• MessageDialog - displays a user message
• ConfirmDialog - asks the user a question, with
  buttons for Yes, No, and Cancel responses

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Input Dialog Boxes

• showInputDialog() method- Creates an input dialog
  box
• Asks a question and uses a text field for
  entering a response
• Two components
• The parent component
• The string component- contains a string or icon
  to be displayed

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Input Dialog Boxes

• showInputDialog() method with four arguments
• Parent component
• String component (prompt)
• The title to be displayed in the title bar
• A class variable describing the type of dialog
  box
• For example
• ERROR_MESSAGE
• INFORMATION_ MESSAGE
• QUESTION_MESSAGE

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Message Dialog Boxes

• Message Dialog Boxes- Uses a simple window to
  display information
• Created with the showMessageDialog() method
• Parent component
• String component

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Message Dialog Boxes

• showMessageDialog() method with four arguments
• Parent component
• String component
• The title to be displayed in the title bar
• A class variable describing the type of dialog
  box
• For example
• ERROR_MESSAGE
• INFORMATION_ MESSAGE
• QUESTION_MESSAGE

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Confirm Dialog Boxes

• showConfirmDialog() method- To create a confirm
  dialog box which displays the options Yes, No,
  and Cancel

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Confirm Dialog Boxes

• A confirm dialog box with 5 components
• Parent component
• String component
• The title to be displayed in the title bar
• An integer that indicates which option button
  will be shown
• An integer that describes the kind of dialog box
  using the class variables ERROR_MESSAGE,
  INFORMATION_MESSAGE, PLAIN_MESSAGE,
  QUESTION_MESSAGE, or WARNING_MESSAGE

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Drawing Flowcharts

• Pseudocode- Programmers use a list of tasks that
  must be accomplished to help them plan a
  programs logic
• Flowchart- The programmer writes the steps in
  diagram form as a series of shapes connected by
  arrows

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Making Decisions with the if and ifelse
Structures

• Making a decision involves choosing between
  alternate courses of action based on some value
  within a program
• The value the decision is based on is always
  Boolean-true or false
• You can use if or ifelse statements to make a
  decision

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if and ifelse statements

• Single alternative- You only perform an action
  based on one alternative
• Dual alternative- Requires two options for a
  course of action
• Provides the mechanism for performing one action
  when a Boolean expression evaluates as true and
  if it evaluates to false a different action
  occurs

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Using Compound Statements in an if or ifelse
Structure

• To execute more than one statement that depends
  on the evaluation of a Boolean expression, use a
  pair of curly braces to place the dependent
  statements within a block

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Nesting if and ifelse statements

• Nesting if and ifelse statements- Statements
  with an if inside another if
• Nested if statements are useful when two
  conditions must be met before some action can
  occur

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Using AND and OR Operators

• AND operator- Used to determine whether two
  expressions are both true
• Written as
• OR operator- Only one of two conditions is true
• Written as
• if (itemsSold gt 3 totalValue gt 1000)
• bonus50

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Using the Switch Statement

• Switch statement- To test a single variable
  against a series of exact integer or character
  values
• The switch statement uses four keywords
• switch - starts the structure and is followed
  immediately by a test expression enclosed in
  parentheses
• case - is followed by one of the possible values
  for the test expression and a colon
• break - optionally terminates a switch structure
  at the end of each case
• default - optionally is used prior to any action
  that should occur if the test variable does not
  match any case

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Using the Conditional and NOT Operators

• Conditional operator- Requires three expressions
  separated with a question mark and a colon
• Is used as an abbreviated version of the ifelse
  structure
• (testExpression) ? true Result false Result

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Using the Conditional and NOT Operators

• NOT operator- To negate the result of any Boolean
  expression
• Written as the exclamation point (!)
• boolean oldEnough (age gt 25)
• if (!oldEnough)
• System.out.println(Too young!)

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Understanding Precedence

• Operations have higher and lower precedences
• The order in which you use operators makes a
  difference
• You can always use parentheses to change
  precedence or make your intentions clear

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