• Multi-Threaded Programming:
  • It can be very difficult to ensure that the right things happen at the right time
• Overcoming this Difficulty:
  • Focuses on how classes should handle events rather than on how or when these events will occur

• Focus On:
  • The events that can occur (e.g., mouse clicks, timing signals, key presses)
  • The classes that can generate events of different kinds (often called event generators)
  • The classes that need to respond to events of different kinds (often called event receivers)
• Do Not Focus On:
  • The order in which events occur

Using an Event Queue

• Central repository for all events
• Ensures that events are processed in the right order
• Decouples the generators and the receivers

Event Listening

Event Bubbling

• EventQueue :
  • Fires events using dispatchEvent()
  • Can coalesce events that are posted to it
• Event Dispatch Thread (EDT):
  • All events are dispatched in this thread
  • Tasks that execute in this thread should "finish quickly"
  • Code can be executed in the event dispatch thread using SwingUtilities.invokedAndWait() and SwingUtilities.invokeLater()

• Why Bother?
  • It is impossible to build interesting event-based applications in Java without using GUI "widgets"
• Our Objective Here:
  • Consider only what we need
• "Widgets" in Swing:
  • Components
  • Containers

• JLabel :
  • Displays a String or Icon or both
• JButton :
  • One of the simplest components that allows for user interaction
  • Contains a String or Icon or both
  • Can be "clicked"

Using a JLabel

javaexamples/app/BadInteractiveRandomMessageSwingApplication.java (Fragment: label0)

Using a JButton

javaexamples/app/BadInteractiveRandomMessageSwingApplication.java (Fragment: button0)

• Top-Level:
  • Simplest example -- JFrame
  • Should not be used as a container; should use the root pane
• Ordinary:
  • Simplest example -- JPanel

Using a JFrame

javaexamples/app/BadInteractiveRandomMessageSwingApplication.java (Fragment: jframe)

Getting the Content Pane

javaexamples/app/BadInteractiveRandomMessageSwingApplication.java (Fragment: contentPane0)

• Defined:
  • The process of positioning and sizing the components in a container
• The "Normal" Approach:
  • Use a LayoutManager
• A Simpler Approach:
  • Use absolute/null layout (i.e., explicitly set the position and size)
  • [Note: (0,0) is at the upper left]

Using Absolute Layout

javaexamples/app/BadInteractiveRandomMessageSwingApplication.java (Fragment: contentPane1) javaexamples/app/BadInteractiveRandomMessageSwingApplication.java (Fragment: label1) javaexamples/app/BadInteractiveRandomMessageSwingApplication.java (Fragment: button1)

• Defined Loosely:
  • A group of cooperating objects with a well-defined entry point (i.e., a method that should be executed first) and, perhaps, an exit point
• Types:
  • Desktops -- applications and applets
  • Servers -- servlets
  • Mobile Devices -- midlets

• Environment:
  • Operating system
• Top-Level Container:
  • JFrame
• Entry Point:
  • main()

• Environment:
  • Browser (or other application)
• Top-Level Container:
  • JApplet
• Entry Point:
  • init() then start()

With a Problem

javaexamples/app/BadRandomMessageApplication.java

Using the Event Dispatch Thread

javaexamples/app/BadRandomMessageSwingApplication.java

Adding Event Handling

javaexamples/app/BadInteractiveRandomMessageSwingApplication.java

Like the Above Applications

javaexamples/app/BadRandomMessageJApplet.java javaexamples/app/BadInteractiveRandomMessageJApplet.java

• Observations:
  • The general structure of applets and applications are very similar
  • The details are different enough that there is no easy way to create an applet and an application with the same functionality without copying and pasting code from one method in one class to another method in another class
• Towards Unification:
  • Create an abstract JApplication class that mimics the programming interface and life-cycle of the JApplet class

The JApplication Class

javaexamples/app/JApplication.java (Fragment: run) javaexamples/app/JApplication.java (Fragment: constructMainWindow2) javaexamples/app/JApplication.java (Fragment: init)

An Example JApplication

javaexamples/app/BadRandomMessageJApplication.java

• A Technical Problem:
  • The "transition" methods are not called in the event dispatch thread
• A Design-Related Problem:
  • If you write a JApplet and JApplication with the same functionality they will have an enormous amount of duplicate code
• An Historical Problem:
  • The RootPaneContainer for a JApplet has access to the start-up parameters whereas the RootPaneContainer for a JApplication (which is a JFrame) does not.

Correcting the Historical Problem

javaexamples/app/MultimediaRootPaneContainer.java

Correcting the Remaining Problems with the Decorator Pattern

The Interface

javaexamples/app/MultimediaApp.java

The MultimediaApplet Class

javaexamples/app/MultimediaApplet.java

An Example App

javaexamples/app/RandomMessageApp.java

The Application and Applet

javaexamples/app/RandomMessageMultimediaApplet.java javaexamples/app/RandomMessageMultimediaApplication.java

Unifying Calls to the "Transition" Methods

javaexamples/app/JApplication.java (Fragment: constructMainWindow3) javaexamples/app/JApplication.java (Fragment: windowOpened) javaexamples/app/JApplication.java (Fragment: windowDeiconified) javaexamples/app/JApplication.java (Fragment: windowIconified) javaexamples/app/JApplication.java (Fragment: windowClosing) javaexamples/app/JApplication.java (Fragment: exit) javaexamples/app/JApplication.java (Fragment: windowClosed)

The Final Design

Loading Resources

javaexamples/io/ResourceFinder.java

• Events that occur at a particular point in time
• Events that occur after a particular interval of time
• Events that recur after a particular interval of time (called fixed-delay execution)
• Events that recur at a particular rate (called fixed-rate execution)

Design of a Simple Metronome

javaexamples/event/MetronomeListener.java

javaexamples/event/Metronome.java (Fragment: listeners) javaexamples/event/Metronome.java (Fragment: notifyListeners) javaexamples/event/Metronome.java (Fragment: MetronomeTickDispatcher) javaexamples/event/Metronome.java (Fragment: start) javaexamples/event/Metronome.java (Fragment: run)

A Simple Example

javaexamples/app/StopWatchApp.java