1 /*
2 * Copyright (c) 2000, 2012, 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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7 * published by the Free Software Foundation. Oracle designates this
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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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20 *
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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 <tt>LinkedHashMap</tt> whose order of iteration is the
59 * order in which its entries were last accessed, from least-recently accessed
60 * to 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 @Override
240 void init() {
241 header = new Entry<>(-1, null, null, null);
242 header.before = header.after = header;
243 }
244
245 /**
246 * Returns <tt>true</tt> if this map maps one or more keys to the
247 * specified value.
248 *
249 * @param value value whose presence in this map is to be tested
250 * @return <tt>true</tt> if this map maps one or more keys to the
251 * specified value
252 */
253 public boolean containsValue(Object value) {
254 // Overridden to take advantage of faster iterator
255 if (value==null) {
256 for (Entry<?,?> e = header.after; e != header; e = e.after)
257 if (e.value==null)
258 return true;
259 } else {
260 for (Entry<?,?> e = header.after; e != header; e = e.after)
261 if (value.equals(e.value))
262 return true;
263 }
264 return false;
265 }
266
267 /**
268 * Returns the value to which the specified key is mapped,
269 * or {@code null} if this map contains no mapping for the key.
270 *
271 * <p>More formally, if this map contains a mapping from a key
272 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
273 * key.equals(k))}, then this method returns {@code v}; otherwise
274 * it returns {@code null}. (There can be at most one such mapping.)
275 *
276 * <p>A return value of {@code null} does not <i>necessarily</i>
277 * indicate that the map contains no mapping for the key; it's also
278 * possible that the map explicitly maps the key to {@code null}.
279 * The {@link #containsKey containsKey} operation may be used to
280 * distinguish these two cases.
281 */
282 public V get(Object key) {
283 Entry<K,V> e = (Entry<K,V>)getEntry(key);
284 if (e == null)
285 return null;
286 e.recordAccess(this);
287 return e.value;
288 }
289
290 /**
291 * Removes all of the mappings from this map.
292 * The map will be empty after this call returns.
293 */
294 public void clear() {
295 super.clear();
296 header.before = header.after = header;
297 }
298
299 /**
300 * LinkedHashMap entry.
301 */
302 private static class Entry<K,V> extends HashMap.Entry<K,V> {
303 // These fields comprise the doubly linked list used for iteration.
304 Entry<K,V> before, after;
305
306 Entry(int hash, K key, V value, Object next) {
307 super(hash, key, value, next);
308 }
309
310 /**
311 * Removes this entry from the linked list.
312 */
313 private void remove() {
314 before.after = after;
315 after.before = before;
316 }
317
318 /**
319 * Inserts this entry before the specified existing entry in the list.
320 */
321 private void addBefore(Entry<K,V> existingEntry) {
322 after = existingEntry;
323 before = existingEntry.before;
324 before.after = this;
325 after.before = this;
326 }
327
328 /**
329 * This method is invoked by the superclass whenever the value
330 * of a pre-existing entry is read by Map.get or modified by Map.put.
331 * If the enclosing Map is access-ordered, it moves the entry
332 * to the end of the list; otherwise, it does nothing.
333 */
334 void recordAccess(HashMap<K,V> m) {
335 LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m;
336 if (lm.accessOrder) {
337 lm.modCount++;
338 remove();
339 addBefore(lm.header);
340 }
341 }
342
343 void recordRemoval(HashMap<K,V> m) {
344 remove();
345 }
346 }
347
348 private abstract class LinkedHashIterator<T> implements Iterator<T> {
349 Entry<K,V> nextEntry = header.after;
350 Entry<K,V> lastReturned = null;
351
352 /**
353 * The modCount value that the iterator believes that the backing
354 * List should have. If this expectation is violated, the iterator
355 * has detected concurrent modification.
356 */
357 int expectedModCount = modCount;
358
359 public boolean hasNext() {
360 return nextEntry != header;
361 }
362
363 public void remove() {
364 if (lastReturned == null)
365 throw new IllegalStateException();
366 if (modCount != expectedModCount)
367 throw new ConcurrentModificationException();
368
369 LinkedHashMap.this.remove(lastReturned.key);
370 lastReturned = null;
371 expectedModCount = modCount;
372 }
373
374 Entry<K,V> nextEntry() {
375 if (modCount != expectedModCount)
376 throw new ConcurrentModificationException();
377 if (nextEntry == header)
378 throw new NoSuchElementException();
379
380 Entry<K,V> e = lastReturned = nextEntry;
381 nextEntry = e.after;
382 return e;
383 }
384 }
385
386 private class KeyIterator extends LinkedHashIterator<K> {
387 public K next() { return nextEntry().getKey(); }
388 }
389
390 private class ValueIterator extends LinkedHashIterator<V> {
391 public V next() { return nextEntry().value; }
392 }
393
394 private class EntryIterator extends LinkedHashIterator<Map.Entry<K,V>> {
395 public Map.Entry<K,V> next() { return nextEntry(); }
396 }
397
398 // These Overrides alter the behavior of superclass view iterator() methods
399 Iterator<K> newKeyIterator() { return new KeyIterator(); }
400 Iterator<V> newValueIterator() { return new ValueIterator(); }
401 Iterator<Map.Entry<K,V>> newEntryIterator() { return new EntryIterator(); }
402
403 /**
404 * This override alters behavior of superclass put method. It causes newly
405 * allocated entry to get inserted at the end of the linked list and
406 * removes the eldest entry if appropriate.
407 */
408 @Override
409 void addEntry(int hash, K key, V value, int bucketIndex, boolean checkIfNeedTree) {
410 super.addEntry(hash, key, value, bucketIndex, checkIfNeedTree);
411
412 // Remove eldest entry if instructed
413 Entry<K,V> eldest = header.after;
414 if (removeEldestEntry(eldest)) {
415 removeEntryForKey(eldest.key);
416 }
417 }
418
419 /*
420 * Create a new LinkedHashMap.Entry and setup the before/after pointers
421 */
422 @Override
423 HashMap.Entry<K,V> newEntry(int hash, K key, V value, Object next) {
424 Entry<K,V> newEntry = new Entry<>(hash, key, value, next);
425 newEntry.addBefore(header);
426 return newEntry;
427 }
428
429 /**
430 * Returns <tt>true</tt> if this map should remove its eldest entry.
431 * This method is invoked by <tt>put</tt> and <tt>putAll</tt> after
432 * inserting a new entry into the map. It provides the implementor
433 * with the opportunity to remove the eldest entry each time a new one
434 * is added. This is useful if the map represents a cache: it allows
435 * the map to reduce memory consumption by deleting stale entries.
436 *
437 * <p>Sample use: this override will allow the map to grow up to 100
438 * entries and then delete the eldest entry each time a new entry is
439 * added, maintaining a steady state of 100 entries.
440 * <pre>
441 * private static final int MAX_ENTRIES = 100;
442 *
443 * protected boolean removeEldestEntry(Map.Entry eldest) {
444 * return size() > MAX_ENTRIES;
445 * }
446 * </pre>
447 *
448 * <p>This method typically does not modify the map in any way,
449 * instead allowing the map to modify itself as directed by its
450 * return value. It <i>is</i> permitted for this method to modify
451 * the map directly, but if it does so, it <i>must</i> return
452 * <tt>false</tt> (indicating that the map should not attempt any
453 * further modification). The effects of returning <tt>true</tt>
454 * after modifying the map from within this method are unspecified.
455 *
456 * <p>This implementation merely returns <tt>false</tt> (so that this
457 * map acts like a normal map - the eldest element is never removed).
458 *
459 * @param eldest The least recently inserted entry in the map, or if
460 * this is an access-ordered map, the least recently accessed
461 * entry. This is the entry that will be removed it this
462 * method returns <tt>true</tt>. If the map was empty prior
463 * to the <tt>put</tt> or <tt>putAll</tt> invocation resulting
464 * in this invocation, this will be the entry that was just
465 * inserted; in other words, if the map contains a single
466 * entry, the eldest entry is also the newest.
467 * @return <tt>true</tt> if the eldest entry should be removed
468 * from the map; <tt>false</tt> if it should be retained.
469 */
470 protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
471 return false;
472 }
473 }