EventQueue

EventQueue is a fundamental component of event-driven programming in GUI systems. It is a data structure and coordinating mechanism that holds events until they can be processed by the program’s event loop. Events may be generated by user input (mouse, keyboard), timers, system notifications, or inter-thread communication. An event queue is typically a first-in, first-out (FIFO) queue, sometimes with support for prioritization or coalescing of related events, and it is designed to be thread-safe so producers from multiple threads can enqueue events.

In Java’s AWT and Swing ecosystems, the EventQueue is responsible for dispatching events to the appropriate components on the Event Dispatch Thread. The EDT runs a loop that retrieves the next event from the queue and invokes the corresponding handler on the target component. Applications commonly post work to the queue from other threads using framework APIs, ensuring long-running tasks do not run on the event thread. If a modal dialog or nested loop occurs, the event loop may temporarily push a nested queue, but still serializes event processing on a single thread.

Cross-platform there are similar constructs: Android uses a Looper and MessageQueue to schedule and dispatch messages on a single thread; Qt uses an event loop managed by QCoreApplication or QApplication. Understanding EventQueue behavior is important for responsiveness, avoiding race conditions, and ensuring that GUI updates occur on the designated thread.