NodeCachingLinkedList

/*
 * 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.list;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.Collection;

/**
 * A {@code List} implementation that stores a cache of internal Node objects
 * in an effort to reduce wasteful object creation.
 * <p>
 * A linked list creates one Node for each item of data added. This can result in
 * a lot of object creation and garbage collection. This implementation seeks to
 * avoid that by maintaining a store of cached nodes.
 * </p>
 * <p>
 * This implementation is suitable for long-lived lists where both add and remove
 * are used. Short-lived lists, or lists which only grow will have worse performance
 * using this class.
 * </p>
 * <p>
 * <b>Note that this implementation is not synchronized.</b>
 * </p>
 *
 * @since 3.0
 */
public class NodeCachingLinkedList<E> extends AbstractLinkedList<E> implements Serializable {

    /** Serialization version */
    private static final long serialVersionUID = 6897789178562232073L;

    /**
     * The default value for {@link #maximumCacheSize}.
     */
    private static final int DEFAULT_MAXIMUM_CACHE_SIZE = 20;

    /**
     * The first cached node, or {@code null} if no nodes are cached.
     * Cached nodes are stored in a singly-linked list with
     * {@code next} pointing to the next element.
     */
    private transient Node<E> firstCachedNode;

    /**
     * The size of the cache.
     */
    private transient int cacheSize;

    /**
     * The maximum size of the cache.
     */
    private int maximumCacheSize;

    /**
     * Constructor that creates.
     */
    public NodeCachingLinkedList() {
        this(DEFAULT_MAXIMUM_CACHE_SIZE);
    }

    /**
     * Constructor that copies the specified collection
     *
     * @param coll  the collection to copy
     */
    public NodeCachingLinkedList(final Collection<? extends E> coll) {
        super(coll);
        this.maximumCacheSize = DEFAULT_MAXIMUM_CACHE_SIZE;
    }

    /**
     * Constructor that species the maximum cache size.
     *
     * @param maximumCacheSize  the maximum cache size
     */
    public NodeCachingLinkedList(final int maximumCacheSize) {
        this.maximumCacheSize = maximumCacheSize;
        init();  // must call init() as use super();
    }

    /**
     * Adds a node to the cache, if the cache isn't full.
     * The node's contents are cleared, so they can be garbage collected.
     *
     * @param node  the node to add to the cache
     */
    protected void addNodeToCache(final Node<E> node) {
        if (isCacheFull()) {
            // don't cache the node.
            return;
        }
        // clear the node's contents and add it to the cache.
        final Node<E> nextCachedNode = firstCachedNode;
        node.previous = null;
        node.next = nextCachedNode;
        node.setValue(null);
        firstCachedNode = node;
        cacheSize++;
    }

    /**
     * Creates a new node, either by reusing one from the cache or creating
     * a new one.
     *
     * @param value  value of the new node
     * @return the newly created node
     */
    @Override
    protected Node<E> createNode(final E value) {
        final Node<E> cachedNode = getNodeFromCache();
        if (cachedNode == null) {
            return super.createNode(value);
        }
        cachedNode.setValue(value);
        return cachedNode;
    }

    /**
     * Gets the maximum size of the cache.
     *
     * @return the maximum cache size
     */
    protected int getMaximumCacheSize() {
        return maximumCacheSize;
    }

    /**
     * Gets a node from the cache. If a node is returned, then the value of
     * {@link #cacheSize} is decreased accordingly. The node that is returned
     * will have {@code null} values for next, previous and element.
     *
     * @return a node, or {@code null} if there are no nodes in the cache.
     */
    protected Node<E> getNodeFromCache() {
        if (cacheSize == 0) {
            return null;
        }
        final Node<E> cachedNode = firstCachedNode;
        firstCachedNode = cachedNode.next;
        cachedNode.next = null; // This should be changed anyway, but defensively
                                // set it to null.
        cacheSize--;
        return cachedNode;
    }

    /**
     * Checks whether the cache is full.
     *
     * @return true if the cache is full
     */
    protected boolean isCacheFull() {
        return cacheSize >= maximumCacheSize;
    }

    /**
     * Deserializes the data held in this object to the stream specified.
     *
     * @param in  the input stream
     * @throws IOException if an error occurs while reading from the stream
     * @throws ClassNotFoundException if an object read from the stream can not be loaded
     */
    private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
        doReadObject(in);
    }

    /**
     * Removes all the nodes from the list, storing as many as required in the
     * cache for reuse.
     */
    @Override
    protected void removeAllNodes() {
        // Add the removed nodes to the cache, then remove the rest.
        // We can add them to the cache before removing them, since
        // {@link AbstractLinkedList.removeAllNodes()} removes the
        // nodes by removing references directly from {@link #header}.
        final int numberOfNodesToCache = Math.min(size, maximumCacheSize - cacheSize);
        Node<E> node = header.next;
        for (int currentIndex = 0; currentIndex < numberOfNodesToCache; currentIndex++) {
            final Node<E> oldNode = node;
            node = node.next;
            addNodeToCache(oldNode);
        }
        super.removeAllNodes();
    }

    /**
     * Removes the node from the list, storing it in the cache for reuse
     * if the cache is not yet full.
     *
     * @param node  the node to remove
     */
    @Override
    protected void removeNode(final Node<E> node) {
        super.removeNode(node);
        addNodeToCache(node);
    }

    /**
     * Sets the maximum size of the cache.
     *
     * @param maximumCacheSize  the new maximum cache size
     */
    protected void setMaximumCacheSize(final int maximumCacheSize) {
        this.maximumCacheSize = maximumCacheSize;
        shrinkCacheToMaximumSize();
    }

    /**
     * Reduce the size of the cache to the maximum, if necessary.
     */
    protected void shrinkCacheToMaximumSize() {
        // Rich Dougherty: This could be more efficient.
        while (cacheSize > maximumCacheSize) {
            getNodeFromCache();
        }
    }

    /**
     * Serializes the data held in this object to the stream specified.
     *
     * @param out  the output stream
     * @throws IOException if an error occurs while writing to the stream
     */
    private void writeObject(final ObjectOutputStream out) throws IOException {
        out.defaultWriteObject();
        doWriteObject(out);
    }

}