The Abstract Window Toolkit

1
Chapter 12

• The Abstract Window Toolkit

2
12.1 Overview

• Javas Abstract Window Toolkit provides classes
  and other tools for building programs that have a
  graphical user interface.
• The term Abstract refers to the AWTs ability
  to run on multiple platforms.
• Building a GUI involves creating abstract
  components such as buttons and windows, which are
  then mapped to concrete components for a
  specific platform.

3
Swing

• Java has a newer library for building graphical
  user interfaces, known as Swing. Swing is more
  powerful and sophisticated than the AWT.
• Swing is built around the existing AWT, so it
  helps to understand the AWT first.
• Swing is similar enough to the AWT that its
  relatively easy to switch if the need arises.
• Many Swing classes correspond to AWT classes. For
  example, Swings JButton class corresponds to the
  AWTs Button class.

4
Creating a Graphical User Interface

• GUI programming in Java is based on three
  concepts
• Components. A component is an object that the
  user can see on the screen andin most
  casesinteract with.
• Containers. A container is a component that can
  hold other components.
• Events. An event is an action triggered by the
  user, such as a key press or mouse click.
• Designing a graphical user interface involves
  creating components, putting them into
  containers, and arranging for the program to
  respond to events.

5
Creating a Graphical User Interface

• Components are objects, so theyre created by
  invoking a constructor.
• A button would be created by using a constructor
  belonging to the Button class.
• The most commonly used constructor has one
  argument (the buttons label)
• Button b new Button("Testing")
• For a component to be visible, it must be added
  to a container (typically a frame) by the add
  method.

6
Creating a Graphical User Interface

• To detect when an event occurs, a special
  listener object can be attached to a component.
• When the user performs an action that involves
  the component, a method belonging to the listener
  object will be called automatically.

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12.2 Frames

• In Java terminology, a frame is a window with a
  title and a border.
• A frame may also have a menu bar.
• Frames play an important role in the AWT because
  a GUI program normally displays a frame when its
  executed.
• The DrawableFrame objects used in previous
  chapters are examples of frames.

8
The Frame Class

• Frames are created using one of the constructors
  in the Frame class.
• One constructor takes a single argument (the
  title to be displayed at the top of the frame)
• Frame f new Frame("Title goes here")
• Although the Frame object now exists, its not
  visible on the screen.
• Before making the frame visible, a method should
  be called to set the size of the frame.
• If desired, the frames location can also be
  specified.

9
Frame Methods

• Many methods used with Frame objects are
  inherited from Window (Frames superclass) or
  from Component (Windows superclass).
• The setSize method sets the width and height of a
  frame
• f.setSize(width, height)
• If a program fails to call setSize or pack before
  displaying a frame, it will assume a default size.

10
Frame Methods

• The size of a frame can change during the
  execution of a program.
• The getSize method returns a frames current
  width and height
• Dimension frameSize f.getSize()
• frameSize.width will contain fs width.
  frameSize.height will contain fs height.

11
Frame Methods

• The setVisible method controls whether or not a
  frame is currently visible on the screen.
• Calling setVisible with true as the argument
  makes a frame visible
• f.setVisible(true)
• Calling it with false as the argument makes the
  frame disappear from the screen
• f.setVisible(false)
• The Frame object still exists it can be made to
  reappear later by calling setVisible again.

12
Creating a Frame

• The FrameTest program creates a Frame object and
  displays it on the screen.
• This program illustrates three key steps
• 1. Using the Frame constructor to create a
  frame.
• 2. Setting the size of the frame.
• 3. Displaying the frame on the screen.

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• FrameTest.java
• // Displays a frame on the screen.
• // WARNING Frame cannot be closed.
• import java.awt.
• public class FrameTest
• public static void main(String args)
• Frame f new Frame("Frame Test")
• f.setSize(150, 100)
• f.setVisible(true)

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Creating a Frame

• Frame created by the FrameTest program
• As with the other AWT components, the appearance
  of a frame depends on the platform.

15
Creating a Frame

• Clicking on the Close button has no effect,
  because theres no action associated with that
  button.
• The frame will have be closed the hard way, by
  killing the program.
• In Windows, click on the DOS window from which
  the program was launched, hold down the Ctrl key,
  and press the letter C.

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Setting the Location of a Frame

• By default, all windows (including frames) are
  displayed in the upper-left corner of the screen,
  which has coordinates (0, 0).
• The setLocation method can be used to specify a
  different location
• f.setLocation(50, 75)
• To find the current location of a frame, call
  getLocation
• Point frameLocation f.getLocation()
• The coordinates of fs upper-left corner will be
  stored in frameLocation.x and frameLocation.y.

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Adding Components to a Frame

• The Frame class is rarely used to create objects
  directly.
• Instead, its customary to define a subclass of
  Frame and then create an instance of the
  subclass.
• This strategy makes it possible to tailor the
  subclass.
• In particular, the constructor for the subclass
  can put components into the frame.

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Adding Components to a Frame

• To add a component to a frame (or any kind of
  container), the add method is used.
• add belongs to the Container class, so its
  inherited by Frame and the other container
  classes.
• An example of adding a button to a frame
• Button b new Button("Testing")
• add(b)
• These statements would normally go in the
  constructor for the frame class.

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Adding Components to a Frame

• ButtonTest is a modified version of FrameTest.
• ButtonTest defines a subclass of Frame named
  ButtonTestFrame, and then creates an instance of
  ButtonTestFrame.
• Actions taken by the ButtonTestFrame constructor
• 1. Invokes the superclass constructor (the
  constructor for Frame), passing it the title of
  the frame.
• 2. Calls setLayout to specify how the components
  inside the frame will be laid out.
• 3. Creates a Button object.
• 4. Calls add to add the button to the frame.

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• ButtonTest.java
• // Displays a frame containing a single button.
• // WARNING Frame cannot be closed.
• import java.awt.
• // Driver class
• public class ButtonTest
• public static void main(String args)
• Frame f new ButtonTestFrame("Button Test")
• f.setSize(150, 100)
• f.setVisible(true)
• // Frame class
• class ButtonTestFrame extends Frame
• public ButtonTestFrame(String title)

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Adding Components to a Frame

• Frame created by the ButtonTest program
• Pressing the Testing button has no effect.

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Adding Components to a Frame

• Instead of calling setSize, the main method in
  ButtonTest could have called pack
• f.pack()
• pack makes the frame just large enough to display
  the components within it
• Regardless of whether setSize or pack is called,
  the user can manually resize the frame.

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Adding Components to a Frame

• Its not necessary to have two separate classes,
  (ButtonTest and ButtonTestFrame).
• By moving the main method from ButtonTest to
  ButtonTestFrame, the program could be condensed
  to one class (ButtonTestFrame).

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12.3 Event Listeners

• When the user performs an action, Java creates an
  object containing information about the event.
• Responding to an event is done by writing a
  method that can be called when the event occurs.
• Steps involved in handling an event
• 1. The user performs an action, causing an event
  to be triggered (or fired).
• 2. An object is created that contains
  information about the event, including an
  indication of which component was involved.
• 3. A method that belongs to a listener object is
  called. The object created in step 2 is passed to
  the method.

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Events

• When an event occurs, an object is created that
  contains information about the event.
• This object will belong to one of several
  different classes, depending on the nature of the
  event.
• These classes all belong to the java.awt.event
  package.
• Java divides events into two groups high-level
  events and low-level events.

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Events

• High-level events
• Class Name
  Description of Event
• ActionEvent A significant action has been
  performed on
• a component (a button was pressed, a list
• item was double-clicked, or the Enter key
• was pressed in a text field).
• AdjustmentEvent The state of an adjustable
  component (such
• as a scrollbar) has changed.
• ItemEvent An item has been selected (or
  deselected)
• within a checkbox, choice menu, or list.
• TextEvent The contents of a text area or text
  field
• have changed.

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Events

• Low-level events include moving the mouse or
  pressing a key.
• One low-level event is WindowEvent, which occurs
  when the status of a window has changed.
• In particular, a WindowEvent occurs when the user
  attempts to close a window.

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Interfaces

• Event-handling requires the use of interfaces.
• An interface looks like a class, except that its
  methods arent fully defined.
• Each method in an interface has a name, a
  parameter list, and a result type, but no body.
• One common interface is named ActionListener
• public interface ActionListener extends
  EventListener
• public void actionPerformed(ActionEvent evt)
• This resembles a class declaration, except that
  the word class has been replaced by interface,
  and the actionPerformed method has no body.

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Interfaces

• An interface is nothing but a pattern that will
  be used later to define real classes.
• A class implements an interface by agreeing to
  provide bodies for all methods in the interface.
• A class that implements the ActionListener
  interface would have to provide a method named
  actionPerformed with one parameter of type
  ActionEvent and a result type of void.

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Interfaces

• The keyword implements is used to tell the
  compiler that a class will implement a particular
  interface.
• A class that implements the ActionListener
  interface
• class class-name implements ActionListener
• public void actionPerformed(ActionEvent evt)
• // Variables, constructors, and methods,
• // if desired
• The class may contain any number of variables,
  constructors, and methods.

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Creating Event Listeners

• To handle an event, its necessary to create an
  event listener object.
• This object will be registered with a
  component when an event occurs that involves the
  component, one of the listeners methods will be
  called.
• An event listener will be an instance of a
  listener class defined by the programmer.

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Creating Event Listeners

• A listener class must implement one of the
  interfaces
• that belong to the java.awt.event package.
• Listener interfaces for high-level events
• Interface Name
  Required Method
• ActionListener actionPerformed(ActionEvent evt)
• AdjustmentListener adjustmentValueChanged(Adjustm
  entEvent evt)
• ItemListener itemStateChanged(ItemEvent evt)
• TextListener textValueChanged(TextEvent evt)
• Each interface contains a single method. The
  access modifier for each method is public, and
  the result
• type is void.

33
Creating Event Listeners

• Theres a similar set of listener interfaces for
  low-level events.
• The listener interface for WindowEvent is named
  WindowListener.

34
Creating Event Listeners

• Pressing a button is an action event, so the
  listener class for a button would need to
  implement the ActionListener interface.
• To implement this interface, the class must
  define a public void method named actionPerformed
  with a parameter of type ActionEvent.
• An example of a listener for an action event
• class ButtonListener implements ActionListener
• public void actionPerformed(ActionEvent evt)

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Creating Event Listeners

• After writing a listener class, the next step is
  to create an instance of the class and connect it
  to a particular component.
• In the simplest case, a single listener object
  will be attached to a single component.
• Suppose that b is a Button object
• Button b new Button("Change Color")
• A listener object can be created by using the
  constructor for the listener class
• ButtonListener listener new ButtonListener()

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Creating Event Listeners

• listener can now be registered as an action
  listener for the button
• b.addActionListener(listener)
• Its sometimes possible to save a statement by
  combining the creation of the listener object
  with the call of addActionListener
• b.addActionListener(new ButtonListener())

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Creating Event Listeners

• Calling addActionListener creates a link between
  the Button object and its listener
• When the user presses the button, the
  ButtonListener objects actionPerformed method
  will be called.

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Creating Event Listeners

• Because ButtonListener implements the
  ActionListener interface, the compiler can verify
  that it has an actionPerformed method.
• The ActionListener interface acts as a sort of
  contract that ButtonListener agrees to honor.
• Its an error to pass an object to
  addActionListener unless the object belongs to a
  class that implements ActionListener.

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Creating Event Listeners

• The ButtonTest program displays a Testing
  button, but pressing the button has no effect.
• Making the button work involves defining a
  listener class that implements the ActionListener
  interface, creating an instance of the class, and
  attaching it to the button.
• The ButtonTest2 program is similar to ButtonTest,
  but the window will close when the button is
  pressed.
• Changes are highlighted in bold.

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• ButtonTest2.java
• // Displays a frame containing a single "Close
  window"
• // button. The frame can be closed by pressing
  the button.
• import java.awt.
• import java.awt.event.
• // Driver class
• public class ButtonTest2
• public static void main(String args)
• Frame f new ButtonTestFrame("Button Test")
• f.setSize(150, 100)
• f.setVisible(true)

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• // Frame class
• class ButtonTestFrame extends Frame
• public ButtonTestFrame(String title)
• super(title)
• setLayout(new FlowLayout())
• Button b new Button("Close window")
• add(b)
• b.addActionListener(new ButtonListener())
• // Listener for button
• class ButtonListener implements ActionListener
• public void actionPerformed(ActionEvent evt)
• System.exit(0)

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Creating Event Listeners

• Frame created by the ButtonTest2 program

43
Creating Event Listeners

• Pressing the Close window button causes a call
  of the actionPerformed method for the buttons
  listener object.
• This method calls System.exit, which causes the
  program to terminate and the frame to disappear.
• When a program terminates, any windows that it
  created are automatically closed.

44
Adapter Classes

• To make the Close button work, a WindowEvent
  listener is needed.
• A class that implements the WindowListener
  interface would have to contain seven methods.
• Theres an easier technique use the
  WindowAdapter class from the java.awt.event
  package.
• This class implements the WindowListener
  interface, although the methods that it provides
  are all empty.

45
Adapter Classes

• The listener class will extend the WindowAdapter
  class and override the windowClosing method.
• It will then inherit all the other methods it
  needs.
• WindowAdapter is an example of an adapter classa
  class that can be extended instead of
  implementing an interface.
• Java provides matching adapter classes for most
  interfaces that have two or more methods.

46
Adapter Classes

• The ButtonTest3 program is a modification of
  ButtonTest2.
• The new WindowCloser class extends WindowAdapter
  and provides a windowClosing method of its own.
• The constructor for ButtonTestFrame now calls
  addWindowListener to install a WindowCloser
  object as a listener for window events.

47

• ButtonTest3.java
• // Displays a frame containing a single "Close
  window"
• // button. The frame can be closed by pressing
  either the
• // "Close window" button or the frame's "close"
  button.
• import java.awt.
• import java.awt.event.
• // Driver class
• public class ButtonTest3
• public static void main(String args)
• Frame f new ButtonTestFrame("Button Test")
• f.setSize(150, 100)
• f.setVisible(true)

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• // Frame class
• class ButtonTestFrame extends Frame
• public ButtonTestFrame(String title)
• super(title)
• setLayout(new FlowLayout())
• Button b new Button("Close window")
• add(b)
• b.addActionListener(new ButtonListener())
• // Attach window listener
• addWindowListener(new WindowCloser())
• // Listener for button
• class ButtonListener implements ActionListener
• public void actionPerformed(ActionEvent evt)
• System.exit(0)

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• // Listener for window
• class WindowCloser extends WindowAdapter
• public void windowClosing(WindowEvent evt)
• System.exit(0)

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Adapter Classes

• When a window event occurs, one of the methods in
  the WindowCloser class will be called.
• If the event is caused by the user attempting to
  close the window, the windowClosing method is
  called, and the program terminates.
• Any other window event will cause one of
  WindowClosers inherited methods to be called.
• These methods are empty, so nothing will happen.

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12.4 Inner Classes

• The ChangeColor program will also have a single
  button.
• Pressing the button will change the background
  color of the frame. The background will initially
  be white

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The ChangeColor Program

• Pressing the button once will change the
  background to black
• Pressing it again will cause the background to
  return to white.

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The setBackground andgetBackground Methods

• Changing the background color of a frame is done
  by calling the setBackground method, which is
  inherited from the Component class
• setBackground(Color.black)
• The getBackground method (also inherited from
  Component) returns the current background color.

54
Writing the ChangeColor Program

• The button listener in ChangeColor will change
  the frames background color instead of causing
  the program to terminate
• class ButtonListener implements ActionListener
• public void actionPerformed(ActionEvent evt)
• if (getBackground() Color.white)
• setBackground(Color.black)
• else
• setBackground(Color.white)

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Writing the ChangeColor Program

• Unfortunately, the compiler wont allow the
  listener to call getBackground or setBackground,
  because those methods dont belong to the
  ButtonListener class or its superclass (Object).
• The actionPerformed method needs to get (and set)
  the background for the frame, not the background
  for the ButtonListener object itself (which
  doesnt have a background, anyway).

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Writing the ChangeColor Program

• Problems of this sort are common when writing a
  listener class because listeners often need
  access to variables or methods that belong to the
  frame.
• Theres an easy way to solve such a problem put
  the listener class inside the frame class.
• A class thats nested inside another class is
  said to be an inner class.
• Inner class methods have access to variables and
  methods in the enclosing class, allowing the
  inner class to serve as a helper for the
  enclosing class.

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• ChangeColor.java
• // Displays a frame containing a single button.
  Pressing the
• // button causes the background of the frame to
  change from
• // white to black or from black to white.
• import java.awt.
• import java.awt.event.
• // Driver class
• public class ChangeColor
• public static void main(String args)
• Frame f new ChangeColorFrame("Change
  Color")
• f.setSize(160, 100)
• f.setVisible(true)

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• // Frame class
• class ChangeColorFrame extends Frame
• public ChangeColorFrame(String title)
• // Set title, layout, and background color
• super(title)
• setLayout(new FlowLayout())
• setBackground(Color.white)
• // Add "Change color" button to frame and
  attach listener
• Button b new Button("Change color")
• add(b)
• b.addActionListener(new ButtonListener())
• // Attach window listener
• addWindowListener(new WindowCloser())

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• // Listener for button
• class ButtonListener implements ActionListener
  
• public void actionPerformed(ActionEvent evt)
  
• if (getBackground() Color.white)
• setBackground(Color.black)
• else
• setBackground(Color.white)
• // Listener for window
• class WindowCloser extends WindowAdapter
• public void windowClosing(WindowEvent evt)
• System.exit(0)

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12.5 Attaching Listeners toMultiple Components

• If a program has two buttons, one possibility is
  to attach the second button to the same
  ButtonListener object
• The ButtonListener objects actionPerformed
  method will be called when either button is
  pressed.

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Using Multiple Listeners

• The other possibility is to create a different
  listener object for the second button

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Using Multiple Listeners

• The second listener could be an instance of a
  different listener class

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Determining the Source of an Event

• If two or more buttons are connected to a single
  listener, its actionPerformed method will need to
  determine which button was pressed.
• Ways to solve this problem
• Compare the source of the event (the component
  that triggered the method call) to see which
  Button object it matches.
• Test the events action command to see which
  button label it matches.

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The getSource Method

• If evt is an instance of any event class, the
  source of the event can be found by calling
  getSource
• Object source evt.getSource()
• Because events can be caused by a variety of
  components, getSource returns an Object
  reference.
• To determine which component fired the event, the
  value returned by getSource can be compared with
  the variables containing the components
• if (source testButton)

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The getActionCommand Method

• The other way to determine the origin of a button
  press is to use the getActionCommand method.
• String label evt.getActionCommand()
• This method can be used only with action events,
  such as button presses.
• In the case of a button press, getActionCommand
  returns the label on the button.
• A statement that checks whether the button
  labeled "Testing" was pressed
• if (label.equals("Testing"))

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Example A Single Listener

• The ChangeColor2 program displays two buttons,
  labeled Lighter and Darker

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Example A Single Listener

• Pressing the Lighter button will lighten the
  background slightly

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Example A Single Listener

• Pressing Darker will darken it
• Each button can be pressed more than once,
  causing the background to become successively
  lighter or darker.

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Example A Single Listener

• Its possible to use a single listener object for
  both buttons. Alternatively, there could be two
  listeners, one for each button.
• The ChangeColor2 program will use a single
  listener.
• The listeners actionPerformed method will
  determine which button was pressed by testing the
  string returned by getActionCommand.
• There are only two buttons, so the string will be
  either "Lighter" or "Darker".

70

• ChangeColor2.java
• // Displays a frame containing two buttons.
  Pressing the
• // "Lighter" button lightens the background of
  the frame.
• // Pressing the "Darker" button darkens the
  background.
• import java.awt.
• import java.awt.event.
• // Driver class
• public class ChangeColor2
• public static void main(String args)
• Frame f new ChangeColorFrame("Change
  Color")
• f.setSize(160, 100)
• f.setVisible(true)

71

• // Frame class
• class ChangeColorFrame extends Frame
• public ChangeColorFrame(String title)
• // Set title, layout, and background color
• super(title)
• setLayout(new FlowLayout())
• setBackground(Color.gray)
• // Create button listener
• ButtonListener listener new
  ButtonListener()
• // Add "Lighter" button to frame and attach
  listener
• Button b new Button("Lighter")
• add(b)
• b.addActionListener(listener)
• // Add "Darker" button to frame and attach
  listener
• b new Button("Darker")
• add(b)

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• // Listener for both buttons
• class ButtonListener implements ActionListener
  
• public void actionPerformed(ActionEvent evt)
  
• Color currentBackground getBackground()
• String buttonLabel evt.getActionCommand()
  
• // Test label on button and change
  background color
• if (buttonLabel.equals("Lighter"))
• setBackground(currentBackground.brighter()
  )
• else
• setBackground(currentBackground.darker())
  
• // Listener for window
• class WindowCloser extends WindowAdapter
• public void windowClosing(WindowEvent evt)
• System.exit(0)

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Example A Single Listener

• The brighter and darker methods belong to the
  Color class.
• brighter returns a brighter version of the Color
  object that called it darker returns a darker
  version.

74
Example Separate Listeners

• The ChangeColor3 program is similar to
  ChangeColor2, except that it uses separate
  listeners for the Lighter and Darker buttons.
• Using separate listeners requires an additional
  listener class.
• On the other hand, the actionPerformed method in
  each class will be very short, because theres no
  need to test which button was pressed.

75

• ChangeColor3.java
• // Displays a frame containing two buttons.
  Pressing the
• // "Lighter" button lightens the background of
  the frame.
• // Pressing the "Darker" button darkens the
  background.
• import java.awt.
• import java.awt.event.
• // Driver class
• public class ChangeColor3
• public static void main(String args)
• Frame f new ChangeColorFrame("Change
  Color")
• f.setSize(160, 100)
• f.setVisible(true)

76

• // Frame class
• class ChangeColorFrame extends Frame
• public ChangeColorFrame(String title)
• // Set title, layout, and background color
• super(title)
• setLayout(new FlowLayout())
• setBackground(Color.gray)
• // Add "Lighter" button to frame and attach
  listener
• Button b new Button("Lighter")
• add(b)
• b.addActionListener(new LighterButtonListener(
  ))
• // Add "Darker" button to frame and attach
  listener
• b new Button("Darker")
• add(b)
• b.addActionListener(new DarkerButtonListener()
  )
• // Attach window listener

77

• // Listener for "Lighter" button
• class LighterButtonListener implements
  ActionListener
• public void actionPerformed(ActionEvent evt)
  
• setBackground(getBackground().brighter())
• // Listener for "Darker" button
• class DarkerButtonListener implements
  ActionListener
• public void actionPerformed(ActionEvent evt)
  
• setBackground(getBackground().darker())
• // Listener for window
• class WindowCloser extends WindowAdapter
• public void windowClosing(WindowEvent evt)
• System.exit(0)

78
12.6 Layout

• The layout of components within a container
  remains a mystery.
• Consider the ChangeColor2 frame
• Why are the buttons placed side by side?
• Why are the buttons centered within the frame?

79
Layout Managers

• These decisions are made by an object known as a
  layout manager.
• Every container has a default layout manager that
  determines the sizes and positions of components
  within the container.
• This layout manager can be replaced if desired.
• One reason that Java uses layout managers is so
  that containers can be resized gracefully.
• Each time a container is resized, the containers
  layout manager determines new sizes and positions
  for the components in the container.

80
Layout Manager Classes

• The java.awt package provides five layout manager
  classes
• Class Name
  Behavior
• BorderLayout Arranges components along the sides
  of the
• container and in the middle.
• CardLayout Arrange components in cards. Only
  one
• card is visible at a time.
• FlowLayout Arranges components in variable-length
  rows.
• GridBagLayout Aligns components horizontally and
  vertically
• components can be of different sizes.
• GridLayout Arranges components in fixed-length
  rows and
• columns.

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Layout Manager Classes

• FlowLayout and GridLayout are the easiest layout
  managers to understand.
• BorderLayout and CardLayout are somewhat harder
  to use.
• GridBagLayout is the most powerful layout
  manager, but its also the most complicated.

82
Layout Manager Classes

• Choosing a layout manager is done by calling the
  setLayout method. (setLayout belongs to the
  Container class, so its inherited by all
  container classes.)
• To select FlowLayout as the layout manager for a
  frame, put the following call of setLayout in the
  frames constructor
• setLayout(new FlowLayout())
• If no layout manager is specified for a frame,
  the default is BorderLayout.

83
The FlowLayout Class

• The FlowLayout layout manager can handle any
  number of components.
• The components are laid out side by side from
  left to right.
• When no more components will fit in a row,
  FlowLayout starts a new row.

84
The FlowLayout Class

• Suppose that a frame containing seven buttons
  uses FlowLayout as its layout manager.
• The number of buttons that can be squeezed into a
  row depends on how wide the frame is

85
The FlowLayout Class

• The simplest way to use FlowLayout is to call its
  no-arg constructor and pass the resulting object
  to setLayout
• setLayout(new FlowLayout())
• By default, components will be separated by five
  pixels of space and centered in each row.

86
The FlowLayout Class

• The alignment can be specified explicitly by
  passing FlowLayout.LEFT, FlowLayout.RIGHT, or
  FlowLayout.CENTER to the constructor
• setLayout(new FlowLayout(FlowLayout.LEFT))
• The horizontal and vertical gaps between
  components can also be passed to the constructor
• setLayout(new FlowLayout(FlowLayout.LEFT, 20,
  10))

87
The GridLayout Class

• The GridLayout layout manager places components
  in rows, with each row (except possibly the last)
  having an equal number of components

88
The GridLayout Class

• If a frame with a GridLayout is resized, the
  components within the frame change size as well

89
The GridLayout Class

• The GridLayout constructor requires that the
  number of rows and columns be specified
• setLayout(new GridLayout(4, 5))
• Components will be arranged in four rows and
  five columns, with no space between components.
• If space is desired between components, two more
  argumentsthe horizontal gap and the vertical
  gapare supplied to the constructor
• setLayout(new GridLayout(4, 5, 20, 10))

90
The GridLayout Class

• GridLayout works best when all components in the
  container are the same kind (all buttons, for
  example), because it forces the components to be
  the same size.
• If the container contains a mixture of
  components, some components may end up appearing
  too large while others look too small.

91
The BorderLayout Class

• The BorderLayout layout manager can handle up to
  five components.
• Four of the components can be positioned against
  the sides of the container, with the fifth
  occupying the center of the container.

92
The BorderLayout Class

• The positions in a BorderLayout are named North,
  South, East, West, and Center

93
The BorderLayout Class

• The North and South components are stretched to
  the width of the container.
• The West and East components are stretched
  vertically to fill the gap between North and
  South.
• The Center component expands in both directions
  to fill any remaining space.

94
The BorderLayout Class

• The no-arg version of the BorderLayout
  constructor leaves no space between components
• setLayout(new BorderLayout())
• A different constructor is used if space is
  needed between components
• setLayout(new BorderLayout(20, 10))

95
The BorderLayout Class

• When a container uses BorderLayout as its layout
  manager, a different version of the add method is
  required
• add("Center", new Button("Test"))
• The first argument must be either "North",
  "South", "East", "West", or "Center".

96
The BorderLayout Class

• If a frame with a BorderLayout is resized, the
  heights of the North and South components
  dont change
• The widths of the East and West components
  also remain the same.

97
The BorderLayout Class

• BorderLayout doesnt require that all five
  positions be used unused positions are filled by
  neighboring components.
• This property makes BorderLayout a surprisingly
  versatile layout tool.

98
Preferred Sizes

• Every component has a preferred size.
• For example, the preferred size of a button is
  determined by the size of the label on the
  buttonthe longer the label, the wider the button.

99
Preferred Sizes

• Each layout manager has a different way of
  dealing with preferred sizes
• FlowLayout Honors the preferred sizes of all
  components.
• GridLayout Ignores the preferred sizes of all
  components.
• BorderLayout Honors the preferred widths of the
  East and West components. Honors the preferred
  heights of the North and South components.
  Ignores the preferred size of the Center
  component.

100
Preferred Sizes

• The layout examples shown earlier illustrate this
  behavior
• The buttons in the FlowLayout example stayed the
  same size (their preferred size), no matter what
  the size of the frame was.
• The buttons in the GridLayout example expanded to
  fill the entire frame.
• In the BorderLayout example, the North and
  South buttons kept their preferred height,
  while the East and West buttons kept their
  preferred width.

101
Panels

• A panelan instance of the Panel classis another
  kind of container.
• A panel is rectangular but has no border.
• When a panel is placed inside another container,
  it blends in seamlessly.
• Each panel has its own layout manager.
• A panel can be used to create a group of
  components that is treated as a single component.

102
Panels

• Panel objects can be created by using the no-arg
  version of the Panel constructor
• Panel p new Panel()
• By default, the layout manager for a panel is
  FlowLayout.
• A different layout manager can be chosen by
  calling setLayout
• p.setLayout(new BorderLayout())
• Passing the layout manager to the Panel
  constructor avoids a separate call of setLayout
• Panel p new Panel(new BorderLayout())

103
Panels

• Once a panel has been created, components are
  added to it by calling the add method
• p.add("Center", new Button("Test"))
• The panel itself will need to be added to a frame
  or other container.

104
Panels

• Consider the problem of creating a frame with the
  following appearance

105
Panels

• The top 12 buttons will need to be grouped into a
  single component using a panel.
• The panel will use a GridLayout to force the
  buttons into four rows and three columns.
• To make sure that this panel is positioned above
  the Dial button, the frame itself will need to
  use a BorderLayout.
• If the Dial button is placed at the South
  position and the panel at the Center position,
  the panel will expand to fill the frame.

106
Panels

• To keep the Dial button at the correct size, it
  will need to be put in a panel of its own, which
  is then placed at the South position.
• This panel will have a FlowLayout manager, which
  will center the button and keep it at its
  preferred size.
• The panel containing the 12 buttons will also
  need to be put inside another panel, to keep the
  buttons from growing if the frame is resized.

107
Panels

• Summary of panels needed
• buttonPanel Contains 12 buttons uses
  GridLayout.
• centerPanel Contains buttonPanel uses
  FlowLayout.
• bottomPanel Contains Dial button uses
  FlowLayout.

108
Panels

• A figure showing the panels as dashed rectangles

109
Panels

• Statements to create the phone layout
• Panel buttonPanel new Panel()
• buttonPanel.setLayout(new GridLayout(4, 3, 10,
  10))
• for (int i 1 i lt 9 i)
• buttonPanel.add(new Button(i ""))
• buttonPanel.add(new Button(""))
• buttonPanel.add(new Button("0"))
• buttonPanel.add(new Button(""))
• Panel centerPanel new Panel()
• centerPanel.add(buttonPanel)
• add("Center", centerPanel)
• Panel bottomPanel new Panel()
• bottomPanel.add(new Button("Dial"))
• add("South", bottomPanel)

110
12.7 Creating and Using Components

• For each component, its important to know three
  things
• How to create the component
• What kind of event(s) it fires
• How to determine the current state of the
  component

111
Checkboxes

• A checkbox is a small box that the user can
  check by clicking with the mouse
• Clicking on the box causes a check mark to
  appear
• Clicking a second time removes the check mark
  from the box.

112
Checkboxes

• A checkbox normally has a label, which is passed
  to the Checkbox constructor as an argument
• Checkbox cb new Checkbox("Enable sounds")
• The no-arg version of the Checkbox constructor
  creates a checkbox without a label
• Checkbox cb new Checkbox()
• By default, a new checkbox is in the off state
  (no check mark).
• Creating a checkbox thats on requires using a
  constructor that takes the state as its second
  argument
• Checkbox cb new Checkbox("Enable sounds",
  true)

113
Checkboxes

• When a checkbox is clicked, an item event occurs.
• Detecting this event requires writing a listener
  class that implements the ItemListener interface.
• Implementing this interface requires writing a
  method named itemStateChanged
• class CheckboxListener implements ItemListener
• public void itemStateChanged(ItemEvent evt)
• The addItemListener method can be used to attach
  a listener to the checkbox
• cb.addItemListener(new CheckboxListener())

114
Checkboxes

• Not every checkbox will require a listener.
• A program may wait for some other event to occur
  and then examine the checkboxes to see which ones
  are currently checked.
• The getState method returns the state of a
  checkbox
• boolean state cb.getState()
• The setState method changes the state of a
  checkbox
• cb.setState(true)

115
Checkbox Groups

• A checkbox group is a collection of checkboxes in
  which only one box can be checked at a time
• Checkboxes that are related in this way are often
  referred to as radio buttons.
• Checkboxes that belong to a group often have a
  different appearance than individual checkboxes.
• Under Windows, boxes in a group are round instead
  of square.

116
Checkbox Groups

• The first step in creating a group of checkboxes
  is to create a CheckboxGroup object
• CheckboxGroup musicGroup new CheckboxGroup()
• The next step is to create the checkboxes,
  supplying the CheckboxGroup object as the second
  argument to the Checkbox constructor
• Checkbox rockBox
• new Checkbox("Rock", musicGroup, true)
• Checkbox jazzBox
• new Checkbox("Jazz", musicGroup, false)
• Checkbox classicalBox
• new Checkbox("Classical", musicGroup, false)

117
Choice Menus

• A choice menu (or popup menu) displays one of
  several items
• When the user presses on the arrow button with
  the mouse, the full list of choices pops up

118
Choice Menus

• Creating a choice menu requires two steps. The
  first step is to create a Choice object
• Choice countryChoice new Choice()
• The second step is to add menu items using the
  add method
• countryChoice.add("U.S.A.")
• countryChoice.add("Canada")
• countryChoice.add("Mexico")
• The order in which the items are added determines
  the order in which theyll appear on the menu.

119
Choice Menus

• When the user pops up the menu and makes a
  choice, an item event occurs.
• As a result, the listener class for a choice menu
  will need to implement the ItemListener
  interface.
• The getSelectedItem method returns the selected
  item
• String itemSelected countryChoice.getSelectedIt
  em()
• The getSelectedIndex method returns the position
  of the selected item
• int itemIndex countryChoice.getSelectedIndex()
  
• The first item in the list has index 0.

120
Labels

• A label is a rectangular area containing a text
  string
• A label has no border around it the user sees
  nothing but the text.
• Labels are often placed next to other components
  to indicate their meaning or function.
• The user cant change a labels text there are
  no events defined for labels.

121
Labels

• One of the Label constructors takes a single
  argument, the text to be displayed within the
  label
• Label lastName new Label("Enter last name")
• By default, the text is left-justified within the
  label.
• The desired alignment can be passed as a second
  argument to the Label constructor
• Label lastName
• new Label("Enter last name", Label.CENTER)
• Possible values are Label.CENTER, Label.LEFT,
  and Label.RIGHT.

122
Labels

• The getText method returns the text of a label
• String labelContents lastName.getText()
• The setText method changes the text of a label
• lastName.setText("Enter first name")

123
Lists

• A list is a rectangle containing a series of
  items
• The user can choose an item by clicking on it

124
Lists

• If not all list items are visible, a scrollbar
  appears to the right of the list

125
Lists

• Creating a list is similar to creating a choice
  menu.
• The first step is to create a List object
• List countryList new List()
• By default, four items will be visible at a time.
  The number of visible items can be specified if
  desired
• List countryList new List(5)
• Once the list has been created, the add method is
  used to add items to it
• countryList.add("U.S.A.")
• countryList.add("Canada")
• countryList.add("Mexico")

126
Lists

• Single-clicking on a list item causes an item
  event.
• Double-clicking causes an action event.
• To determine which item was selected, either
  getSelectedIndex or getSelectedItem can be called.

127
Scrollbars

• A scrollbar is a sliding bar.
• Scrollbars can be either horizontal
• or vertical

128
Scrollbars

• Each scrollbar represents a number chosen from a
  range of integers, such as 0 to 100 or 32 to 212.
• The width of the sliding portion of the scrollbar
  (the scroll box or bubble) must be at least 1
  (measured in the scrollbars own units, not in
  pixels), but it can be wider if desired.

129
Scrollbars

• The largest value that the user can select is the
  maximum value of the scrollbars range minus the
  width of the scroll box.
• If the scrollbar has a range of 0 to 100, and the
  scroll box has a width of 10, then the largest
  value that the user can select is 100 10 90.

130
Scrollbars

• Ways for the user to change the value of a
  scrollbar
• Drag the scroll box to a different position.
• Click on the arrow buttons, which changes the
  value by a small amount, known as the unit
  increment. (By default, the unit increment is
  1.)
• Click in the area between an arrow button and the
  scroll box, which changes the value by a larger
  amount, known as the block increment. (By
  default, the block increment is 10.)

131
Scrollbars

• One Scrollbar constructor has five arguments
• Scrollbar sb
• new Scrollbar(Scrollbar.HORIZONTAL, 50, 1, 0,
  100)
• The first argument (Scrollbar.HORIZONTAL or
  Scrollbar.VERTICAL) specifies the scrollbars
  orientation.
• The fourth and fifth arguments specify the
  minimum and maximum values of the scrollbars
  range.
• The second argument is the initial value of the
  scrollbar.
• The third argument is the width of the scroll
  box, which must be at least 1.

132
Scrollbars

• When the user adjusts a scrollbar, an adjustment
  event occurs.
• Handling the event requires writing a class that
  implements the AdjustmentListener interface.
• This class must contain a method named
  adjustmentValueChanged
• class ScrollbarListener implements
  AdjustmentListener
• public void adjustmentValueChanged(AdjustmentEve
  nt evt)

133
Scrollbars

• The addAdjustmentListener method is used to
  attach a listener to the scrollbar
• sb.addAdjustmentListener(new ScrollbarListener())
  
• The getValue method returns the current value of
  a scrollbar
• int value sb.getValue()
• The setValue method changes the value of a
• scrollbar
• sb.setValue(newValue)

134
Text Areas

• A text area is capable of displaying multiple
  lines of text
• Scrollbars at the bottom and right side make it
  possible for the user to view text thats not
  otherwise visible.

135
Text Areas

• There are four ways to create a TextArea object,
  depending on
• Whether or not text is to be displayed initially.
• Whether the number of rows and columns is
  specified.

136
Text Areas

• Assume that quote is the following string
• String quote
• "To be, or not to be that is the question\n"
  
• "Whether 'tis nobler in the mind to suffer\n"
• "The slings and arrows of outrageous
  fortune,\n"
• "Or to take arms against a sea of troubles,\n"
  
• "And by opposing end them? To die to sleep\n"
  
• "No more and, by a sleep to say we end\n"
• "The heartache and the thousand natural
  shocks\n"
• "That flesh is heir to, 'tis a consummation\n"
  
• "Devoutly to be wish'd. To die, to sleep\n"
• "To sleep perchance to dream ay, there's the
  rub\n"
• "For in that sleep of death what dreams may
  come"
• Notice that new-line characters are used to
  separate lines.

137
Text Areas

• The no-arg constructor creates an empty text area
  with a default size
• TextArea ta new TextArea()
• To create a nonempty text area, a string
  containing the desired text is passed as an
  argument
• TextArea ta new TextArea(quote)
• The number of rows and columns can be specified
• TextArea ta new TextArea(10, 20)
• Its also possible to specify values for both the
  text and the rows and columns
• TextArea ta new TextArea(quote, 10, 20)

138
Text Areas

• A text area can be made editable or not editable
  by calling setEditable and passing either true or
  false
• ta.setEditable(false) // Not editable
• Text areas are editable by default.
• A text event occurs whenever the user changes any
  of the text in a text area.

139
Text Areas

• Detecting a text event requires writing a class
  that implements the TextListener interface.
• Implementing this interface involves writing a
  method named textValueChanged
• class TextAreaListener implements TextListener
• public void textValueChanged(TextEvent evt)
• The addTextListener method attaches a listener to
  a text area
• ta.addTextListener(new TextAreaListener())

140
Text Areas

• The getText method returns the current contents
  of a text area
• String text ta.getText()
• getText returns a single string, with new-line
  characters marking breaks between lines.
• The setText method replaces the contents of a
  text area
• ta.setText("Line 1\nLine 2\nLine 3")
• The append method adds text to the end of a text
  area
• ta.append("\nLine 4")

141
Text Fields

• A text field contains a single line of text
• The TextField class has four constructors, whose
  arguments specify the contents of the text field
  and/or the number of columns in the text field
• TextField tf new TextField()
• TextField tf new TextField("Your name here")
• TextField tf new TextField(40)
• TextField tf new TextField("Your name here",
  40)

142
Text Fields

• Text fields are editable by default.
• A text field can be made not editable by calling
  setEditable with false as the argument
• tf.setEditable(false) // Not editable
• A text field can fire text events, which occur
  when the user modifies the contents of the text
  field.
• An action event occurs when the user presses the
  Enter key after entering data into a text field.
• Both text events and action events are possible
  only if the text field is editable.

143
Text Fields

• The methods for text fields, which include
  getText and setText, are similar to those for
  text areas.
• This similarity isnt surprising, because the
  TextArea and TextField classes inherit much of
  their behavior from their superclass,
  TextComponent.
• Text fields dont support the append method,
  however.

144
12.8 Examples

• The ConvertTemp, ShowDefinition, and PickColor
  programs illustrate the use of various GUI
  components.

145
Using Labels and Text FieldsTemperature
Conversion

• ConvertTemp is a GUI version of the Chapter 2
  program that converts Fahrenheit temperatures to
  Celsius.
• The new version will also be able to convert
  temperatures from Celsius to Fahrenheit.
• ConvertTemp will display the following frame

146
Using Labels and Text FieldsTemperature
Conversion

• If the user enters a value in the Fahrenheit
  field and presses the Enter key, the
  corresponding Celsius temperature will appear in
  the Celsius field

147
Using Labels and Text FieldsTemperature
Conversion

• Likewise, if the user enters a value in the
  Celsius field and presses the Enter key, the
  corresponding Fahrenheit temperature will appear
  in the Fahrenheit field
• Temperatures displayed by the program will be
  rounded to two decimal places.

148
Using Labels and Text FieldsTemperature
Conversion

• Designing this program requires confronting two
  issues layout and event-handling.
• The GUI components will be two labels and two
  text fields.
• The layout manager can be GridLayout with two
  rows and two columns.
• GridLayout will make the labels and text fields
  all the same size.
• The text fields will need to be declared as
  instance variables so that a listener will be
  able to modify one of the text fields when the
  other is changed.

149
Using Labels and Text FieldsTemperature
Conversion

• ConvertTemp will need at least two listeners.
• One listener will cause the program to terminate
  when the user closes the frame.
• Another listener will be called when the user
  enters data into either one of the text fields.
• Although one listener is enough for this purpose,
  the program is easier to understand if two
  listeners are used, one for each field.

150
Using Labels and Text FieldsTemperature
Conversion

• The listener classes, FahrenheitListener and
  CelsiusListener, will each have an
  actionPerformed method.
• Actions taken by this method
• Retrieve the users input from one of the text
  fields and convert it to double form.
• Convert this number from one temperature scale to
  the other.
• Round the result to two decimal places and
  display it in the other text field.
• Convert.toDouble (from the jpb package) is used
  to convert the users input into a double value.

151

• ConvertTemp.java
• // Converts a Fahrenheit temperature entered by
  the user to
• // Celsius, or vice versa
• import java.awt.
• import java.awt.event.
• import jpb.
• // Driver class
• public class ConvertTemp
• public static void main(String args)
• Frame frame
• new ConvertTempFrame("Temperature
  Conversion")
• frame.setSize(150, 75)
• frame.setVisible(true)

152

• // Frame class
• class ConvertTempFrame extends Frame
• private TextField fahrenField new
  TextField()
• private TextField celsiusField new
  TextField()
• // Constructor
• public ConvertTempFrame(String title)
• // Set title for frame and choose layout
• super(title)
• setLayout(new GridLayout(2, 2))
• // Add Fahrenheit label and text field to
  frame attach
• // listener to text field
• add(new Label("Fahrenheit"))