Event-Driven Programming and Basic GUI Objects

1
Chapter 7

• Event-Driven Programming and Basic GUI Objects

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Topics

• Graphical User Interfaces
• Inheritance
• GUI components
• Interfaces
• Events and Listeners

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Introduction

• This chapter covers the graphical user interface
  (GUI). In Java, GUI-based programs are
  implemented by using classes from the javax.swing
  and java.awt packages.
• The Swing classes provide greater compatibility
  across different operating systems. They are
  fully implemented in Java, and behave the same on
  different operating systems.

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GUI Components

• The javax.swing package contains a number of GUI
  components.
• JFrame
• JMenu
• JLabel
• JButton
• JTextField
• ImageIcon
• JTextArea

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Sample GUI Program
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Introduction

• AWT classes are implemented by using the native
  GUI objects.
• Swing classes support many new functionalities
  not supported by AWT counterparts.
• Do not mix the counterparts in the same program
  because of their differences in implementation.

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Introduction

• To build an effective GUI using objects from the
  Swing and AWT packages, we must learn a new style
  of program control called event-driven
  programming.
• An event occurs when the user interacts with a
  GUI object.
• In event-driven programs, we program objects to
  respond to these events by defining
  event-handling methods.

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Creating a Subclass of JFrame

• As we know from Chapter 1, inheritance is a
  feature we use to define a more specialized class
  from an existing class.
• The existing class is the superclass and the
  specialized class is the subclass.

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Inheritance hierarchy of JOptionPane

• From the API documentation.
• java.lang.Object
• - java.awt.Component
• - java.awt.Container
• - javax.swing.JComponent
• - javax.swing.JOptionPane

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Creating a Subclass of JFrame

• To create a customized user interface, we often
  define a subclass of the JFrame class.
• The JFrame class contains rudimentary
  functionalities to support features found in any
  frame window.

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Default JFrame Window

• A default JFrame window appears at the top left
  corner of the screen.
• You may not notice this because it is so small.
• It looks different on computers running different
  operating systems

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Creating a Subclass of JFrame

• To define a subclass of another class, we declare
  the subclass with the reserved word extends.
• class Ch7JFrameSubclass1 extends JFrame
• ...

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Creating a Subclass of JFrame

• We will also add the following default
  characteristics
• The title is set to My First Subclass.
• The program terminates when the close box is
  clicked.
• The size of the frame is 300 pixels wide by 200
  pixels high.
• The frame is positioned at screen coordinate
  (150, 250).
• These properties are set inside the default
  constructor.

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Size and Location of a JFrame

• How an instance of Ch7JFrameSubclass1 will appear
  on the screen.

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Inheritance

• Every method of a superclass is inherited by its
  subclass.
• Because the subclass-superclass relationships are
  formed into an inheritance hierarchy, a subclass
  inherits all methods defined in its ancestor
  classes.

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Another Subclass of JFrame

• Next we will define another subclass called
  Ch7JFrameSubclass2, which will have a white
  background.
• We will define this class as an instantiable main
  class so we dont have to define a separate main
  class.

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Using the Content Pane

• To make the background white, we must access the
  frames content pane, the area of the frame
  excluding the title and menu bars and the border.
• We access the content pane by calling the frames
  getContentPane method. We change the background
  color by calling the content panes setBackground
  method.

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Ch7JFrameSubclass2
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Using Buttons in a JFrame

• There are two approaches to placing GUI objects
  on a frames content pane.
• One approach uses a layout manager, an object
  that controls the placement of the GUI objects.
• The other approach uses absolute positioning to
  explicitly specify the position and size of GUI
  objects on the content pane.

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Absolute Positioning of Components

• To use absolute positioning, set the layout
  manager of a frames content pane to none by
  passing null to the setLayout method.
• contentPane.setLayout(null)
• Set the size and position of a button by calling
  the buttons setBounds method
• okButton.setBounds( 75, 125, 80, 30)
• The first two arguments specify the buttons
  position.
• The last two arguments specify the width and
  height of the button.
• To make a button appear on the frame, add it to
  the content pane by calling the add method.
• contentPane.add(okButton)

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Button Placement
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Handling Button Events

• An action involving a GUI object, such as
  clicking a button, is called an event.
• The mechanism to process events is called event
  handling.
• The event-handling model of Java is based on the
  concept known as the delegation-based event
  model.
• With this model, event handling is implemented by
  two types of objects
• event source objects
• event listener objects.

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Event Handling in Java
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Event Sources and Listeners

• An event source object is a GUI object where an
  event occurs. An event source generates events.
• An event listener object is an object that
  includes a method that gets executed in response
  to the generated events. When an event is
  generated, the system notifies the relevant event
  listener objects.

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Events

• There are many different kinds of events, but the
  most common one is an action event.
• For the generated events to be processed, we must
  associate, or register, event listeners to the
  event sources.
• If event sources have no registered listeners,
  the events they generate are ignored.

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Listener Classes

• An object that can be registered as an action
  listener must be an instance of a class that is
  declared specifically for the purpose. We call
  such classes action listener classes.
• To associate an action listener to an action
  event source, we call the event sources
  addActionListener method with the action listener
  as its argument.

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Listeners and Event Sources

• A single listener can be associated to multiple
  event sources.
• Likewise, multiple listeners can be associated to
  a single event source.

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Event Handling

• When an event source generates an event, the
  system checks for matching registered listeners.
• If there is no matching listener, the event is
  ignored.
• If there is a matching listener, the system
  notifies the listener by calling the appropriate
  method in the listener.
• In the case of action events, the method called
  is actionPerformed.
• How does the system know it can call this method
  for any listener class?

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Interfaces

• Like a class, an interface is a reference data
  type, but unlike a class, an interface includes
  only constants and abstract methods.
• An abstract method has only the method header, or
  prototype it has no method body.
• An interface cannot be instantiated.
• You use an interface by writing a class that
  implements an interface.

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Implementing Interfaces

• A class that implements a Java interface must
  provide the method body for all the abstract
  methods defined in the interface.
• By requiring an object we pass as an argument to
  the addActionListener method to be an instance of
  a class that implements the ActionListener
  interface, the system ensures that this object
  will include the necessary actionPerformed
  method.
• We use ActionListener as a parameter type in the
  addActionListener method
• An object from any class that implements
  ActionListener can be used as an argument.

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ActionListener Classes

• ActionListener is an interface, not a class.
• To ensure that the programmer includes the
  necessary actionPerformed method in an action
  listener class, the class must implement the
  ActionListener interface.
• import java.awt.event.
• class ButtonHandler implements ActionListener
• ...
• In the actionPerformed method, we need to define
  what should happen when an event occurs.

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The actionPerformed method

• To change the title of the frame, depending on
  which button is clicked, we use the
  actionPerformed method. The method model is
• public void actionPerformed(ActionEvent evt)
• String buttonText
• get the text of the event source
• JFrame frame
• the frame that contains this event source
• frame.setTitle(You clicked buttonText)

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Finding the Source of an Event

• We can get the text from the event source using
  the getActionCommand method of the action event
  object evt
• String buttonText
• evt.getActionCommand()
• We can get a reference to the event source itself
  using the getSource method of the action event
  object evt.
• JButton clickedButton
• (JButton) evt.getSource()
• String buttonText clickedButton.getText()
• Note that the object returned by the getSource
  method may be an instance of any Component class,
  so we type cast the returned object to a proper
  class in order to use the desired method.

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Identifying the Frame

• To find the frame that contains the event source,
  we
• get the root pane to which the event source
  belongs, then
• get the frame that contains this root pane.
• JRootPane rootPane clickedButton.getRootPane()
• Frame frame (JFrame) rootPane.getParent()

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Chapter7JButtonFrame

• A sample window when it is first opened and after
  the CANCEL button is clicked.

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Frames as Listeners

• A frame window contains nested layers of panes.
• The topmost pane is called the root pane.
• The frame can be the event listener for the GUI
  objects it contains.
• cancelButton.addActionListener(this)
• okButton.addActionListener(this)
• The Frame can call its setTitle method directly.
• The frame class must implement ActionListener

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Classes for Text

• The Swing GUI classes JLabel, JTextField, and
  JTextArea all deal with text.
• A JLabel object displays uneditable (by the user)
  text.
• A JTextField object allows the user to enter a
  single line of text.
• A JTextArea object allows the user to enter
  multiple lines of text. It can also be used for
  displaying multiple lines of uneditable text.

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JTextField Class

• An instance of JTextField generates action events
  when the object is active and the user presses
  the ENTER key.
• The actionPerformed method must determine which
  of the three event sources in our example (two
  buttons and one text field) generated the event.

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What class does an Object Come From?

• The instanceof operator determines the class to
  which the event source belongs. The general model
  is thus
• if (event.getSource() instanceof JButton )
• //event source is either cancelButton
• //or okButton
• ...
• else //event source must be inputLine
• ...

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JTextField

• The getText method of JTextField may be used to
  retrieve the text that the user entered.
• public void actionPerformed(ActionEvent event)
• if (event.getSource() instanceof JButton)
• JButton clickedButton
• (JButton) event.getSource()
• String buttonText clickedButton.getText()
• setTitle(You clicked buttonText)
• else //the event source is inputLine
• setTitle(You entered
• inputLine.getText() )

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JLabel

• A JLabel object is useful for displaying a label
  indicating the purpose of another object, such as
  a JTextField object.
• prompt new JLabel(Please enter your name)

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Images in JLabel Objects

• JLabel objects may also be used to display
  images.
• They can display text, image or both
• To create a JLabel with an image instead of text,
  we pass an ImageIcon object instead of a string.
• To create the ImageIcon object, we must specify
  the filename of the image.

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JLabel

• We declare the data member
• private JLabel image
• then create it in the constructor as
• public Ch7TextFrame2
• ...
• image new JLabel(new ImageIcon(cat.gif)
• //note that this assumes the .gif is
• //in the same directory as the program
• image.setBounds(10, 20, 50, 50)
• contentPane.add(image)
• ...

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Ch7TextFrame2

• A window with one text JLabel, one image JLabel,
  one JTextField, and two JButton objects.

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JTextArea

• Next we will declare a JTextArea object
• private JTextArea textArea
• and add statements to create it
• textArea new JTextArea()
• textArea.setBounds(50, 5, 200, 135)
• textArea.setBorder(BorderFactory.createLineBorder(
  Color.red))
• textArea.setEditable(false)
• contentPane.add(textArea)

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Putting Borders on Components

• by default the rectangle indicating the boundary
  of the JTextArea object is not shown on the
  screen.
• We call the createLineBorder method of the
  BorderFactory class, because
• We disable editing of the text area in the
  statement
• textArea.setEditable(false)

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Modifying Text in a JTextArea

• The append method allows us to add text to the
  text area without replacing old content.
• Using the setText method of JTextArea will
  replace old content with new content.
• See Ch7TextFrame3

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Scroll Bars

• To add scroll bars to the JTextArea object, we
  amend the earlier code as follows
• textArea new JTextArea()
• textArea.setEditable(false)
• JScrollPane scrollText new JScrollPane(textArea)
  
• textArea.setBounds(50, 5, 200, 135)
• textArea.setBorder(BorderFactory.createLine
• Border(Color.red))
• contentPane.add(scrollText)
• See TextFrame4

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Menus

• The javax.swing package contains three useful
  menu-related classes
• JMenuBar is a bar where the menus are placed.
• JMenu (such as File or Edit) is a single item in
  the MenuBar.
• JMenuItem (such as Copy, Cut, or Paste) is an
  individual menu choice in the Menu.

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Using Menus

• When a MenuItem is selected, it generates a menu
  action event.
• We process menu selections by registering an
  action listener to menu items.

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How to Implement Menus

1. Create a JMenuBar object and attach it to a
   frame.
2. Create a JMenu object.
3. Create JMenuItem objects and add them to the
   JMenu object.
4. Attach the JMenu object to the JMenuBar object.

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Creating a Menu

• We create a fileMenu object as
• fileMenu new Menu(File)
• The argument to the Menu constructor is the name
  of the menu.
• Menu items appear from the top in the order in
  which they were added to the menu.

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Creating MenuItems

• To create and add a menu item New to fileMenu, we
  execute
• item new JMenuItem(New)
• item.addActionListener(this)
• fileMenu.add(item)
• And to add a horizontal line as a separator
  between menu items, we execute
• fileMenu.addSeparator()

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Creating the Menu Bar

• We create a JMenuBar object
• JMenuBar menuBar new JMenuBar()
• Then call the frames setMenuBar method to attach
  it to the frame
• setMenuBar(menuBar)
• Then add the two JMenu objects to the menu bar.
• menuBar.add(fileMenu)
• menuBar.add(editMenu)