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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

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 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

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 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 */

package java.util;

import java.io.*;

/**

 * <p>Hash table and linked list implementation of the <tt>Map</tt> interface,

 * with predictable iteration order.  This implementation differs from

 * <tt>HashMap</tt> in that it maintains a doubly-linked list running through

 * all of its entries.  This linked list defines the iteration ordering,

 * which is normally the order in which keys were inserted into the map

 * (<i>insertion-order</i>).  Note that insertion order is not affected

 * if a key is <i>re-inserted</i> into the map.  (A key <tt>k</tt> is

 * reinserted into a map <tt>m</tt> if <tt>m.put(k, v)</tt> is invoked when

 * <tt>m.containsKey(k)</tt> would return <tt>true</tt> immediately prior to

 * the invocation.)

 *

 * <p>This implementation spares its clients from the unspecified, generally

 * chaotic ordering provided by {@link HashMap} (and {@link Hashtable}),

 * without incurring the increased cost associated with {@link TreeMap}.  It

 * can be used to produce a copy of a map that has the same order as the

 * original, regardless of the original map's implementation:

 * <pre>

 *     void foo(Map m) {

 *         Map copy = new LinkedHashMap(m);

 *         ...

 *     }

 * </pre>

 * This technique is particularly useful if a module takes a map on input,

 * copies it, and later returns results whose order is determined by that of

 * the copy.  (Clients generally appreciate having things returned in the same

 * order they were presented.)

 *

 * <p>A special {@link #LinkedHashMap(int,float,boolean) constructor} is

 * provided to create a <tt>LinkedHashMap</tt> whose order of iteration is the

 * order in which its entries were last accessed, from least-recently accessed

 * to most-recently (<i>access-order</i>).  This kind of map is well-suited to

 * building LRU caches.  Invoking the <tt>put</tt> or <tt>get</tt> method

 * results in an access to the corresponding entry (assuming it exists after

 * the invocation completes).  The <tt>putAll</tt> method generates one entry

 * access for each mapping in the specified map, in the order that key-value

 * mappings are provided by the specified map's entry set iterator.  <i>No

 * other methods generate entry accesses.</i> In particular, operations on

 * collection-views do <i>not</i> affect the order of iteration of the backing

 * map.

 *

 * <p>The {@link #removeEldestEntry(Map.Entry)} method may be overridden to

 * impose a policy for removing stale mappings automatically when new mappings

 * are added to the map.

 *

 * <p>This class provides all of the optional <tt>Map</tt> operations, and

 * permits null elements.  Like <tt>HashMap</tt>, it provides constant-time

 * performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and

 * <tt>remove</tt>), assuming the hash function disperses elements

 * properly among the buckets.  Performance is likely to be just slightly

 * below that of <tt>HashMap</tt>, due to the added expense of maintaining the

 * linked list, with one exception: Iteration over the collection-views

 * of a <tt>LinkedHashMap</tt> requires time proportional to the <i>size</i>

 * of the map, regardless of its capacity.  Iteration over a <tt>HashMap</tt>

 * is likely to be more expensive, requiring time proportional to its

 * <i>capacity</i>.

 *

 * <p>A linked hash map has two parameters that affect its performance:

 * <i>initial capacity</i> and <i>load factor</i>.  They are defined precisely

 * as for <tt>HashMap</tt>.  Note, however, that the penalty for choosing an

 * excessively high value for initial capacity is less severe for this class

 * than for <tt>HashMap</tt>, as iteration times for this class are unaffected

 * by capacity.

 *

 * <p><strong>Note that this implementation is not synchronized.</strong>

 * If multiple threads access a linked hash map concurrently, and at least

 * one of the threads modifies the map structurally, it <em>must</em> be

 * synchronized externally.  This is typically accomplished by

 * synchronizing on some object that naturally encapsulates the map.

 *

 * If no such object exists, the map should be "wrapped" using the

 * {@link Collections#synchronizedMap Collections.synchronizedMap}

 * method.  This is best done at creation time, to prevent accidental

 * unsynchronized access to the map:<pre>

 *   Map m = Collections.synchronizedMap(new LinkedHashMap(...));</pre>

 *

 * A structural modification is any operation that adds or deletes one or more

 * mappings or, in the case of access-ordered linked hash maps, affects

 * iteration order.  In insertion-ordered linked hash maps, merely changing

 * the value associated with a key that is already contained in the map is not

 * a structural modification.  <strong>In access-ordered linked hash maps,

 * merely querying the map with <tt>get</tt> is a structural

 * modification.</strong>)

 *

 * <p>The iterators returned by the <tt>iterator</tt> method of the collections

 * returned by all of this class's collection view methods are

 * <em>fail-fast</em>: if the map is structurally modified at any time after

 * the iterator is created, in any way except through the iterator's own

 * <tt>remove</tt> method, the iterator will throw a {@link

 * ConcurrentModificationException}.  Thus, in the face of concurrent

 * modification, the iterator fails quickly and cleanly, rather than risking

 * arbitrary, non-deterministic behavior at an undetermined time in the future.

 *

 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed

 * as it is, generally speaking, impossible to make any hard guarantees in the

 * presence of unsynchronized concurrent modification.  Fail-fast iterators

 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.

 * Therefore, it would be wrong to write a program that depended on this

 * exception for its correctness:   <i>the fail-fast behavior of iterators

 * should be used only to detect bugs.</i>

 *

 * <p>This class is a member of the

 * <a href="{@docRoot}/../technotes/guides/collections/index.html">

 * Java Collections Framework</a>.

 *

 * @param <K> the type of keys maintained by this map

 * @param <V> the type of mapped values

 *

 * @author  Josh Bloch

 * @see     Object#hashCode()

 * @see     Collection

 * @see     Map

 * @see     HashMap

 * @see     TreeMap

 * @see     Hashtable

 * @since   1.4

 */

public class LinkedHashMap<K,V>

    extends HashMap<K,V>

    implements Map<K,V>

{

    private static final long serialVersionUID = 3801124242820219131L;

    /**

     * The head of the doubly linked list.

     */

    private transient Entry<K,V> header;

    /**

     * The iteration ordering method for this linked hash map: <tt>true</tt>

     * for access-order, <tt>false</tt> for insertion-order.

     *

     * @serial

     */

    private final boolean accessOrder;

    /**

     * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance

     * with the specified initial capacity and load factor.

     *

     * @param  initialCapacity the initial capacity

     * @param  loadFactor      the load factor

     * @throws IllegalArgumentException if the initial capacity is negative

     *         or the load factor is nonpositive

     */

    public LinkedHashMap(int initialCapacity, float loadFactor) {

        super(initialCapacity, loadFactor);

        accessOrder = false;

    }

    /**

     * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance

     * with the specified initial capacity and a default load factor (0.75).

     *

     * @param  initialCapacity the initial capacity

     * @throws IllegalArgumentException if the initial capacity is negative

     */

    public LinkedHashMap(int initialCapacity) {

        super(initialCapacity);

        accessOrder = false;

    }

    /**

     * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance

     * with the default initial capacity (16) and load factor (0.75).

     */

    public LinkedHashMap() {

        super();

        accessOrder = false;

    }

    /**

     * Constructs an insertion-ordered <tt>LinkedHashMap</tt> instance with

     * the same mappings as the specified map.  The <tt>LinkedHashMap</tt>

     * instance is created with a default load factor (0.75) and an initial

     * capacity sufficient to hold the mappings in the specified map.

     *

     * @param  m the map whose mappings are to be placed in this map

     * @throws NullPointerException if the specified map is null

     */

    public LinkedHashMap(Map<? extends K, ? extends V> m) {

        super(m);

        accessOrder = false;

    }

    /**

     * Constructs an empty <tt>LinkedHashMap</tt> instance with the

     * specified initial capacity, load factor and ordering mode.

     *

     * @param  initialCapacity the initial capacity

     * @param  loadFactor      the load factor

     * @param  accessOrder     the ordering mode - <tt>true</tt> for

     *         access-order, <tt>false</tt> for insertion-order

     * @throws IllegalArgumentException if the initial capacity is negative

     *         or the load factor is nonpositive

     */

    public LinkedHashMap(int initialCapacity,

                         float loadFactor,

                         boolean accessOrder) {

        super(initialCapacity, loadFactor);

        this.accessOrder = accessOrder;

    }

    /**

     * Called by superclass constructors and pseudoconstructors (clone,

     * readObject) before any entries are inserted into the map.  Initializes

     * the chain.

     */

    @Override

    void init() {

        header = new Entry<>(-1, null, null, null);

        header.before = header.after = header;

    }

    /**

     * Returns <tt>true</tt> if this map maps one or more keys to the

     * specified value.

     *

     * @param value value whose presence in this map is to be tested

     * @return <tt>true</tt> if this map maps one or more keys to the

     *         specified value

     */

    public boolean containsValue(Object value) {

        // Overridden to take advantage of faster iterator

        if (value==null) {

            for (Entry<?,?> e = header.after; e != header; e = e.after)

                if (e.value==null)

                    return true;

        } else {

            for (Entry<?,?> e = header.after; e != header; e = e.after)

                if (value.equals(e.value))

                    return true;

        }

        return false;

    }

    /**

     * Returns the value to which the specified key is mapped,

     * or {@code null} if this map contains no mapping for the key.

     *

     * <p>More formally, if this map contains a mapping from a key

     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :

     * key.equals(k))}, then this method returns {@code v}; otherwise

     * it returns {@code null}.  (There can be at most one such mapping.)

     *

     * <p>A return value of {@code null} does not <i>necessarily</i>

     * indicate that the map contains no mapping for the key; it's also

     * possible that the map explicitly maps the key to {@code null}.

     * The {@link #containsKey containsKey} operation may be used to

     * distinguish these two cases.

     */

    public V get(Object key) {

        Entry<K,V> e = (Entry<K,V>)getEntry(key);

        if (e == null)

            return null;

        e.recordAccess(this);

        return e.value;

    }

    /**

     * Removes all of the mappings from this map.

     * The map will be empty after this call returns.

     */

    public void clear() {

        super.clear();

        header.before = header.after = header;

    }

    /**

     * LinkedHashMap entry.

     */

    private static class Entry<K,V> extends HashMap.Entry<K,V> {

        // These fields comprise the doubly linked list used for iteration.

        Entry<K,V> before, after;

        Entry(int hash, K key, V value, Object next) {

            super(hash, key, value, next);

        }

        /**

         * Removes this entry from the linked list.

         */

        private void remove() {

            before.after = after;

            after.before = before;

        }

        /**

         * Inserts this entry before the specified existing entry in the list.

         */

        private void addBefore(Entry<K,V> existingEntry) {

            after  = existingEntry;

            before = existingEntry.before;

            before.after = this;

            after.before = this;

        }

        /**

         * This method is invoked by the superclass whenever the value

         * of a pre-existing entry is read by Map.get or modified by Map.put.

         * If the enclosing Map is access-ordered, it moves the entry

         * to the end of the list; otherwise, it does nothing.

         */

        void recordAccess(HashMap<K,V> m) {

            LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m;

            if (lm.accessOrder) {

                lm.modCount++;

                remove();

                addBefore(lm.header);

            }

        }

        void recordRemoval(HashMap<K,V> m) {

            remove();

        }

    }

    private abstract class LinkedHashIterator<T> implements Iterator<T> {

        Entry<K,V> nextEntry    = header.after;

        Entry<K,V> lastReturned = null;

        /**

         * The modCount value that the iterator believes that the backing

         * List should have.  If this expectation is violated, the iterator

         * has detected concurrent modification.

         */

        int expectedModCount = modCount;

        public boolean hasNext() {

            return nextEntry != header;

        }

        public void remove() {

            if (lastReturned == null)

                throw new IllegalStateException();

            if (modCount != expectedModCount)

                throw new ConcurrentModificationException();

            LinkedHashMap.this.remove(lastReturned.key);

            lastReturned = null;

            expectedModCount = modCount;

        }

        Entry<K,V> nextEntry() {

            if (modCount != expectedModCount)

                throw new ConcurrentModificationException();

            if (nextEntry == header)

                throw new NoSuchElementException();

            Entry<K,V> e = lastReturned = nextEntry;

            nextEntry = e.after;

            return e;

        }

    }

    private class KeyIterator extends LinkedHashIterator<K> {

        public K next() { return nextEntry().getKey(); }

    }

    private class ValueIterator extends LinkedHashIterator<V> {

        public V next() { return nextEntry().value; }

    }

    private class EntryIterator extends LinkedHashIterator<Map.Entry<K,V>> {

        public Map.Entry<K,V> next() { return nextEntry(); }

    }

    // These Overrides alter the behavior of superclass view iterator() methods

    Iterator<K> newKeyIterator()   { return new KeyIterator();   }

    Iterator<V> newValueIterator() { return new ValueIterator(); }

    Iterator<Map.Entry<K,V>> newEntryIterator() { return new EntryIterator(); }

    /**

     * This override alters behavior of superclass put method. It causes newly

     * allocated entry to get inserted at the end of the linked list and

     * removes the eldest entry if appropriate.

     */

    @Override

    void addEntry(int hash, K key, V value, int bucketIndex, boolean checkIfNeedTree) {

        super.addEntry(hash, key, value, bucketIndex, checkIfNeedTree);

        // Remove eldest entry if instructed

        Entry<K,V> eldest = header.after;

        if (removeEldestEntry(eldest)) {

            removeEntryForKey(eldest.key);

        }

    }

    /*

     * Create a new LinkedHashMap.Entry and setup the before/after pointers

     */

    @Override

    HashMap.Entry<K,V> newEntry(int hash, K key, V value, Object next) {

        Entry<K,V> newEntry = new Entry<>(hash, key, value, next);

        newEntry.addBefore(header);

        return newEntry;

    }

    /**

     * Returns <tt>true</tt> if this map should remove its eldest entry.

     * This method is invoked by <tt>put</tt> and <tt>putAll</tt> after

     * inserting a new entry into the map.  It provides the implementor

     * with the opportunity to remove the eldest entry each time a new one

     * is added.  This is useful if the map represents a cache: it allows

     * the map to reduce memory consumption by deleting stale entries.

     *

     * <p>Sample use: this override will allow the map to grow up to 100

     * entries and then delete the eldest entry each time a new entry is

     * added, maintaining a steady state of 100 entries.

     * <pre>{@code

     *     private static final int MAX_ENTRIES = 100;

     *

     *     protected boolean removeEldestEntry(Map.Entry eldest) {

     *        return size() > MAX_ENTRIES;

     *     }

     * }</pre>

     *

     * <p>This method typically does not modify the map in any way,

     * instead allowing the map to modify itself as directed by its

     * return value.  It <i>is</i> permitted for this method to modify

     * the map directly, but if it does so, it <i>must</i> return

     * <tt>false</tt> (indicating that the map should not attempt any

     * further modification).  The effects of returning <tt>true</tt>

     * after modifying the map from within this method are unspecified.

     *

     * <p>This implementation merely returns <tt>false</tt> (so that this

     * map acts like a normal map - the eldest element is never removed).

     *

     * @param    eldest The least recently inserted entry in the map, or if

     *           this is an access-ordered map, the least recently accessed

     *           entry.  This is the entry that will be removed it this

     *           method returns <tt>true</tt>.  If the map was empty prior

     *           to the <tt>put</tt> or <tt>putAll</tt> invocation resulting

     *           in this invocation, this will be the entry that was just

     *           inserted; in other words, if the map contains a single

     *           entry, the eldest entry is also the newest.

     * @return   <tt>true</tt> if the eldest entry should be removed

     *           from the map; <tt>false</tt> if it should be retained.

     */

    protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {

        return false;

    }

}