Creating Menus and Button Arrays Using the Abstract Windows Toolkit

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Chapter 6

• Creating Menus and Button Arrays Using the
  Abstract Windows Toolkit

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Chapter Objectives

• Create and implement private variables
• Include a menu system in a GUI application
• Manipulate Button arrays
• Move data in and out of the system clipboard
• Differentiate between the getActionCommand() and
  get Source() methods

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Chapter Objectives

• Write code to search for which component was
  clicked
• Use multiple layout managers
• Program multiple case solutions
• Change the icon in a Java programs title bar
• Access methods from the Toolkit class

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Introduction

• Create a Calculator application
• Stand-alone
• Portable
• Incorporate a Menu system
• Clear display
• Exit the application
• Read about the program
• Copy and paste data using the system clipboard
• Implement as a GUI
• Array of Button objects, TextField, flags

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Calculator Application
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Problem Analysis

• Standard window with ability to resize
• Minimize, Maximum, Close buttons
• 10 numeric buttons, 4 operator buttons, a decimal
  point, and an equal button
• LCD screen
• Displays current number
• Clears display when an operator button is clicked
• Displays answer when equal button is clicked
• Program remembers operators and operands
• Menu bar with drop-down menu of commands

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Design the Solution

• Size and shape of a handheld calculator
• Title bar with caption and icon
• TextField for number display
• BorderLayout layout manager
• TextField in North region
• Buttons in Center region
• Array of buttons placed in Panel using a
  GridLayout
• Buttons have a label with the number or symbol
• Menu system with commands

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Program Design

• Button clicks
• Numeric or decimal point button clicks display in
  the TextField
• Operator button clicks store the current
  TextField number and the operator
• Equal sign clicks display the result
• Declaration and construction of components
• Majority of logic in ActionListener event
• Listens and responds to a user click
• Menu options have individual if statements
• Keypad buttons share code using a case structure
  

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Private Variables

• Ensures that a driver class cannot access a
  variable directly
• Local in scope
• Used only in the processes of the class in which
  they are declared
• Protects class from potential user corruption
• Displays encapsulation
• Principle of least privilege

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Overview of coding process

• Import statements
• java.awt.datatransfer package moves data in and
  out of the system clipboard
• Extends Frame and implements ActionListener
• Constructor method
• Creates Frame
• Embeds components
• Initializes variables
• Establishes layout manager
• Sets TextField display
• Main() method creates an instance of Calculator

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Creating Menus
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Using Menus

• File, Edit, and About commands
• File contains Exit command
• Edit contains Clear, Copy, and Paste commands
• About contains About Calculator command
• Common practice to use the prefix mnu for each
  item with menu bar commands
• Horizontal separator between Clear and Copy
• Commands on the menu bar only display their menu
• This action is polymorphic

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Initializing the Calculator Variables

• TextField is read-only
• setEditable() is initialized to false
• Panel contains keypad Buttons
• Variables first, op1, clearText and lastOp are
  initialized
• calcPattern is a DecimalFormat object
• Initialized to hold a pattern of up to eight
  digits before and after decimal point
• No commas for easier parsing

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Button Arrays and the Keypad

• Use an array for all Buttons on the keypad
• Allows the same code for multiple buttons
• Facilitates searching for buttons
• A for loop is used to construct and label all the
  numeric buttons
• Use the valueOf() method to convert the index to
  a String label for the Button
• Explicitly assign other Buttons
• Set keypad layout and spacing with GridLayout
  constructor

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Adding Components to the Interface

• Add constructed Buttons to keypad Panel
• Add buttons row by row from the upper-left
  corner, using a for loop for sequentially ordered
  buttons

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Adding Components to the Interface

• Add ActionListener to each Button
• Use the length property of the array in the for
  loop to make the code more genericThe keyword,
  this, is used to refer to the Button
• Add Panel and TextField to Frame
• Frame will add Buttons all at once

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The addWindowListener() Method

• Registers the listener with the Frame
• Allows Frame events to be sent to the listener
• Creates a new instance of WindowAdapter inside
  the argument of addWindowListener
• Overrides the windowClosing() method of
  WindowAdapter to terminate the program when the
  Close button is clicked
• WindowListener is an interface
• The code creates a WindowAdapter object, which
  implements the interface

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The System Clipboard

• Most operating systems have a clipboard
• Temporary memory reserved for user storage
• Can hold more than 20 data pieces of varying
  types
• Data can be cut, copied, or pasted across
  applications
• The Clipboard class implements the system
  clipboard to store clipboard contents
• Methods from the Toolkit class and Transferable
  interface are used to perform the cut, copy, and
  paste operations
• A Toolkit object binds components to native
  implementations
• A Transferable object resides in a buffer to
  transfer objects between applications

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Selecting Text

• Copy data to the clipboard by selecting data
• Normally done by dragging the mouse or using
  SHIFTARROW
• Can be done in this application by selecting the
  Copy command, since only the TextField can be
  copied
• The StringSelection class holds the TextField
  data
• The setContents() method transfers the data to
  the clipboard
• The first argument is the data
• The second argument is the current system and
  user

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The actionPerformed() Method

• In order to make the menu system and keypad
  functional, an actionPerformed() method must be
  coded
• ActionListener requires an actionPerformed method
• When a button or menu option is clicked, the
  ActionListener activates the corresponding
  actionPerformed() method
• actionPerformed() takes one argument
• Usually identified by e, which represents the
  object passed to it from ActionListener

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Searching for Component Clicks

• setActionCommand() establishes a keyword command
  for each menu item
• getActionCommand() retrieves this keyword
• Used to look at individual keywords associated
  with menu items
• getSource() is associated with ActionEvent
  objects and can be used to compare a components
  variable name
• Used to look for which of the individual buttons
  was clicked

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Exit and Clear Commands
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Copy and Paste Commands

• The getSystemClipBoard() method is called from
  within a reference to the operating system
  toolkit using the getDefaultToolkit() method
• The period delimiter is used to link the two
  methods together
• Assign a variable name to the current clipboard
  each time it is implemented
• Transfer data to a buffer first and then attempt
  to convert the number to a String
• Use try blocks to catch non-numeric data
• The Throwable superclass catches any exception

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About Calculator Command
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Searching the Numeric Buttons

• Test for button click
• Sequentially search the array
• If a match is found, use the case statements to
  determine the action
• One break statement can serve for multiple cases
• Set foundKey to true to avoid unnecessary looping
  
• To display a number with multiple digits,
  concatenate the result of the getLabel() method
  with the previous data in TextField

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Searching for the First Operator Button Click

• If the first flag is true
• Save the TextField value in op1
• Save the operator index in lastOp
• Set first to false

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Searching for Subsequent Operator Clicks

• If the first flag is false
• A switch structure determines which operator was
  clicked
• Each case statement performs the corresponding
  calculation
• The result is stored in op1
• The DecimalFormat variable formats the result
  before the lcd TextField displays it
• The clearText flag is set to true to prepare for
  a new calculation

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Searching for the Equal Button

• If the equal button is clicked
• The first flag is set to true

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Coding the main() Method

• Constructs an instance of Calculator
• Sets attributes with Frame methods

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Compiling, Running, and Testing the Application

• Compile the application
• Test the application using all possible inputs,
  including invalid data
• Test the keypad functionality by performing
  calculations with different operators and
  multiple operands
• Test the menu system by clicking on the menu
  commands
• Test with the system clipboard by copying and
  pasting across applications

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Chapter Summary

• Create and implement private variables
• Include a menu system in a GUI application
• Manipulate Button arrays
• Move data in and out of the system clipboard
• Differentiate between the getActionCommand() and
  get Source() methods

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Chapter Summary

• Write code to search for which component was
  clicked
• Use multiple layout managers
• Program multiple case solutions
• Change the icon in a Java programs title bar
• Access methods from the Toolkit class

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Chapter 6 Complete

• Creating Menus and Button Arrays Using the
  Abstract Windows Toolkit