Package org.apache.commons.collections4

Class QueueUtils

  • public class QueueUtils
extends Object
    Provides utility methods and decorators for Queue instances.
    Since:
    4.0

    • 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