Module java.desktop
Package java.awt

# Class EventQueue

java.lang.Object
java.awt.EventQueue

public class EventQueue extends Object
`EventQueue` is a platform-independent class that queues events, both from the underlying peer classes and from trusted application classes.

It encapsulates asynchronous event dispatch machinery which extracts events from the queue and dispatches them by calling `dispatchEvent(AWTEvent)` method on this `EventQueue` with the event to be dispatched as an argument. The particular behavior of this machinery is implementation-dependent. The only requirements are that events which were actually enqueued to this queue (note that events being posted to the `EventQueue` can be coalesced) are dispatched:

Sequentially.
  That is, it is not permitted that several events from this queue are dispatched simultaneously.
In the same order as they are enqueued.
  That is, if `AWTEvent` A is enqueued to the `EventQueue` before `AWTEvent` B then event B will not be dispatched before event A.

Some browsers partition applets in different code bases into separate contexts, and establish walls between these contexts. In such a scenario, there will be one `EventQueue` per context. Other browsers place all applets into the same context, implying that there will be only a single, global `EventQueue` for all applets. This behavior is implementation-dependent. Consult your browser's documentation for more information.

For information on the threading issues of the event dispatch machinery, see AWT Threading Issues.

Since:
  1.1

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `EventQueue()`
  Initializes a new instance of `EventQueue`.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `SecondaryLoop`
  `createSecondaryLoop()`
  Creates a new `secondary loop` associated with this event queue.
  `protected void`
  `dispatchEvent(AWTEvent event)`
  Dispatches an event.
  `static AWTEvent`
  `getCurrentEvent()`
  Returns the event currently being dispatched by the `EventQueue` associated with the calling thread.
  `static long`
  `getMostRecentEventTime()`
  Returns the timestamp of the most recent event that had a timestamp, and that was dispatched from the `EventQueue` associated with the calling thread.
  `AWTEvent`
  `getNextEvent()`
  Removes an event from the `EventQueue` and returns it.
  `static void`
  `invokeAndWait(Runnable runnable)`
  Causes `runnable` to have its `run` method called in the `dispatch thread` of `the system EventQueue`.
  `static void`
  `invokeLater(Runnable runnable)`
  Causes `runnable` to have its `run` method called in the `dispatch thread` of `the system EventQueue`.
  `static boolean`
  `isDispatchThread()`
  Returns true if the calling thread is `the current AWT EventQueue`'s dispatch thread.
  `AWTEvent`
  `peekEvent()`
  Returns the first event on the `EventQueue` without removing it.
  `AWTEvent`
  `peekEvent(int id)`
  Returns the first event with the specified id, if any.
  `protected void`
  `pop()`
  Stops dispatching events using this `EventQueue`.
  `void`
  `postEvent(AWTEvent theEvent)`
  Posts a 1.1-style event to the `EventQueue`.
  `void`
  `push(EventQueue newEventQueue)`
  Replaces the existing `EventQueue` with the specified one.

  ### Methods declared in class java.lang.Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- ## Constructor Details

  - ### EventQueue

    public EventQueue()
    Initializes a new instance of `EventQueue`.

- ## Method Details

  - ### postEvent

    public void postEvent(AWTEvent theEvent)
    Posts a 1.1-style event to the `EventQueue`. If there is an existing event on the queue with the same ID and event source, the source `Component`'s `coalesceEvents` method will be called.
    Parameters:
      `theEvent` - an instance of `java.awt.AWTEvent`, or a subclass of it
    Throws:
      `NullPointerException` - if `theEvent` is `null`

  - ### getNextEvent

    public AWTEvent getNextEvent() throws InterruptedException
    Removes an event from the `EventQueue` and returns it. This method will block until an event has been posted by another thread.
    Returns:
      the next `AWTEvent`
    Throws:
      `InterruptedException` - if any thread has interrupted this thread

  - ### peekEvent

    public AWTEvent peekEvent()
    Returns the first event on the `EventQueue` without removing it.
    Returns:
      the first event

  - ### peekEvent

    public AWTEvent peekEvent(int id)
    Returns the first event with the specified id, if any.
    Parameters:
      `id` - the id of the type of event desired
    Returns:
      the first event of the specified id or `null` if there is no such event

  - ### dispatchEvent

    protected void dispatchEvent(AWTEvent event)
    Dispatches an event. The manner in which the event is dispatched depends upon the type of the event and the type of the event's source object:

    Event types, source types, and dispatch methods
    Event Type | Source Type | Dispatched To
    --- | --- | ---
    ActiveEvent | Any | event.dispatch()
    Other | Component | source.dispatchEvent(AWTEvent)
    Other | MenuComponent | source.dispatchEvent(AWTEvent)
    Other | Other | No action (ignored)

    Parameters:
      `event` - an instance of `java.awt.AWTEvent`, or a subclass of it
    Throws:
      `NullPointerException` - if `event` is `null`
    Since:
      1.2

  - ### getMostRecentEventTime

    public static long getMostRecentEventTime()
    Returns the timestamp of the most recent event that had a timestamp, and that was dispatched from the `EventQueue` associated with the calling thread. If an event with a timestamp is currently being dispatched, its timestamp will be returned. If no events have yet been dispatched, the EventQueue's initialization time will be returned instead.In the current version of the JDK, only `InputEvent`s, `ActionEvent`s, and `InvocationEvent`s have timestamps; however, future versions of the JDK may add timestamps to additional event types. Note that this method should only be invoked from an application's `event dispatching thread`. If this method is invoked from another thread, the current system time (as reported by `System.currentTimeMillis()`) will be returned instead.
    Returns:
      the timestamp of the last `InputEvent`, `ActionEvent`, or `InvocationEvent` to be dispatched, or `System.currentTimeMillis()` if this method is invoked on a thread other than an event dispatching thread
    Since:
      1.4
    See Also:
      - `InputEvent.getWhen()`
      - `ActionEvent.getWhen()`
      - `InvocationEvent.getWhen()`
      - `isDispatchThread()`

  - ### getCurrentEvent

    public static AWTEvent getCurrentEvent()
    Returns the event currently being dispatched by the `EventQueue` associated with the calling thread. This is useful if a method needs access to the event, but was not designed to receive a reference to it as an argument. Note that this method should only be invoked from an application's event dispatching thread. If this method is invoked from another thread, null will be returned.
    Returns:
      the event currently being dispatched, or null if this method is invoked on a thread other than an event dispatching thread
    Since:
      1.4

  - ### push

    public void push(EventQueue newEventQueue)
    Replaces the existing `EventQueue` with the specified one. Any pending events are transferred to the new `EventQueue` for processing by it.
    Parameters:
      `newEventQueue` - an `EventQueue` (or subclass thereof) instance to be use
    Throws:
      `NullPointerException` - if `newEventQueue` is `null`
    Since:
      1.2
    See Also:
      - `pop()`

  - ### pop

    protected void pop() throws EmptyStackException
    Stops dispatching events using this `EventQueue`. Any pending events are transferred to the previous `EventQueue` for processing.

    Warning: To avoid deadlock, do not declare this method synchronized in a subclass.

    Throws:
      `EmptyStackException` - if no previous push was made on this `EventQueue`
    Since:
      1.2
    See Also:
      - `push(java.awt.EventQueue)`

  - ### createSecondaryLoop

    public SecondaryLoop createSecondaryLoop()
    Creates a new `secondary loop` associated with this event queue. Use the `SecondaryLoop.enter()` and `SecondaryLoop.exit()` methods to start and stop the event loop and dispatch the events from this queue.
    Returns:
      secondaryLoop A new secondary loop object, which can be used to launch a new nested event loop and dispatch events from this queue
    Since:
      1.7
    See Also:
      - `SecondaryLoop.enter()`
      - `SecondaryLoop.exit()`

  - ### isDispatchThread

    public static boolean isDispatchThread()
    Returns true if the calling thread is `the current AWT EventQueue`'s dispatch thread. Use this method to ensure that a particular task is being executed (or not being) there.

    Note: use the `invokeLater(java.lang.Runnable)` or `invokeAndWait(java.lang.Runnable)` methods to execute a task in `the current AWT EventQueue`'s dispatch thread.

    Returns:
      true if running in `the current AWT EventQueue`'s dispatch thread
    Since:
      1.2
    See Also:
      - `invokeLater(java.lang.Runnable)`
      - `invokeAndWait(java.lang.Runnable)`
      - `Toolkit.getSystemEventQueue()`

  - ### invokeLater

    public static void invokeLater(Runnable runnable)
    Causes `runnable` to have its `run` method called in the `dispatch thread` of `the system EventQueue`. This will happen after all pending events are processed.
    Parameters:
      `runnable` - the `Runnable` whose `run` method should be executed asynchronously in the `event dispatch thread` of `the system EventQueue`
    Since:
      1.2
    See Also:
      - `invokeAndWait(java.lang.Runnable)`
      - `Toolkit.getSystemEventQueue()`
      - `isDispatchThread()`

  - ### invokeAndWait

    public static void invokeAndWait(Runnable runnable) throws InterruptedException, InvocationTargetException
    Causes `runnable` to have its `run` method called in the `dispatch thread` of `the system EventQueue`. This will happen after all pending events are processed. The call blocks until this has happened. This method will throw an Error if called from the `event dispatcher thread`.
    Parameters:
      `runnable` - the `Runnable` whose `run` method should be executed synchronously in the `event dispatch thread` of `the system EventQueue`
    Throws:
      `InterruptedException` - if any thread has interrupted this thread
      `InvocationTargetException` - if an throwable is thrown when running `runnable`
    Since:
      1.2
    See Also:
      - `invokeLater(java.lang.Runnable)`
      - `Toolkit.getSystemEventQueue()`
      - `isDispatchThread()`