CIS3931 Intro to JAVA

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CIS3931 - Intro to JAVA

• Lecture Notes Set 8
• 9-June-05

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JAVA GUI Classes

• Based on fundamental classes in the Application
  Windowing Toolkit (AWT).
• JAVA releases gt 1.1.2 now use Swing, which is a
  more versatile version of AWT.
• Swing is the accepted standard for GUI
  programming in JAVA

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Graphical Components

• GUI consists of graphical components such as
• Windows
• Buttons
• Menus
• Text Fields

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Events

• An event is an action performed by the user
• Examples
• Clicking on a button to choose a program option
• Making a choice from a menu
• Entereing text in a text field
• Dragging a scroll bar
• Clicking on a windows close button

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Event-Driven Programming

• GUIs are generally interactive programs.
• A event-driven program is one that responses to
  events generated by the user.
• In JAVA, the Swing components generate the events
  (w/ user interaction) and the JAVA methods
  respond to the events.

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The Swing Package

• Swing contains all of the components necessary
  for writing a GUI program.
• No need to manually program buttons, windows,
  menus, etc.
• Simply call the constructor for a component

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Parts of a GUI Program

• Commonly divided into three parts
• 1. Graphical components
• Make up the visual part of the GUI
• 2. Listener methods
• Receive events and handle/respond to them
• 3. Application methods
• Perform the useful work of the program

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GUI Program Flow

• GUI Program displays the graphical components
• Interactive graphical components are handled by
  listener methods
• Listener methods respond to events generated by
  the interactive graphical components and call
  application methods
• Application methods perform the necessary
  functions of the program.

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GUI Container Classes

• GUI program consists of a collection of graphical
  components that are placed inside of one or more
  windows.
• Components are contained by a particular windows.
• Containers are objects that hold other GUI
  components.

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Swing Frames

• GUI programs are created by extending the class
  JFrame.
• JFrame class holds the basic GUI functionality.
• We will be discussing the following
• The JFrame class
• Extending the JFrame class
• The paint() method
• The drawString() method

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Frames

• Frame in JAVA Window
• The frame holds all the GUI components
• GUI programs can have one more more frames.

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The smallest GUI frame program

• import java.awt.
• import javax.swing.
• public class TestFrame1
• public static void main ( String args )
• JFrame frame new JFrame("Test Frame 1")
• frame.setSize(200,100)
• frame.setVisible( true )

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Program explanation

• //Call the new constructor for JFrame and set
• //the title Test Frame 1
• JFrame frame new JFrame(Test Frame 1)
• //Set the initial size to 200w x 100h pixels
• frame.setSize(200,100)
• //Make the frame visible (default is invisible)
• frame.setVisible(true)

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setVisible(boolean)

• All windows are initially constructed with
  setVisible(false)
• Object exists in memory, but it not drawn to the
  screen
• setVisible(true) draws the frame to the screen.
• setVisible(false) makes the frame invisible (but
  retains the frame in program memory)

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Closing the frame

• Hitting the x in the top right corner closes
  the frame in the example program
• HOWEVER The program continues to run!
• The close window event must be handled
  somewhere in the program to allow the program to
  properly exit (discussed later in this class).

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Frame Dimensions

• setSize() can be called anywhere in the program
  to change the frame size
• Example
• import java.awt.
• import javax.swing.
• public class TestFrame1
• public static void main ( String args )
• int height100, width200
• JFrame frame new JFrame("Test Frame 1")
• frame.setSize( width, height )
• frame.setVisible( true )
• frame.setSize( width50, height75 )

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Extending JFrame

• GUIs are usually written in a class that extends
  the JFrame
• Main program then calls this class to run the GUI.

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Extending JFrame example

• import java.awt.
• import javax.swing.
• class MyFrame extends JFrame
• // paint() is called automatically by the
  system to display your customizations to the
  frame.
• public void paint ( Graphics g )
• g.drawString("A MyFrame object",
  10, 50 ) // draw a String at location x10 y50
• public class TestFrame2
• public static void main ( String args )
• MyFrame frame new MyFrame() //
  construct a MyFrame object
• frame.setSize( 150, 100 )
  // set it to 150 wide by 100 high
• frame.setVisible( true )
  // ask it to become visible

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Example explanation

• MyFrame extends JFrame
• MyFrame does everything JFrame does, with the
  addition of whatever methods are in MyFrame.
• A MyFrame is constructed, which actually
  constructs a JFrame in addition to whatever you
  added in MyFrame.

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Event Listeners

• Users generate events by interacting with a GUI
  component.
• Examples of event generation
• Moving the mouse
• Clicking the mouse
• Clicking on a button
• Typing some text into a text area

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Event Listeners

• Event Listener Objects allow the program to
  respond to an event.
• Listener methods are contained in Event Listener
  Objects to handle various types of events.
• Events can be ignored (no listener ignore event)

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Event Objects

• Events in JAVA are represented as Objects (event
  objects)
• Event Objects are sent to the listener registered
  to the GUI component

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Responding to Events

• To respond to events, a program must
• 1. Create an event listener object for the type
  of event
• 2. Register the listener object to the GUI
  component that generates the event.

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Event Creation

• JAVA doesnt know which events will be ignored
  so, it must create an event object for every event

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WindowAdapter Class

• WindowAdapter listener for events generator by
  the class Window and its decendants (JFrame,
  Frame )
• Includes listener methods for every type of
  Window event.
• By default, these methods receive an event, but
  do nothing. In order to make them do something,
  you have to extend WindowAdaptor and override its
  methods.

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Creating a listener for a frame object

• public class WindowQuitter extends WindowAdapter
• // override a method of WindowAdapter
• public void windowClosing( WindowEvent e )
• System.exit( 0 ) // what to do for this
  event

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Adding the window listener to the GUI program

• See GUItester.java

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Questions?