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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.GwtCompatible;
import com.google.common.annotations.J2ktIncompatible;
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 */ @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 @J2ktIncompatible // Incompatible return type change. Use inherited implementation 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; }





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