1 /*
2 * Copyright (c) 2000, 2010, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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).
16 *
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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 }