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/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.commons.collections4;
import java.util.LinkedList;
import java.util.Queue;
import org.apache.commons.collections4.queue.PredicatedQueue;
import org.apache.commons.collections4.queue.SynchronizedQueue;
import org.apache.commons.collections4.queue.TransformedQueue;
import org.apache.commons.collections4.queue.UnmodifiableQueue;
/**
* Provides utility methods and decorators for {@link Queue} instances.
*
* @since 4.0
*/
public class QueueUtils {
/**
* An empty unmodifiable queue.
*/
@SuppressWarnings("rawtypes") // OK, empty queue is compatible with any type
public static final Queue EMPTY_QUEUE = UnmodifiableQueue.unmodifiableQueue(new LinkedList<>());
/**
* QueueUtils should not normally be instantiated.
*/
private QueueUtils() {}
//-----------------------------------------------------------------------
/**
* 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.
*
* @param the element type
* @param queue the queue to synchronize, must not be null
* @return a synchronized queue backed by that queue
* @throws NullPointerException if the queue is null
* @since 4.2
*/
public static Queue synchronizedQueue(final Queue queue) {
return SynchronizedQueue.synchronizedQueue(queue);
}
/**
* Returns an unmodifiable queue backed by the given queue.
*
* @param the type of the elements in the queue
* @param queue the queue to make unmodifiable, must not be null
* @return an unmodifiable queue backed by that queue
* @throws NullPointerException if the queue is null
*/
public static Queue unmodifiableQueue(final Queue queue) {
return UnmodifiableQueue.unmodifiableQueue(queue);
}
/**
* 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.
*
* @param the type of the elements in the queue
* @param queue the queue to predicate, must not be null
* @param predicate the predicate used to evaluate new elements, must not be null
* @return a predicated queue
* @throws NullPointerException if the queue or predicate is null
*/
public static Queue predicatedQueue(final Queue queue, final Predicate predicate) {
return PredicatedQueue.predicatedQueue(queue, predicate);
}
/**
* 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 {@link TransformedQueue#transformedQueue}.
*
* @param the type of the elements in the queue
* @param queue the queue to predicate, must not be null
* @param transformer the transformer for the queue, must not be null
* @return a transformed queue backed by the given queue
* @throws NullPointerException if the queue or transformer is null
*/
public static Queue transformingQueue(final Queue queue,
final Transformer transformer) {
return TransformedQueue.transformingQueue(queue, transformer);
}
/**
* Get an empty Queue.
*
* @param the type of the elements in the queue
* @return an empty {@link Queue}
*/
@SuppressWarnings("unchecked") // OK, empty queue is compatible with any type
public static Queue emptyQueue() {
return EMPTY_QUEUE;
}
}