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/*
* Copyright (C) 2012 The Guava Authors
*
* Licensed 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 com.google.common.collect;
import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.io.Serializable;
import java.util.ArrayDeque;
import java.util.Collection;
import java.util.Queue;
/**
* A non-blocking queue which automatically evicts elements from the head of the queue when
* attempting to add new elements onto the queue and it is full. This queue orders elements FIFO
* (first-in-first-out). This data structure is logically equivalent to a circular buffer (i.e.,
* cyclic buffer or ring buffer).
*
* An evicting queue must be configured with a maximum size. Each time an element is added to a
* full queue, the queue automatically removes its head element. This is different from conventional
* bounded queues, which either block or reject new elements when full.
*
*
This class is not thread-safe, and does not accept null elements.
*
* @author Kurt Alfred Kluever
* @since 15.0
*/
@Beta
@GwtCompatible
@ElementTypesAreNonnullByDefault
public final class EvictingQueue extends ForwardingQueue implements Serializable {
private final Queue delegate;
@VisibleForTesting final int maxSize;
private EvictingQueue(int maxSize) {
checkArgument(maxSize >= 0, "maxSize (%s) must >= 0", maxSize);
this.delegate = new ArrayDeque<>(maxSize);
this.maxSize = maxSize;
}
/**
* Creates and returns a new evicting queue that will hold up to {@code maxSize} elements.
*
* When {@code maxSize} is zero, elements will be evicted immediately after being added to the
* queue.
*/
public static EvictingQueue create(int maxSize) {
return new EvictingQueue<>(maxSize);
}
/**
* Returns the number of additional elements that this queue can accept without evicting; zero if
* the queue is currently full.
*
* @since 16.0
*/
public int remainingCapacity() {
return maxSize - size();
}
@Override
protected Queue delegate() {
return delegate;
}
/**
* Adds the given element to this queue. If the queue is currently full, the element at the head
* of the queue is evicted to make room.
*
* @return {@code true} always
*/
@Override
@CanIgnoreReturnValue
public boolean offer(E e) {
return add(e);
}
/**
* Adds the given element to this queue. If the queue is currently full, the element at the head
* of the queue is evicted to make room.
*
* @return {@code true} always
*/
@Override
@CanIgnoreReturnValue
public boolean add(E e) {
checkNotNull(e); // check before removing
if (maxSize == 0) {
return true;
}
if (size() == maxSize) {
delegate.remove();
}
delegate.add(e);
return true;
}
@Override
@CanIgnoreReturnValue
public boolean addAll(Collection collection) {
int size = collection.size();
if (size >= maxSize) {
clear();
return Iterables.addAll(this, Iterables.skip(collection, size - maxSize));
}
return standardAddAll(collection);
}
@Override
public Object[] toArray() {
/*
* If we could, we'd declare the no-arg `Collection.toArray()` to return "Object[] but elements
* have the same nullness as E." Since we can't, we declare it to return nullable elements, and
* we can override it in our non-null-guaranteeing subtypes to present a better signature to
* their users.
*
* However, the checker *we* use has this special knowledge about `Collection.toArray()` anyway,
* so in our implementation code, we can rely on that. That's why the expression below
* type-checks.
*/
return super.toArray();
}
private static final long serialVersionUID = 0L;
}