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; 025import com.google.errorprone.annotations.CanIgnoreReturnValue; 026import java.io.Serializable; 027import java.util.ArrayDeque; 028import java.util.Collection; 029import java.util.Queue; 030 031/** 032 * A non-blocking queue which automatically evicts elements from the head of the queue when 033 * attempting to add new elements onto the queue and it is full. This queue orders elements FIFO 034 * (first-in-first-out). This data structure is logically equivalent to a circular buffer (i.e., 035 * cyclic buffer or ring buffer). 036 * 037 * <p>An evicting queue must be configured with a maximum size. Each time an element is added to a 038 * full queue, the queue automatically removes its head element. This is different from conventional 039 * bounded queues, which either block or reject new elements when full. 040 * 041 * <p>This class is not thread-safe, and does not accept null elements. 042 * 043 * @author Kurt Alfred Kluever 044 * @since 15.0 045 */ 046@Beta 047@GwtCompatible 048@ElementTypesAreNonnullByDefault 049public final class EvictingQueue<E> extends ForwardingQueue<E> implements Serializable { 050 051 private final Queue<E> delegate; 052 053 @VisibleForTesting final int maxSize; 054 055 private EvictingQueue(int maxSize) { 056 checkArgument(maxSize >= 0, "maxSize (%s) must >= 0", maxSize); 057 this.delegate = new ArrayDeque<E>(maxSize); 058 this.maxSize = maxSize; 059 } 060 061 /** 062 * Creates and returns a new evicting queue that will hold up to {@code maxSize} elements. 063 * 064 * <p>When {@code maxSize} is zero, elements will be evicted immediately after being added to the 065 * queue. 066 */ 067 public static <E> EvictingQueue<E> create(int maxSize) { 068 return new EvictingQueue<E>(maxSize); 069 } 070 071 /** 072 * Returns the number of additional elements that this queue can accept without evicting; zero if 073 * the queue is currently full. 074 * 075 * @since 16.0 076 */ 077 public int remainingCapacity() { 078 return maxSize - size(); 079 } 080 081 @Override 082 protected Queue<E> delegate() { 083 return delegate; 084 } 085 086 /** 087 * Adds the given element to this queue. If the queue is currently full, the element at the head 088 * of the queue is evicted to make room. 089 * 090 * @return {@code true} always 091 */ 092 @Override 093 @CanIgnoreReturnValue 094 public boolean offer(E e) { 095 return add(e); 096 } 097 098 /** 099 * Adds the given element to this queue. If the queue is currently full, the element at the head 100 * of the queue is evicted to make room. 101 * 102 * @return {@code true} always 103 */ 104 @Override 105 @CanIgnoreReturnValue 106 public boolean add(E e) { 107 checkNotNull(e); // check before removing 108 if (maxSize == 0) { 109 return true; 110 } 111 if (size() == maxSize) { 112 delegate.remove(); 113 } 114 delegate.add(e); 115 return true; 116 } 117 118 @Override 119 @CanIgnoreReturnValue 120 public boolean addAll(Collection<? extends E> collection) { 121 int size = collection.size(); 122 if (size >= maxSize) { 123 clear(); 124 return Iterables.addAll(this, Iterables.skip(collection, size - maxSize)); 125 } 126 return standardAddAll(collection); 127 } 128 129 @Override 130 public Object[] toArray() { 131 /* 132 * If we could, we'd declare the no-arg `Collection.toArray()` to return "Object[] but elements 133 * have the same nullness as E." Since we can't, we declare it to return nullable elements, and 134 * we can override it in our non-null-guaranteeing subtypes to present a better signature to 135 * their users. 136 * 137 * However, the checker *we* use has this special knowledge about `Collection.toArray()` anyway, 138 * so in our implementation code, we can rely on that. That's why the expression below 139 * type-checks. 140 */ 141 return super.toArray(); 142 } 143 144 private static final long serialVersionUID = 0L; 145}