Event Queue
(This slide added in 397.)

• When an event occurs, the operating system notifies the Java VM.
• The VM takes the information from the OS and creates an event.
• Specifically, it constructs an AWTEvent object.
• When constructing an AWTEvent, it must pass the source object (the event source) into the constructor.  This tells us a lot - it tells us what object caused the event to happen (a specific JButton or JRadioButton, for example), and it can guide the JVM to find the listeners, as discussed below.
• The event is put on something called an event queue.  The event queue is actually an object, java.awt.EventQueue.
• So, events that magically appear in your program are actually constructed internally by the JVM and thrown on the EventQueue.
• We don't need to know how the EventQueue works internally, that is the point of encapsulation.  But, for academic reasons, we can speculate.
• The EventQueue must hold multiple AWTEvent objects.  Thus, we assume it implements a List object of some type, perhaps a Vector.
• Then, it must be able to add and remove elements from that List.
• We rarely need to manipulate the EventQueue manually - but we can if we want to.  Here are some useful methods of the EventQueue:
• AWTEvent peekEvent() allows us to see what event is next in the queue without actually removing it.
• AWTEvent getNextEvent() gets the next event and removes it from the EventQueue.
• void postEvent(AWTEvent anEvent) adds anEvent to the EventQueue.


 Now What Do We Do?
 
 

Created by:  Brandan Jones January 4, 2002