001/*
002 * Copyright (C) 2012 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import static com.google.common.base.Preconditions.checkArgument;
020import static com.google.common.base.Preconditions.checkNotNull;
021
022import com.google.common.annotations.Beta;
023import com.google.common.annotations.GwtCompatible;
024import com.google.common.annotations.VisibleForTesting;
025
026import java.io.Serializable;
027import java.util.Collection;
028import java.util.Queue;
029
030/**
031 * A non-blocking queue which automatically evicts elements from the head of the queue when
032 * attempting to add new elements onto the queue and it is full. This data structure is logically
033 * equivalent to a circular buffer (i.e., cyclic buffer or ring buffer).
034 *
035 * <p>An evicting queue must be configured with a maximum size. Each time an element is added
036 * to a full queue, the queue automatically removes its head element. This is different from
037 * conventional bounded queues, which either block or reject new elements when full.
038 *
039 * <p>This class is not thread-safe, and does not accept null elements.
040 *
041 * @author Kurt Alfred Kluever
042 * @since 15.0
043 */
044@Beta
045@GwtCompatible
046public final class EvictingQueue<E> extends ForwardingQueue<E> implements Serializable {
047
048  private final Queue<E> delegate;
049
050  @VisibleForTesting final int maxSize;
051
052  private EvictingQueue(int maxSize) {
053    checkArgument(maxSize >= 0, "maxSize (%s) must >= 0", maxSize);
054    this.delegate = Platform.newFastestQueue(maxSize);
055    this.maxSize = maxSize;
056  }
057
058  /**
059   * Creates and returns a new evicting queue that will hold up to {@code maxSize} elements.
060   *
061   * <p>When {@code maxSize} is zero, elements will be evicted immediately after being added to the
062   * queue.
063   */
064  public static <E> EvictingQueue<E> create(int maxSize) {
065    return new EvictingQueue<E>(maxSize);
066  }
067
068  /**
069   * Returns the number of additional elements that this queue can accept without evicting;
070   * zero if the queue is currently full.
071   *
072   * @since 16.0
073   */
074  public int remainingCapacity() {
075    return maxSize - size();
076  }
077
078  @Override
079  protected Queue<E> delegate() {
080    return delegate;
081  }
082
083  /**
084   * Adds the given element to this queue. If the queue is currently full, the element at the head
085   * of the queue is evicted to make room.
086   *
087   * @return {@code true} always
088   */
089  @Override
090  public boolean offer(E e) {
091    return add(e);
092  }
093
094  /**
095   * Adds the given element to this queue. If the queue is currently full, the element at the head
096   * of the queue is evicted to make room.
097   *
098   * @return {@code true} always
099   */
100  @Override
101  public boolean add(E e) {
102    checkNotNull(e); // check before removing
103    if (maxSize == 0) {
104      return true;
105    }
106    if (size() == maxSize) {
107      delegate.remove();
108    }
109    delegate.add(e);
110    return true;
111  }
112
113  @Override
114  public boolean addAll(Collection<? extends E> collection) {
115    return standardAddAll(collection);
116  }
117
118  @Override
119  public boolean contains(Object object) {
120    return delegate().contains(checkNotNull(object));
121  }
122
123  @Override
124  public boolean remove(Object object) {
125    return delegate().remove(checkNotNull(object));
126  }
127
128  // TODO(kak): Do we want to checkNotNull each element in containsAll, removeAll, and retainAll?
129
130  private static final long serialVersionUID = 0L;
131}