Apache Commons Collections 4.4 · org.apache.commons.collections4.QueueUtils

Modifier and Type	Field and Description
public static final Queue<E>	EMPTY_QUEUE An empty unmodifiable queue.

Access	Constructor and Description
private	QueueUtils() `QueueUtils` should not normally be instantiated.

Method Summary

Modifier and Type	Method and Description
public static < the type of the elements in the queue E> Queue<E>	Returns: an empty `Queue` emptyQueue() Get an empty `Queue`.
public static < the type of the elements in the queue E> Queue<E>	Returns: a predicated queue predicatedQueue(final Queue<E> the queue to predicate, must not be null queue, final Predicate<? super E> the predicate used to evaluate new elements, must not be null predicate) Returns a predicated (validating) queue backed by the given queue.
public static < the element type E> Queue<E>	Returns: a synchronized queue backed by that queue synchronizedQueue(final Queue<E> the queue to synchronize, must not be null queue) Returns a synchronized (thread-safe) queue backed by the given queue.
public static < the type of the elements in the queue E> Queue<E>	Returns: a transformed queue backed by the given queue transformingQueue(final Queue<E> the queue to predicate, must not be null queue, final Transformer<? super E, ? extends E> the transformer for the queue, must not be null transformer) Returns a transformed queue backed by the given queue.
public static < the type of the elements in the queue E> Queue<E>	Returns: an unmodifiable queue backed by that queue unmodifiableQueue(final Queue<? extends E> the queue to make unmodifiable, must not be null queue) Returns an unmodifiable queue backed by the given queue.

Inherited from java.lang.Object:: clone equals finalize getClass hashCode notify notifyAll toString wait wait wait

EMPTY_QUEUE	back to summary
public static final Queue<E> EMPTY_QUEUE An empty unmodifiable queue. Annotations @SuppressWarnings:rawtypes

QueueUtils	back to summary
private QueueUtils() `QueueUtils` should not normally be instantiated.

Method Detail

emptyQueue back to summary

emptyQueue	back to summary
public static <E> Queue<E> emptyQueue() Get an empty `Queue`. Parameters <E> the type of the elements in the queue Returns:Queue<E> an empty `Queue` Annotations @SuppressWarnings:unchecked

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

Get an empty Queue.

Parameters

<E>: the type of the elements in the queue

Returns:Queue<E>

an empty Queue

Annotations

@SuppressWarnings:unchecked

predicatedQueue	back to summary
public static <E> Queue<E> predicatedQueue(final Queue<E> queue, final 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. Parameters <E> the type of the elements in the queue queue:Queue<E> the queue to predicate, must not be null predicate:Predicate<? super E> the predicate used to evaluate new elements, must not be null Returns:Queue<E> a predicated queue Exceptions NullPointerException: if the queue or predicate is null

predicatedQueue

back to summary

public static <E> Queue<E> predicatedQueue(final Queue<E> queue, final 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.

Parameters

<E>: the type of the elements in the queue
queue:Queue<E>: the queue to predicate, must not be null
predicate:Predicate<? super E>: the predicate used to evaluate new elements, must not be null

Returns:Queue<E>

a predicated queue

Exceptions

NullPointerException:: if the queue or predicate is null

synchronizedQueue	back to summary
public static <E> Queue<E> synchronizedQueue(final 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. Parameters <E> the element type queue:Queue<E> the queue to synchronize, must not be null Returns:Queue<E> a synchronized queue backed by that queue Exceptions NullPointerException: if the queue is null Since 4.2

synchronizedQueue

back to summary

public static <E> Queue<E> synchronizedQueue(final 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.

Parameters

<E>: the element type
queue:Queue<E>: the queue to synchronize, must not be null

Returns:Queue<E>

a synchronized queue backed by that queue

Exceptions

NullPointerException:: if the queue is null

Since

4.2

transformingQueue back to summary

transformingQueue	back to summary
public static <E> Queue<E> transformingQueue(final Queue<E> queue, final 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`. Parameters <E> the type of the elements in the queue queue:Queue<E> the queue to predicate, must not be null transformer:Transformer<? super E, ? extends E> the transformer for the queue, must not be null Returns:Queue<E> a transformed queue backed by the given queue Exceptions NullPointerException: if the queue or transformer is null

public static <E> Queue<E> transformingQueue(final Queue<E> queue, final 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.

Parameters

<E>: the type of the elements in the queue
queue:Queue<E>: the queue to predicate, must not be null
transformer:Transformer<? super E, ? extends E>: the transformer for the queue, must not be null

Returns:Queue<E>

a transformed queue backed by the given queue

Exceptions

NullPointerException:: if the queue or transformer is null

unmodifiableQueue	back to summary
public static <E> Queue<E> unmodifiableQueue(final Queue<? extends E> queue) Returns an unmodifiable queue backed by the given queue. Parameters <E> the type of the elements in the queue queue:Queue<? extends E> the queue to make unmodifiable, must not be null Returns:Queue<E> an unmodifiable queue backed by that queue Exceptions NullPointerException: if the queue is null

unmodifiableQueue

back to summary

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

Returns an unmodifiable queue backed by the given queue.

Parameters

<E>: the type of the elements in the queue
queue:Queue<? extends E>: the queue to make unmodifiable, must not be null

Returns:Queue<E>

an unmodifiable queue backed by that queue

Exceptions

NullPointerException:: if the queue is null

public Class QueueUtils

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