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 }
参考链接 http://blog.csdn.net/ns_code/article/details/37867985 https://www.cnblogs.com/xiaoxi/p/6170590.html
时间： 2024-10-25 08:12:49