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    4    *
    5    * This code is free software; you can redistribute it and/or modify it
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    9    * by Oracle in the LICENSE file that accompanied this code.
   10    *
   11    * This code is distributed in the hope that it will be useful, but WITHOUT
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   13    * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   14    * version 2 for more details (a copy is included in the LICENSE file that
   15    * accompanied this code).
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   19    * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   20    *
   21    * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   22    * or visit www.oracle.com if you need additional information or have any
   23    * questions.
   24    */
   25   
   26   package java.util;
   27   import java.io;
   28   
   29   /**
   30    * <p>Hash table and linked list implementation of the <tt>Map</tt> interface,
   31    * with predictable iteration order.  This implementation differs from
   32    * <tt>HashMap</tt> in that it maintains a doubly-linked list running through
   33    * all of its entries.  This linked list defines the iteration ordering,
   34    * which is normally the order in which keys were inserted into the map
   35    * (<i>insertion-order</i>).  Note that insertion order is not affected
   36    * if a key is <i>re-inserted</i> into the map.  (A key <tt>k</tt> is
   37    * reinserted into a map <tt>m</tt> if <tt>m.put(k, v)</tt> is invoked when
   38    * <tt>m.containsKey(k)</tt> would return <tt>true</tt> immediately prior to
   39    * the invocation.)
   40    *
   41    * <p>This implementation spares its clients from the unspecified, generally
   42    * chaotic ordering provided by {@link HashMap} (and {@link Hashtable}),
   43    * without incurring the increased cost associated with {@link TreeMap}.  It
   44    * can be used to produce a copy of a map that has the same order as the
   45    * original, regardless of the original map's implementation:
   46    * <pre>
   47    *     void foo(Map m) {
   48    *         Map copy = new LinkedHashMap(m);
   49    *         ...
   50    *     }
   51    * </pre>
   52    * This technique is particularly useful if a module takes a map on input,
   53    * copies it, and later returns results whose order is determined by that of
   54    * the copy.  (Clients generally appreciate having things returned in the same
   55    * order they were presented.)
   56    *
   57    * <p>A special {@link #LinkedHashMap(int,float,boolean) constructor} is
   58    * provided to create a linked hash map whose order of iteration is the order
   59    * in which its entries were last accessed, from least-recently accessed to
   60    * most-recently (<i>access-order</i>).  This kind of map is well-suited to
   61    * building LRU caches.  Invoking the <tt>put</tt> or <tt>get</tt> method
   62    * results in an access to the corresponding entry (assuming it exists after
   63    * the invocation completes).  The <tt>putAll</tt> method generates one entry
   64    * access for each mapping in the specified map, in the order that key-value
   65    * mappings are provided by the specified map's entry set iterator.  <i>No
   66    * other methods generate entry accesses.</i> In particular, operations on
   67    * collection-views do <i>not</i> affect the order of iteration of the backing
   68    * map.
   69    *
   70    * <p>The {@link #removeEldestEntry(Map.Entry)} method may be overridden to
   71    * impose a policy for removing stale mappings automatically when new mappings
   72    * are added to the map.
   73    *
   74    * <p>This class provides all of the optional <tt>Map</tt> operations, and
   75    * permits null elements.  Like <tt>HashMap</tt>, it provides constant-time
   76    * performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and
   77    * <tt>remove</tt>), assuming the hash function disperses elements
   78    * properly among the buckets.  Performance is likely to be just slightly
   79    * below that of <tt>HashMap</tt>, due to the added expense of maintaining the
   80    * linked list, with one exception: Iteration over the collection-views
   81    * of a <tt>LinkedHashMap</tt> requires time proportional to the <i>size</i>
   82    * of the map, regardless of its capacity.  Iteration over a <tt>HashMap</tt>
   83    * is likely to be more expensive, requiring time proportional to its
   84    * <i>capacity</i>.
   85    *
   86    * <p>A linked hash map has two parameters that affect its performance:
   87    * <i>initial capacity</i> and <i>load factor</i>.  They are defined precisely
   88    * as for <tt>HashMap</tt>.  Note, however, that the penalty for choosing an
   89    * excessively high value for initial capacity is less severe for this class
   90    * than for <tt>HashMap</tt>, as iteration times for this class are unaffected
   91    * by capacity.
   92    *
   93    * <p><strong>Note that this implementation is not synchronized.</strong>
   94    * If multiple threads access a linked hash map concurrently, and at least
   95    * one of the threads modifies the map structurally, it <em>must</em> be
   96    * synchronized externally.  This is typically accomplished by
   97    * synchronizing on some object that naturally encapsulates the map.
   98    *
   99    * If no such object exists, the map should be "wrapped" using the
  100    * {@link Collections#synchronizedMap Collections.synchronizedMap}
  101    * method.  This is best done at creation time, to prevent accidental
  102    * unsynchronized access to the map:<pre>
  103    *   Map m = Collections.synchronizedMap(new LinkedHashMap(...));</pre>
  104    *
  105    * A structural modification is any operation that adds or deletes one or more
  106    * mappings or, in the case of access-ordered linked hash maps, affects
  107    * iteration order.  In insertion-ordered linked hash maps, merely changing
  108    * the value associated with a key that is already contained in the map is not
  109    * a structural modification.  <strong>In access-ordered linked hash maps,
  110    * merely querying the map with <tt>get</tt> is a structural
  111    * modification.</strong>)
  112    *
  113    * <p>The iterators returned by the <tt>iterator</tt> method of the collections
  114    * returned by all of this class's collection view methods are
  115    * <em>fail-fast</em>: if the map is structurally modified at any time after
  116    * the iterator is created, in any way except through the iterator's own
  117    * <tt>remove</tt> method, the iterator will throw a {@link
  118    * ConcurrentModificationException}.  Thus, in the face of concurrent
  119    * modification, the iterator fails quickly and cleanly, rather than risking
  120    * arbitrary, non-deterministic behavior at an undetermined time in the future.
  121    *
  122    * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
  123    * as it is, generally speaking, impossible to make any hard guarantees in the
  124    * presence of unsynchronized concurrent modification.  Fail-fast iterators
  125    * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
  126    * Therefore, it would be wrong to write a program that depended on this
  127    * exception for its correctness:   <i>the fail-fast behavior of iterators
  128    * should be used only to detect bugs.</i>
  129    *
  130    * <p>This class is a member of the
  131    * <a href="{@docRoot}/../technotes/guides/collections/index.html">
  132    * Java Collections Framework</a>.
  133    *
  134    * @param <K> the type of keys maintained by this map
  135    * @param <V> the type of mapped values
  136    *
  137    * @author  Josh Bloch
  138    * @see     Object#hashCode()
  139    * @see     Collection
  140    * @see     Map
  141    * @see     HashMap
  142    * @see     TreeMap
  143    * @see     Hashtable
  144    * @since   1.4
  145    */
  146   
  147   public class LinkedHashMap<K,V>
  148       extends HashMap<K,V>
  149       implements Map<K,V>
  150   {
  151   
  152       private static final long serialVersionUID = 3801124242820219131L;
  153   
  154       /**
  155        * The head of the doubly linked list.
  156        */
  157       private transient Entry<K,V> header;
  158   
  159       /**
  160        * The iteration ordering method for this linked hash map: <tt>true</tt>
  161        * for access-order, <tt>false</tt> for insertion-order.
  162        *
  163        * @serial
  164        */
  165       private final boolean accessOrder;
  166   
  167       /**
  168        * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
  169        * with the specified initial capacity and load factor.
  170        *
  171        * @param  initialCapacity the initial capacity
  172        * @param  loadFactor      the load factor
  173        * @throws IllegalArgumentException if the initial capacity is negative
  174        *         or the load factor is nonpositive
  175        */
  176       public LinkedHashMap(int initialCapacity, float loadFactor) {
  177           super(initialCapacity, loadFactor);
  178           accessOrder = false;
  179       }
  180   
  181       /**
  182        * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
  183        * with the specified initial capacity and a default load factor (0.75).
  184        *
  185        * @param  initialCapacity the initial capacity
  186        * @throws IllegalArgumentException if the initial capacity is negative
  187        */
  188       public LinkedHashMap(int initialCapacity) {
  189           super(initialCapacity);
  190           accessOrder = false;
  191       }
  192   
  193       /**
  194        * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
  195        * with the default initial capacity (16) and load factor (0.75).
  196        */
  197       public LinkedHashMap() {
  198           super();
  199           accessOrder = false;
  200       }
  201   
  202       /**
  203        * Constructs an insertion-ordered <tt>LinkedHashMap</tt> instance with
  204        * the same mappings as the specified map.  The <tt>LinkedHashMap</tt>
  205        * instance is created with a default load factor (0.75) and an initial
  206        * capacity sufficient to hold the mappings in the specified map.
  207        *
  208        * @param  m the map whose mappings are to be placed in this map
  209        * @throws NullPointerException if the specified map is null
  210        */
  211       public LinkedHashMap(Map<? extends K, ? extends V> m) {
  212           super(m);
  213           accessOrder = false;
  214       }
  215   
  216       /**
  217        * Constructs an empty <tt>LinkedHashMap</tt> instance with the
  218        * specified initial capacity, load factor and ordering mode.
  219        *
  220        * @param  initialCapacity the initial capacity
  221        * @param  loadFactor      the load factor
  222        * @param  accessOrder     the ordering mode - <tt>true</tt> for
  223        *         access-order, <tt>false</tt> for insertion-order
  224        * @throws IllegalArgumentException if the initial capacity is negative
  225        *         or the load factor is nonpositive
  226        */
  227       public LinkedHashMap(int initialCapacity,
  228                            float loadFactor,
  229                            boolean accessOrder) {
  230           super(initialCapacity, loadFactor);
  231           this.accessOrder = accessOrder;
  232       }
  233   
  234       /**
  235        * Called by superclass constructors and pseudoconstructors (clone,
  236        * readObject) before any entries are inserted into the map.  Initializes
  237        * the chain.
  238        */
  239       void init() {
  240           header = new Entry<>(-1, null, null, null);
  241           header.before = header.after = header;
  242       }
  243   
  244       /**
  245        * Transfers all entries to new table array.  This method is called
  246        * by superclass resize.  It is overridden for performance, as it is
  247        * faster to iterate using our linked list.
  248        */
  249       void transfer(HashMap.Entry[] newTable) {
  250           int newCapacity = newTable.length;
  251           for (Entry<K,V> e = header.after; e != header; e = e.after) {
  252               int index = indexFor(e.hash, newCapacity);
  253               e.next = newTable[index];
  254               newTable[index] = e;
  255           }
  256       }
  257   
  258   
  259       /**
  260        * Returns <tt>true</tt> if this map maps one or more keys to the
  261        * specified value.
  262        *
  263        * @param value value whose presence in this map is to be tested
  264        * @return <tt>true</tt> if this map maps one or more keys to the
  265        *         specified value
  266        */
  267       public boolean containsValue(Object value) {
  268           // Overridden to take advantage of faster iterator
  269           if (value==null) {
  270               for (Entry e = header.after; e != header; e = e.after)
  271                   if (e.value==null)
  272                       return true;
  273           } else {
  274               for (Entry e = header.after; e != header; e = e.after)
  275                   if (value.equals(e.value))
  276                       return true;
  277           }
  278           return false;
  279       }
  280   
  281       /**
  282        * Returns the value to which the specified key is mapped,
  283        * or {@code null} if this map contains no mapping for the key.
  284        *
  285        * <p>More formally, if this map contains a mapping from a key
  286        * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
  287        * key.equals(k))}, then this method returns {@code v}; otherwise
  288        * it returns {@code null}.  (There can be at most one such mapping.)
  289        *
  290        * <p>A return value of {@code null} does not <i>necessarily</i>
  291        * indicate that the map contains no mapping for the key; it's also
  292        * possible that the map explicitly maps the key to {@code null}.
  293        * The {@link #containsKey containsKey} operation may be used to
  294        * distinguish these two cases.
  295        */
  296       public V get(Object key) {
  297           Entry<K,V> e = (Entry<K,V>)getEntry(key);
  298           if (e == null)
  299               return null;
  300           e.recordAccess(this);
  301           return e.value;
  302       }
  303   
  304       /**
  305        * Removes all of the mappings from this map.
  306        * The map will be empty after this call returns.
  307        */
  308       public void clear() {
  309           super.clear();
  310           header.before = header.after = header;
  311       }
  312   
  313       /**
  314        * LinkedHashMap entry.
  315        */
  316       private static class Entry<K,V> extends HashMap.Entry<K,V> {
  317           // These fields comprise the doubly linked list used for iteration.
  318           Entry<K,V> before, after;
  319   
  320           Entry(int hash, K key, V value, HashMap.Entry<K,V> next) {
  321               super(hash, key, value, next);
  322           }
  323   
  324           /**
  325            * Removes this entry from the linked list.
  326            */
  327           private void remove() {
  328               before.after = after;
  329               after.before = before;
  330           }
  331   
  332           /**
  333            * Inserts this entry before the specified existing entry in the list.
  334            */
  335           private void addBefore(Entry<K,V> existingEntry) {
  336               after  = existingEntry;
  337               before = existingEntry.before;
  338               before.after = this;
  339               after.before = this;
  340           }
  341   
  342           /**
  343            * This method is invoked by the superclass whenever the value
  344            * of a pre-existing entry is read by Map.get or modified by Map.set.
  345            * If the enclosing Map is access-ordered, it moves the entry
  346            * to the end of the list; otherwise, it does nothing.
  347            */
  348           void recordAccess(HashMap<K,V> m) {
  349               LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m;
  350               if (lm.accessOrder) {
  351                   lm.modCount++;
  352                   remove();
  353                   addBefore(lm.header);
  354               }
  355           }
  356   
  357           void recordRemoval(HashMap<K,V> m) {
  358               remove();
  359           }
  360       }
  361   
  362       private abstract class LinkedHashIterator<T> implements Iterator<T> {
  363           Entry<K,V> nextEntry    = header.after;
  364           Entry<K,V> lastReturned = null;
  365   
  366           /**
  367            * The modCount value that the iterator believes that the backing
  368            * List should have.  If this expectation is violated, the iterator
  369            * has detected concurrent modification.
  370            */
  371           int expectedModCount = modCount;
  372   
  373           public boolean hasNext() {
  374               return nextEntry != header;
  375           }
  376   
  377           public void remove() {
  378               if (lastReturned == null)
  379                   throw new IllegalStateException();
  380               if (modCount != expectedModCount)
  381                   throw new ConcurrentModificationException();
  382   
  383               LinkedHashMap.this.remove(lastReturned.key);
  384               lastReturned = null;
  385               expectedModCount = modCount;
  386           }
  387   
  388           Entry<K,V> nextEntry() {
  389               if (modCount != expectedModCount)
  390                   throw new ConcurrentModificationException();
  391               if (nextEntry == header)
  392                   throw new NoSuchElementException();
  393   
  394               Entry<K,V> e = lastReturned = nextEntry;
  395               nextEntry = e.after;
  396               return e;
  397           }
  398       }
  399   
  400       private class KeyIterator extends LinkedHashIterator<K> {
  401           public K next() { return nextEntry().getKey(); }
  402       }
  403   
  404       private class ValueIterator extends LinkedHashIterator<V> {
  405           public V next() { return nextEntry().value; }
  406       }
  407   
  408       private class EntryIterator extends LinkedHashIterator<Map.Entry<K,V>> {
  409           public Map.Entry<K,V> next() { return nextEntry(); }
  410       }
  411   
  412       // These Overrides alter the behavior of superclass view iterator() methods
  413       Iterator<K> newKeyIterator()   { return new KeyIterator();   }
  414       Iterator<V> newValueIterator() { return new ValueIterator(); }
  415       Iterator<Map.Entry<K,V>> newEntryIterator() { return new EntryIterator(); }
  416   
  417       /**
  418        * This override alters behavior of superclass put method. It causes newly
  419        * allocated entry to get inserted at the end of the linked list and
  420        * removes the eldest entry if appropriate.
  421        */
  422       void addEntry(int hash, K key, V value, int bucketIndex) {
  423           createEntry(hash, key, value, bucketIndex);
  424   
  425           // Remove eldest entry if instructed, else grow capacity if appropriate
  426           Entry<K,V> eldest = header.after;
  427           if (removeEldestEntry(eldest)) {
  428               removeEntryForKey(eldest.key);
  429           } else {
  430               if (size >= threshold)
  431                   resize(2 * table.length);
  432           }
  433       }
  434   
  435       /**
  436        * This override differs from addEntry in that it doesn't resize the
  437        * table or remove the eldest entry.
  438        */
  439       void createEntry(int hash, K key, V value, int bucketIndex) {
  440           HashMap.Entry<K,V> old = table[bucketIndex];
  441           Entry<K,V> e = new Entry<>(hash, key, value, old);
  442           table[bucketIndex] = e;
  443           e.addBefore(header);
  444           size++;
  445       }
  446   
  447       /**
  448        * Returns <tt>true</tt> if this map should remove its eldest entry.
  449        * This method is invoked by <tt>put</tt> and <tt>putAll</tt> after
  450        * inserting a new entry into the map.  It provides the implementor
  451        * with the opportunity to remove the eldest entry each time a new one
  452        * is added.  This is useful if the map represents a cache: it allows
  453        * the map to reduce memory consumption by deleting stale entries.
  454        *
  455        * <p>Sample use: this override will allow the map to grow up to 100
  456        * entries and then delete the eldest entry each time a new entry is
  457        * added, maintaining a steady state of 100 entries.
  458        * <pre>
  459        *     private static final int MAX_ENTRIES = 100;
  460        *
  461        *     protected boolean removeEldestEntry(Map.Entry eldest) {
  462        *        return size() > MAX_ENTRIES;
  463        *     }
  464        * </pre>
  465        *
  466        * <p>This method typically does not modify the map in any way,
  467        * instead allowing the map to modify itself as directed by its
  468        * return value.  It <i>is</i> permitted for this method to modify
  469        * the map directly, but if it does so, it <i>must</i> return
  470        * <tt>false</tt> (indicating that the map should not attempt any
  471        * further modification).  The effects of returning <tt>true</tt>
  472        * after modifying the map from within this method are unspecified.
  473        *
  474        * <p>This implementation merely returns <tt>false</tt> (so that this
  475        * map acts like a normal map - the eldest element is never removed).
  476        *
  477        * @param    eldest The least recently inserted entry in the map, or if
  478        *           this is an access-ordered map, the least recently accessed
  479        *           entry.  This is the entry that will be removed it this
  480        *           method returns <tt>true</tt>.  If the map was empty prior
  481        *           to the <tt>put</tt> or <tt>putAll</tt> invocation resulting
  482        *           in this invocation, this will be the entry that was just
  483        *           inserted; in other words, if the map contains a single
  484        *           entry, the eldest entry is also the newest.
  485        * @return   <tt>true</tt> if the eldest entry should be removed
  486        *           from the map; <tt>false</tt> if it should be retained.
  487        */
  488       protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
  489           return false;
  490       }
  491   }

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