java源码解析---LinkedHashMap

  1 package explain;
  2 
  3 import java.util.function.Consumer;
  4 import java.util.AbstractCollection;
  5 import java.util.AbstractSet;
  6 import java.util.HashMap;
  7 import java.util.Iterator;
  8 import java.util.Map;
  9 import java.util.Set;
 10 import java.util.function.BiConsumer;
 11 import java.util.function.BiFunction;
 12 import java.io.IOException;
 13 
 14 /**
 15  * <p>Hash table and linked list implementation of the <tt>Map</tt> interface,
 16  * with predictable iteration order.  This implementation differs from
 17  * <tt>HashMap</tt> in that it maintains a doubly-linked list running through
 18  * all of its entries.  This linked list defines the iteration ordering,
 19  * which is normally the order in which keys were inserted into the map
 20  * (<i>insertion-order</i>).  Note that insertion order is not affected
 21  * if a key is <i>re-inserted</i> into the map.  (A key <tt>k</tt> is
 22  * reinserted into a map <tt>m</tt> if <tt>m.put(k, v)</tt> is invoked when
 23  * <tt>m.containsKey(k)</tt> would return <tt>true</tt> immediately prior to
 24  * the invocation.)
 25  *  Map接口的哈希表和链接列表实现，具有可预知的迭代顺序。此实现与HashMap的不同之处在于，
 26     后者维护着一个运行于所有条目的双重链接列表。
 27     此链接列表定义了迭代顺序，该迭代顺序通常就是将键插入到映射中的顺序（插入顺序）。
 28     注意，如果在映射中重新插入键，则插入顺序不受影响。会按照第一次插入的顺序，再次插入到之前的位置
 29    （如果在调用m.put(k,v) 前 m.containsKey(k) 返回了true，则调用时会将键k重新插入到映射m中）
 30  * <p>This implementation spares its clients from the unspecified, generally
 31  * chaotic ordering provided by {@link HashMap} (and {@link Hashtable}),
 32  * without incurring the increased cost associated with {@link TreeMap}.  It
 33  * can be used to produce a copy of a map that has the same order as the
 34  * original, regardless of the original map‘s implementation:
 35    此实现可以让客户避免未指定的、由 HashMap（及 Hashtable）所提供的通常为杂乱无章的排序工作，同时无需增加与 TreeMap 相关的成本。      
       使用它可以生成一个与原来顺序相同的映射副本，而与原映射的实现无关：
 36  * <pre>
 37  *     void foo(Map m) {
 38  *         Map copy = new LinkedHashMap(m);
 39  *         ...
 40  *     }
 41  * </pre>
 42  * This technique is particularly useful if a module takes a map on input,
 43  * copies it, and later returns results whose order is determined by that of
 44  * the copy.  (Clients generally appreciate having things returned in the same
 45  * order they were presented.)
 46  * 如果模块通过输入得到一个映射，复制这个映射，然后返回由此副本确定其顺序的结果，这种情况下这项技术特别有用。
 47  （客户通常期望返回的内容与其出现的顺序相同。） 原样返回
 48  * <p>A special {@link #LinkedHashMap(int,float,boolean) constructor} is
 49  * provided to create a linked hash map whose order of iteration is the order
 50  * in which its entries were last accessed, from least-recently accessed to
 51  * most-recently (<i>access-order</i>).  This kind of map is well-suited to
 52  * building LRU caches.  Invoking the {@code put}, {@code putIfAbsent},
 53  * {@code get}, {@code getOrDefault}, {@code compute}, {@code computeIfAbsent},
 54  * {@code computeIfPresent}, or {@code merge} methods results
 55  * in an access to the corresponding entry (assuming it exists after the
 56  * invocation completes). The {@code replace} methods only result in an access
 57  * of the entry if the value is replaced.  The {@code putAll} method generates one
 58  * entry access for each mapping in the specified map, in the order that
 59  * key-value mappings are provided by the specified map‘s entry set iterator.
 60  * <i>No other methods generate entry accesses.</i>  In particular, operations
 61  * on collection-views do <i>not</i> affect the order of iteration of the
 62  * backing map.
 63  提供特殊的构造方法来创建链接哈希映射，该哈希映射的迭代顺序就是最后访问其条目的顺序，
 64  从近期访问最少到近期访问最多的顺序（访问顺序）。这种映射很适合构建 LRU 缓存。调用put或get 方法将会访问相应的条目（假定调用完成后它还存在）。
     putAll 方法以指定映射的条目集迭代器提供的键-值映射关系的顺序，为指定映射的每个映射关系生成一个条目访问。
 65  任何其他方法均不生成条目访问。特别是，collection 视图上的操作不 影响底层映射的迭代顺序
 66  *
 67  * <p>The {@link #removeEldestEntry(Map.Entry)} method may be overridden to
 68  * impose a policy for removing stale mappings automatically when new mappings
 69  * are added to the map.
 70  可以重写 removeEldestEntry(Map.Entry) 方法来实施策略，以便在将新映射关系添加到映射时自动移除旧的映射关系。
 71  * <p>This class provides all of the optional <tt>Map</tt> operations, and
 72  * permits null elements.  Like <tt>HashMap</tt>, it provides constant-time
 73  * performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and
 74  * <tt>remove</tt>), assuming the hash function disperses elements
 75  * properly among the buckets.  Performance is likely to be just slightly
 76  * below that of <tt>HashMap</tt>, due to the added expense of maintaining the
 77  * linked list, with one exception: Iteration over the collection-views
 78  * of a <tt>LinkedHashMap</tt> requires time proportional to the <i>size</i>
 79  * of the map, regardless of its capacity.  Iteration over a <tt>HashMap</tt>
 80  * is likely to be more expensive, requiring time proportional to its
 81  * <i>capacity</i>.
 82  *此类提供所有可选的 Map 操作，并且允许 null 元素。与 HashMap 一样，
 83   它可以为基本操作（add、contains和remove）提供稳定的性能，假定哈希函数将元素正确分布到桶中。
 84   由于增加了维护链接列表的开支，其性能很可能比 HashMap 稍逊一筹，不过这一点例外：LinkedHashMap 的 collection 视图迭代所需时间与映射的大小成比例。
      HashMap 迭代时间很可能开支较大，因为它所需要的时间与其容量 成比例。
 85  * <p>A linked hash map has two parameters that affect its performance:
 86  * <i>initial capacity</i> and <i>load factor</i>.  They are defined precisely
 87  * as for <tt>HashMap</tt>.  Note, however, that the penalty for choosing an
 88  * excessively high value for initial capacity is less severe for this class
 89  * than for <tt>HashMap</tt>, as iteration times for this class are unaffected
 90  * by capacity.
 91  *链接的哈希映射具有两个影响其性能的参数：初始容量和加载因子。它们的定义与 HashMap 极其相似。要注意，为初始容量选择非常高的值对此类的影响比对 HashMap 要小，
      因为此类的迭代时间不受容量的影响。
 92  * <p><strong>Note that this implementation is not synchronized.</strong>
 93  * If multiple threads access a linked hash map concurrently, and at least
 94  * one of the threads modifies the map structurally, it <em>must</em> be
 95  * synchronized externally.  This is typically accomplished by
 96  * synchronizing on some object that naturally encapsulates the map.
 97  *
 98  * If no such object exists, the map should be "wrapped" using the
 99  * {@link Collections#synchronizedMap Collections.synchronizedMap}
100  * method.  This is best done at creation time, to prevent accidental
101  * unsynchronized access to the map:<pre>
102  *   Map m = Collections.synchronizedMap(new LinkedHashMap(...));</pre>
103  * 注意，此实现不是同步的。如果多个线程同时访问链接的哈希映射，而其中至少一个线程从结构上修改了该映射，则它必须 保持外部同步。
       这一般通过对自然封装该映射的对象进行同步操作来完成。如果不存在这样的对象，则应该使用 Collections.synchronizedMap 方法来“包装”该映射。最好在创建时完成这一操作，
       以防止对映射的意外的非同步访问：
104  * A structural modification is any operation that adds or deletes one or more
105  * mappings or, in the case of access-ordered linked hash maps, affects
106  * iteration order.  In insertion-ordered linked hash maps, merely changing
107  * the value associated with a key that is already contained in the map is not
108  * a structural modification.  <strong>In access-ordered linked hash maps,
109  * merely querying the map with <tt>get</tt> is a structural modification.
110  * </strong>)
111  结构修改是指添加或删除一个或多个映射关系，或者在按访问顺序链接的哈希映射中影响迭代顺序的任何操作。
112  在按插入顺序链接的哈希映射中，仅更改与映射中已包含键关联的值不是结构修改。在按访问顺序链接的哈希映射中，仅利用 get 查询映射是结构修改。）1.6和1.8有区别
113  *当accessOrder为true时，在迭代器方式遍历map时，不允许使用get操作，因为在迭代器模式中不允许修改集合的结构，一般情况下get操作是只读的，
      但是本模式下，get操作需要修改map的链表结构，以将最近访问的元素放置到链表末尾。
114  
115  * <p>The iterators returned by the <tt>iterator</tt> method of the collections
116  * returned by all of this class‘s collection view methods are
117  * <em>fail-fast</em>: if the map is structurally modified at any time after
118  * the iterator is created, in any way except through the iterator‘s own
119  * <tt>remove</tt> method, the iterator will throw a {@link
120  * ConcurrentModificationException}.  Thus, in the face of concurrent
121  * modification, the iterator fails quickly and cleanly, rather than risking
122  * arbitrary, non-deterministic behavior at an undetermined time in the future.
123  *Collection（由此类的所有 collection 视图方法所返回）的 iterator 方法返回的迭代器都是快速失败 的：在迭代器创建之后，如果从结构上对映射进行修改，
     除非通过迭代器自身的 remove方法，其他任何时间任何方式的修改，迭代器都将抛出ConcurrentModificationException。
124  因此，面对并发的修改，迭代器很快就会完全失败，
125  而不冒将来不确定的时间任意发生不确定行为的风险。
126  * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
127  * as it is, generally speaking, impossible to make any hard guarantees in the
128  * presence of unsynchronized concurrent modification.  Fail-fast iterators
129  * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
130  * Therefore, it would be wrong to write a program that depended on this
131  * exception for its correctness:   <i>the fail-fast behavior of iterators
132  * should be used only to detect bugs.</i>
133  *注意，迭代器的快速失败行为无法得到保证，因为一般来说，不可能对是否出现不同步并发修改做出任何硬性保证。
134   快速失败迭代器会尽最大努力抛出ConcurrentModificationException。因此，编写依赖于此异常的程序的方式是错误的，
135   正确做法是：迭代器的快速失败行为应该仅用于检测程序错误
136 
137  *
138  * <p>This class is a member of the
139  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
140  * Java Collections Framework</a>.
141  *
142  * @implNote
143  * The spliterators returned by the spliterator method of the collections
144  * returned by all of this class‘s collection view methods are created from
145  * the iterators of the corresponding collections.
146  *
147  * @param <K> the type of keys maintained by this map
148  * @param <V> the type of mapped values
149  *
150  * @author  Josh Bloch
151  * @see     Object#hashCode()
152  * @see     Collection
153  * @see     Map
154  * @see     HashMap
155  * @see     TreeMap
156  * @see     Hashtable
157  * @since   1.4
158  */
159 
160 public class LinkedHashMap<K,V>
161     extends HashMap<K,V>
162     implements Map<K,V>
163 {
164 
165     private static final long serialVersionUID = 3801124242820219131L;
166 
167     /**
168      * The head of the doubly linked list.
169      * 双向链表的头
170      */
171     private transient Entry<K,V> header;
172 
173     /**
174      * The iteration ordering method for this linked hash map: <tt>true</tt>
175      * for access-order, <tt>false</tt> for insertion-order.
176      * LinkedHashMap 迭代的排序方法 当为True时，表示按访问顺序排序
177        当为false时，表示按插入顺序排序，默认是按插入顺序排序
178      * @serial
179      */
180     private final boolean accessOrder;
181 
182     /**
183      * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
184      * with the specified initial capacity and load factor.
185      * 构造一个空的按插入顺序排序的LinkedHashMap,并指定初始化容量和负载因子，调用父类的构造器构造
186      * @param  initialCapacity the initial capacity
187      * @param  loadFactor      the load factor
188      * @throws IllegalArgumentException if the initial capacity is negative
189      *         or the load factor is nonpositive
190      */
191     public LinkedHashMap(int initialCapacity, float loadFactor) {
192         super(initialCapacity, loadFactor);
193         accessOrder = false;
194     }
195 
196     /**
197      * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
198      * with the specified initial capacity and a default load factor (0.75).
199      * 构造一个空的按插入顺序排序的LinkedHashMap,并指定初始化容量和默认的负载因子0.75，调用父类的构造器构造
200      * @param  initialCapacity the initial capacity
201      * @throws IllegalArgumentException if the initial capacity is negative
202      */
203     public LinkedHashMap(int initialCapacity) {
204     super(initialCapacity);
205         accessOrder = false;
206     }
207 
208     /**
209      * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
210      * with the default initial capacity (16) and load factor (0.75).
211      构造一个空的按插入顺序排序的LinkedHashMap,并指定默认初始化容量16和默认的负载因子0.75，调用父类的构造器构造
212      */
213     public LinkedHashMap() {
214     super();
215         accessOrder = false;
216     }
217 
218     /**
219      * Constructs an insertion-ordered <tt>LinkedHashMap</tt> instance with
220      * the same mappings as the specified map.  The <tt>LinkedHashMap</tt>
221      * instance is created with a default load factor (0.75) and an initial
222      * capacity sufficient to hold the mappings in the specified map.
223      * 用指定的Map构造一个按插入顺序排序的LinkedHashMap，默认的负载因子0.75和足够容纳给定map的初始化容量
224      * @param  m the map whose mappings are to be placed in this map
225      * @throws NullPointerException if the specified map is null
226      */
227     public LinkedHashMap(Map<? extends K, ? extends V> m) {
228         super(m);
229         accessOrder = false;
230     }
231 
232     /**
233      * Constructs an empty <tt>LinkedHashMap</tt> instance with the
234      * specified initial capacity, load factor and ordering mode.
235      * 构造一个空的按插入顺序排序的LinkedHashMap，并指定初始化容量,负载因子和排序方式
236      * @param  initialCapacity the initial capacity
237      * @param  loadFactor      the load factor
238      * @param  accessOrder     the ordering mode - <tt>true</tt> for
239      *         access-order, <tt>false</tt> for insertion-order
240      * @throws IllegalArgumentException if the initial capacity is negative
241      *         or the load factor is nonpositive
242      */
243     public LinkedHashMap(int initialCapacity,
244              float loadFactor,
245                          boolean accessOrder) {
246         super(initialCapacity, loadFactor);
247         this.accessOrder = accessOrder;
248     }
249 
250     /**
251      * Called by superclass constructors and pseudoconstructors (clone,
252      * readObject) before any entries are inserted into the map.  Initializes
253      * the chain.
254        覆写父类的init方法，在父类中都是空方法，在任何键值对被放入map之前在父类的构造器和伪构造器中调用
255        初始化双向链表，初始化一个空的节点，hashcode的值是-1，并不存放在哈希表中，作为双向循环链表的头部，
256        循环的开始节点或者可以认为也是结束的标志
257      */
258     void init() {
259         header = new Entry<K,V>(-1, null, null, null);
260         header.before = header.after = header;
261     }
262 
263     /**
264      * Transfers all entries to new table array.  This method is called
265      * by superclass resize.  It is overridden for performance, as it is
266      * faster to iterate using our linked list.
267        将所有的键值对复制到新的哈希表中，该方法在父类的扩容被调用
268        该方法被重写是因为他的性能，因为我们用维护的链表遍历速度更快
269      */
270     void transfer(HashMap.Entry[] newTable) {
271         int newCapacity = newTable.length;
272         for (Entry<K,V> e = header.after; e != header; e = e.after) {
273             int index = indexFor(e.hash, newCapacity);
274             e.next = newTable[index];
275             newTable[index] = e;
276         }
277     }
278 
279 
280     /**
281      * Returns <tt>true</tt> if this map maps one or more keys to the
282      * specified value.
283      *
284      * @param value value whose presence in this map is to be tested
285      * @return <tt>true</tt> if this map maps one or more keys to the
286      *         specified value
287      */
288     public boolean containsValue(Object value) {
289         // Overridden to take advantage of faster iterator
290         //重写是为了性能，链表的遍历速度更快
291         if (value==null) {
292             for (Entry e = header.after; e != header; e = e.after)
293                 if (e.value==null)
294                     return true;
295         } else {
296             for (Entry e = header.after; e != header; e = e.after)
297                 if (value.equals(e.value))
298                     return true;
299         }
300         return false;
301     }
302 
303     /**
304      * Returns the value to which the specified key is mapped,
305      * or {@code null} if this map contains no mapping for the key.
306      * 根据指定的Map的Key返回value值，如果没有返回null，也可能是key对于的Value就是null
307      * <p>More formally, if this map contains a mapping from a key
308      * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
309      * key.equals(k))}, then this method returns {@code v}; otherwise
310      * it returns {@code null}.  (There can be at most one such mapping.)
311      * 更正式点说，如果存在这样个map(key==null ? k==null : key.equals(k)
312      * 返回value，否则返回null，最多只有一个这个的键值对
313      * <p>A return value of {@code null} does not <i>necessarily</i>
314      * indicate that the map contains no mapping for the key; it‘s also
315      * possible that the map explicitly maps the key to {@code null}.
316      * The {@link #containsKey containsKey} operation may be used to
317      * distinguish these two cases.
318      并不一定会返回null，如果返回null值，表示不存在指定key的map，或者可能是该key的Value就是null
319      可以通过containsKey操作来判别
320      */
321     public V get(Object key) {
322         Entry<K,V> e = (Entry<K,V>)getEntry(key);
323         if (e == null)
324             return null;
325         e.recordAccess(this);
326         return e.value;
327     }
328 
329     /**
330      * Removes all of the mappings from this map.
331      * The map will be empty after this call returns.
332        删除所有键值对
333      */
334     public void clear() {
335         super.clear();
336         header.before = header.after = header;
337     }
338 
339     /**
340      * LinkedHashMap entry.
341        LinkedHashMap的键值对对象，继承了HashMap的属性hash，key，value，next，并且新增属性 before和after，前四个属性是为了维护哈希表，before和after属性是为了维护双向循环链表
342      */
343     private static class Entry<K,V> extends HashMap.Entry<K,V> {
344         // These fields comprise the doubly linked list used for iteration.
345         //用来对双向循环链表进行遍历
346         Entry<K,V> before, after;
347 
348     Entry(int hash, K key, V value, HashMap.Entry<K,V> next) {
349             super(hash, key, value, next);
350         }
351 
352         /**
353          * Removes this entry from the linked list.
354            从双向链表中移除当前节点
355          */
356         private void remove() {
357             before.after = after;
358             after.before = before;
359         }
360 
361         /**
362          * Inserts this entry before the specified existing entry in the list.
363            在已存在的双向链表中的某个节点之前，插入当前节点
364          */
365         private void addBefore(Entry<K,V> existingEntry) {
366             after  = existingEntry;
367             before = existingEntry.before;
368             before.after = this;
369             after.before = this;
370         }
371 
372         /**
373          * This method is invoked by the superclass whenever the value
374          * of a pre-existing entry is read by Map.get or modified by Map.set.
375            该方法在父类中通过Map.get获取或者通过Map.set修改已经存在的键值对
376          * If the enclosing Map is access-ordered, it moves the entry
377          * to the end of the list; otherwise, it does nothing.
378            如果封装的Map是访问排序的，该方法会将当前的键值对移到链表的结尾，其他情况，什么也不做
379          */
380         void recordAccess(HashMap<K,V> m) {
381             LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m;
382             if (lm.accessOrder) {
383                 lm.modCount++;
384                 remove();
385                 addBefore(lm.header);
386             }
387         }
388          //从链表中移除当前键值对
389         void recordRemoval(HashMap<K,V> m) {
390             remove();
391         }
392     }
393     //实现迭代器，不多说
394     private abstract class LinkedHashIterator<T> implements Iterator<T> {
395     Entry<K,V> nextEntry    = header.after;
396     Entry<K,V> lastReturned = null;
397 
398     /**
399      * The modCount value that the iterator believes that the backing
400      * List should have.  If this expectation is violated, the iterator
401      * has detected concurrent modification.
402      */
403     int expectedModCount = modCount;
404 
405     public boolean hasNext() {
406             return nextEntry != header;
407     }
408 
409     public void remove() {
410         if (lastReturned == null)
411         throw new IllegalStateException();
412         if (modCount != expectedModCount)
413         throw new ConcurrentModificationException();
414 
415             LinkedHashMap.this.remove(lastReturned.key);
416             lastReturned = null;
417             expectedModCount = modCount;
418     }
419 
420     Entry<K,V> nextEntry() {
421         if (modCount != expectedModCount)
422         throw new ConcurrentModificationException();
423             if (nextEntry == header)
424                 throw new NoSuchElementException();
425 
426             Entry<K,V> e = lastReturned = nextEntry;
427             nextEntry = e.after;
428             return e;
429     }
430     }
431 
432     private class KeyIterator extends LinkedHashIterator<K> {
433     public K next() { return nextEntry().getKey(); }
434     }
435 
436     private class ValueIterator extends LinkedHashIterator<V> {
437     public V next() { return nextEntry().value; }
438     }
439 
440     private class EntryIterator extends LinkedHashIterator<Map.Entry<K,V>> {
441     public Map.Entry<K,V> next() { return nextEntry(); }
442     }
443 
444     // These Overrides alter the behavior of superclass view iterator() methods
445     Iterator<K> newKeyIterator()   { return new KeyIterator();   }
446     Iterator<V> newValueIterator() { return new ValueIterator(); }
447     Iterator<Map.Entry<K,V>> newEntryIterator() { return new EntryIterator(); }
448 
449     /**
450      * This override alters behavior of superclass put method. It causes newly
451      * allocated entry to get inserted at the end of the linked list and
452      * removes the eldest entry if appropriate.
453        修改父类的put方法，最近放入的键值对，放在链表的尾部，如果条件允许删除最不经常使用的元素，也就是
454        在访问排序的模式下，链表最前面的那一个元素
455      */
456     void addEntry(int hash, K key, V value, int bucketIndex) {
457         createEntry(hash, key, value, bucketIndex);
458 
459         // Remove eldest entry if instructed, else grow capacity if appropriate
460         //如果被允许，删除最不常用的键值对，否则，就进行扩容
461         Entry<K,V> eldest = header.after;
462         if (removeEldestEntry(eldest)) {
463             removeEntryForKey(eldest.key);
464         } else {
465             if (size >= threshold)
466                 resize(2 * table.length);
467         }
468     }
469 
470     /**
471      * This override differs from addEntry in that it doesn‘t resize the
472      * table or remove the eldest entry.
473        这重写与addEntry，因为不用扩容和删除最不常用的节点
474      */
475     void createEntry(int hash, K key, V value, int bucketIndex) {
476         HashMap.Entry<K,V> old = table[bucketIndex];
477     Entry<K,V> e = new Entry<K,V>(hash, key, value, old);
478         table[bucketIndex] = e;
479         //将新增的键值对放到链表的尾部
480         e.addBefore(header);
481         size++;
482     }
483 
484     /**
485      * Returns <tt>true</tt> if this map should remove its eldest entry.
486      * This method is invoked by <tt>put</tt> and <tt>putAll</tt> after
487      * inserting a new entry into the map.  It provides the implementor
488      * with the opportunity to remove the eldest entry each time a new one
489      * is added.  This is useful if the map represents a cache: it allows
490      * the map to reduce memory consumption by deleting stale entries.
491      * 该方法为了是先LRU（Least Recently Used）缓存
492      * <p>Sample use: this override will allow the map to grow up to 100
493      * entries and then delete the eldest entry each time a new entry is
494      * added, maintaining a steady state of 100 entries.
495      * <pre>
496      *     private static final int MAX_ENTRIES = 100;
497      *
498      *     protected boolean removeEldestEntry(Map.Entry eldest) {
499      *        return size() > MAX_ENTRIES;
500      *     }
501      * </pre>
502      *
503      * <p>This method typically does not modify the map in any way,
504      * instead allowing the map to modify itself as directed by its
505      * return value.  It <i>is</i> permitted for this method to modify
506      * the map directly, but if it does so, it <i>must</i> return
507      * <tt>false</tt> (indicating that the map should not attempt any
508      * further modification).  The effects of returning <tt>true</tt>
509      * after modifying the map from within this method are unspecified.
510      *
511      * <p>This implementation merely returns <tt>false</tt> (so that this
512      * map acts like a normal map - the eldest element is never removed).
513      *
514      * @param    eldest The least recently inserted entry in the map, or if
515      *           this is an access-ordered map, the least recently accessed
516      *           entry.  This is the entry that will be removed it this
517      *           method returns <tt>true</tt>.  If the map was empty prior
518      *           to the <tt>put</tt> or <tt>putAll</tt> invocation resulting
519      *           in this invocation, this will be the entry that was just
520      *           inserted; in other words, if the map contains a single
521      *           entry, the eldest entry is also the newest.
522      * @return   <tt>true</tt> if the eldest entry should be removed
523      *           from the map; <tt>false</tt> if it should be retained.
524      */
525     protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
526         return false;
527     }
528 }