org.apache.commons.collections4

## Class QueueUtils

- java.lang.Object
- - org.apache.commons.collections4.QueueUtils

-     public class QueueUtils
      extends Object

  Provides utility methods and decorators for `Queue` instances.
  Since:
    4.0

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static Queue` | `EMPTY_QUEUE` An empty unmodifiable queue.

  - ### Method Summary

    All Methods Static Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `static <E> Queue<E>` | `emptyQueue()` Get an empty `Queue`.
    `static <E> Queue<E>` | `predicatedQueue(Queue<E> queue, Predicate<? super E> predicate)` Returns a predicated (validating) queue backed by the given queue.
    `static <E> Queue<E>` | `synchronizedQueue(Queue<E> queue)` Returns a synchronized (thread-safe) queue backed by the given queue.
    `static <E> Queue<E>` | `transformingQueue(Queue<E> queue, Transformer<? super E,? extends E> transformer)` Returns a transformed queue backed by the given queue.
    `static <E> Queue<E>` | `unmodifiableQueue(Queue<? extends E> queue)` Returns an unmodifiable queue backed by the given queue.

    - ### Methods inherited from class java.lang.Object

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

- - ### Field Detail

    - #### EMPTY_QUEUE

          public static final Queue EMPTY_QUEUE

      An empty unmodifiable queue.

  - ### Method Detail

    - #### synchronizedQueue

          public static <E> Queue<E> synchronizedQueue(Queue<E> queue)

      Returns a synchronized (thread-safe) queue backed by the given queue. In order to guarantee serial access, it is critical that all access to the backing queue is accomplished through the returned queue.

      It is imperative that the user manually synchronize on the returned queue when iterating over it:

           Queue queue = QueueUtils.synchronizedQueue(new CircularFifoQueue());
           ...
           synchronized(queue) {
               Iterator i = queue.iterator(); // Must be in synchronized block
               while (i.hasNext())
                   foo(i.next());
               }
           }

      Failure to follow this advice may result in non-deterministic behavior.
      Type Parameters:
        `E` - the element type
      Parameters:
        `queue` - the queue to synchronize, must not be null
      Returns:
        a synchronized queue backed by that queue
      Throws:
        `NullPointerException` - if the queue is null
      Since:
        4.2

    - #### unmodifiableQueue

          public static <E> Queue<E> unmodifiableQueue(Queue<? extends E> queue)

      Returns an unmodifiable queue backed by the given queue.
      Type Parameters:
        `E` - the type of the elements in the queue
      Parameters:
        `queue` - the queue to make unmodifiable, must not be null
      Returns:
        an unmodifiable queue backed by that queue
      Throws:
        `NullPointerException` - if the queue is null

    - #### predicatedQueue

          public static <E> Queue<E> predicatedQueue(Queue<E> queue,
                                                     Predicate<? super E> predicate)

      Returns a predicated (validating) queue backed by the given queue.

      Only objects that pass the test in the given predicate can be added to the queue. Trying to add an invalid object results in an IllegalArgumentException. It is important not to use the original queue after invoking this method, as it is a backdoor for adding invalid objects.

      Type Parameters:
        `E` - the type of the elements in the queue
      Parameters:
        `queue` - the queue to predicate, must not be null
        `predicate` - the predicate used to evaluate new elements, must not be null
      Returns:
        a predicated queue
      Throws:
        `NullPointerException` - if the queue or predicate is null

    - #### transformingQueue

          public static <E> Queue<E> transformingQueue(Queue<E> queue,
                                                       Transformer<? super E,? extends E> transformer)

      Returns a transformed queue backed by the given queue.

      Each object is passed through the transformer as it is added to the Queue. It is important not to use the original queue after invoking this method, as it is a backdoor for adding untransformed objects.

      Existing entries in the specified queue will not be transformed. If you want that behaviour, see `TransformedQueue.transformedQueue(java.util.Queue<E>, org.apache.commons.collections4.Transformer<? super E, ? extends E>)`.

      Type Parameters:
        `E` - the type of the elements in the queue
      Parameters:
        `queue` - the queue to predicate, must not be null
        `transformer` - the transformer for the queue, must not be null
      Returns:
        a transformed queue backed by the given queue
      Throws:
        `NullPointerException` - if the queue or transformer is null

    - #### emptyQueue

          public static <E> Queue<E> emptyQueue()

      Get an empty `Queue`.
      Type Parameters:
        `E` - the type of the elements in the queue
      Returns:
        an empty `Queue`

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