LinkedHashMap.java revision 7a0fb3b1203df27e0bb3e5b1ee3346851f9af38d
1/* 2 * Copyright (C) 2014 The Android Open Source Project 3 * Copyright (c) 2000, 2010, Oracle and/or its affiliates. All rights reserved. 4 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 5 * 6 * This code is free software; you can redistribute it and/or modify it 7 * under the terms of the GNU General Public License version 2 only, as 8 * published by the Free Software Foundation. Oracle designates this 9 * particular file as subject to the "Classpath" exception as provided 10 * by Oracle in the LICENSE file that accompanied this code. 11 * 12 * This code is distributed in the hope that it will be useful, but WITHOUT 13 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 14 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 15 * version 2 for more details (a copy is included in the LICENSE file that 16 * accompanied this code). 17 * 18 * You should have received a copy of the GNU General Public License version 19 * 2 along with this work; if not, write to the Free Software Foundation, 20 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 21 * 22 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 23 * or visit www.oracle.com if you need additional information or have any 24 * questions. 25 */ 26 27package java.util; 28 29import java.io.*; 30import java.util.function.Consumer; 31import java.util.function.BiConsumer; 32 33/** 34 * <p>Hash table and linked list implementation of the <tt>Map</tt> interface, 35 * with predictable iteration order. This implementation differs from 36 * <tt>HashMap</tt> in that it maintains a doubly-linked list running through 37 * all of its entries. This linked list defines the iteration ordering, 38 * which is normally the order in which keys were inserted into the map 39 * (<i>insertion-order</i>). Note that insertion order is not affected 40 * if a key is <i>re-inserted</i> into the map. (A key <tt>k</tt> is 41 * reinserted into a map <tt>m</tt> if <tt>m.put(k, v)</tt> is invoked when 42 * <tt>m.containsKey(k)</tt> would return <tt>true</tt> immediately prior to 43 * the invocation.) 44 * 45 * <p>This implementation spares its clients from the unspecified, generally 46 * chaotic ordering provided by {@link HashMap} (and {@link Hashtable}), 47 * without incurring the increased cost associated with {@link TreeMap}. It 48 * can be used to produce a copy of a map that has the same order as the 49 * original, regardless of the original map's implementation: 50 * <pre> 51 * void foo(Map m) { 52 * Map copy = new LinkedHashMap(m); 53 * ... 54 * } 55 * </pre> 56 * This technique is particularly useful if a module takes a map on input, 57 * copies it, and later returns results whose order is determined by that of 58 * the copy. (Clients generally appreciate having things returned in the same 59 * order they were presented.) 60 * 61 * <p>A special {@link #LinkedHashMap(int,float,boolean) constructor} is 62 * provided to create a linked hash map whose order of iteration is the order 63 * in which its entries were last accessed, from least-recently accessed to 64 * most-recently (<i>access-order</i>). This kind of map is well-suited to 65 * building LRU caches. Invoking the {@code put}, {@code putIfAbsent}, 66 * {@code get}, {@code getOrDefault}, {@code compute}, {@code computeIfAbsent}, 67 * {@code computeIfPresent}, or {@code merge} methods results 68 * in an access to the corresponding entry (assuming it exists after the 69 * invocation completes). The {@code replace} methods only result in an access 70 * of the entry if the value is replaced. The {@code putAll} method generates one 71 * entry access for each mapping in the specified map, in the order that 72 * key-value mappings are provided by the specified map's entry set iterator. 73 * <i>No other methods generate entry accesses.</i> In particular, operations 74 * on collection-views do <i>not</i> affect the order of iteration of the 75 * backing map. * 76 * <p>The {@link #removeEldestEntry(Map.Entry)} method may be overridden to 77 * impose a policy for removing stale mappings automatically when new mappings 78 * are added to the map. 79 * 80 * <p>This class provides all of the optional <tt>Map</tt> operations, and 81 * permits null elements. Like <tt>HashMap</tt>, it provides constant-time 82 * performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and 83 * <tt>remove</tt>), assuming the hash function disperses elements 84 * properly among the buckets. Performance is likely to be just slightly 85 * below that of <tt>HashMap</tt>, due to the added expense of maintaining the 86 * linked list, with one exception: Iteration over the collection-views 87 * of a <tt>LinkedHashMap</tt> requires time proportional to the <i>size</i> 88 * of the map, regardless of its capacity. Iteration over a <tt>HashMap</tt> 89 * is likely to be more expensive, requiring time proportional to its 90 * <i>capacity</i>. 91 * 92 * <p>A linked hash map has two parameters that affect its performance: 93 * <i>initial capacity</i> and <i>load factor</i>. They are defined precisely 94 * as for <tt>HashMap</tt>. Note, however, that the penalty for choosing an 95 * excessively high value for initial capacity is less severe for this class 96 * than for <tt>HashMap</tt>, as iteration times for this class are unaffected 97 * by capacity. 98 * 99 * <p><strong>Note that this implementation is not synchronized.</strong> 100 * If multiple threads access a linked hash map concurrently, and at least 101 * one of the threads modifies the map structurally, it <em>must</em> be 102 * synchronized externally. This is typically accomplished by 103 * synchronizing on some object that naturally encapsulates the map. 104 * 105 * If no such object exists, the map should be "wrapped" using the 106 * {@link Collections#synchronizedMap Collections.synchronizedMap} 107 * method. This is best done at creation time, to prevent accidental 108 * unsynchronized access to the map:<pre> 109 * Map m = Collections.synchronizedMap(new LinkedHashMap(...));</pre> 110 * 111 * A structural modification is any operation that adds or deletes one or more 112 * mappings or, in the case of access-ordered linked hash maps, affects 113 * iteration order. In insertion-ordered linked hash maps, merely changing 114 * the value associated with a key that is already contained in the map is not 115 * a structural modification. <strong>In access-ordered linked hash maps, 116 * merely querying the map with <tt>get</tt> is a structural 117 * modification.</strong>) 118 * 119 * <p>The iterators returned by the <tt>iterator</tt> method of the collections 120 * returned by all of this class's collection view methods are 121 * <em>fail-fast</em>: if the map is structurally modified at any time after 122 * the iterator is created, in any way except through the iterator's own 123 * <tt>remove</tt> method, the iterator will throw a {@link 124 * ConcurrentModificationException}. Thus, in the face of concurrent 125 * modification, the iterator fails quickly and cleanly, rather than risking 126 * arbitrary, non-deterministic behavior at an undetermined time in the future. 127 * 128 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed 129 * as it is, generally speaking, impossible to make any hard guarantees in the 130 * presence of unsynchronized concurrent modification. Fail-fast iterators 131 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis. 132 * Therefore, it would be wrong to write a program that depended on this 133 * exception for its correctness: <i>the fail-fast behavior of iterators 134 * should be used only to detect bugs.</i> 135 * 136 * <p>The spliterators returned by the spliterator method of the collections 137 * returned by all of this class's collection view methods are 138 * <em><a href="Spliterator.html#binding">late-binding</a></em>, 139 * <em>fail-fast</em>, and additionally report {@link Spliterator#ORDERED}. 140 * 141 * <p>This class is a member of the 142 * <a href="{@docRoot}/../technotes/guides/collections/index.html"> 143 * Java Collections Framework</a>. 144 * 145 * @implNote 146 * The spliterators returned by the spliterator method of the collections 147 * returned by all of this class's collection view methods are created from 148 * the iterators of the corresponding collections. 149 * 150 * @param <K> the type of keys maintained by this map 151 * @param <V> the type of mapped values 152 * 153 * @author Josh Bloch 154 * @see Object#hashCode() 155 * @see Collection 156 * @see Map 157 * @see HashMap 158 * @see TreeMap 159 * @see Hashtable 160 * @since 1.4 161 */ 162 163public class LinkedHashMap<K,V> 164 extends HashMap<K,V> 165 implements Map<K,V> 166{ 167 168 private static final long serialVersionUID = 3801124242820219131L; 169 170 /** 171 * The head of the doubly linked list. 172 */ 173 private transient LinkedHashMapEntry<K,V> header; 174 175 /** 176 * The iteration ordering method for this linked hash map: <tt>true</tt> 177 * for access-order, <tt>false</tt> for insertion-order. 178 * 179 * @serial 180 */ 181 private final boolean accessOrder; 182 183 /** 184 * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance 185 * with the specified initial capacity and load factor. 186 * 187 * @param initialCapacity the initial capacity 188 * @param loadFactor the load factor 189 * @throws IllegalArgumentException if the initial capacity is negative 190 * or the load factor is nonpositive 191 */ 192 public LinkedHashMap(int initialCapacity, float loadFactor) { 193 super(initialCapacity, loadFactor); 194 accessOrder = false; 195 } 196 197 /** 198 * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance 199 * with the specified initial capacity and a default load factor (0.75). 200 * 201 * @param initialCapacity the initial capacity 202 * @throws IllegalArgumentException if the initial capacity is negative 203 */ 204 public LinkedHashMap(int initialCapacity) { 205 super(initialCapacity); 206 accessOrder = false; 207 } 208 209 /** 210 * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance 211 * with the default initial capacity (16) and load factor (0.75). 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 * 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 * 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 */ 255 @Override 256 void init() { 257 header = new LinkedHashMapEntry<>(-1, null, null, null); 258 header.before = header.after = header; 259 } 260 261 /** 262 * Transfers all entries to new table array. This method is called 263 * by superclass resize. It is overridden for performance, as it is 264 * faster to iterate using our linked list. 265 */ 266 @Override 267 void transfer(HashMapEntry[] newTable, boolean rehash) { 268 int newCapacity = newTable.length; 269 for (LinkedHashMapEntry<K,V> e = header.after; e != header; e = e.after) { 270 if (rehash) 271 e.hash = (e.key == null) ? 0 : hash(e.key); 272 int index = indexFor(e.hash, newCapacity); 273 e.next = newTable[index]; 274 newTable[index] = e; 275 } 276 } 277 278 279 /** 280 * Returns <tt>true</tt> if this map maps one or more keys to the 281 * specified value. 282 * 283 * @param value value whose presence in this map is to be tested 284 * @return <tt>true</tt> if this map maps one or more keys to the 285 * specified value 286 */ 287 public boolean containsValue(Object value) { 288 // Overridden to take advantage of faster iterator 289 if (value==null) { 290 for (LinkedHashMapEntry e = header.after; e != header; e = e.after) 291 if (e.value==null) 292 return true; 293 } else { 294 for (LinkedHashMapEntry e = header.after; e != header; e = e.after) 295 if (value.equals(e.value)) 296 return true; 297 } 298 return false; 299 } 300 301 /** 302 * Returns the value to which the specified key is mapped, 303 * or {@code null} if this map contains no mapping for the key. 304 * 305 * <p>More formally, if this map contains a mapping from a key 306 * {@code k} to a value {@code v} such that {@code (key==null ? k==null : 307 * key.equals(k))}, then this method returns {@code v}; otherwise 308 * it returns {@code null}. (There can be at most one such mapping.) 309 * 310 * <p>A return value of {@code null} does not <i>necessarily</i> 311 * indicate that the map contains no mapping for the key; it's also 312 * possible that the map explicitly maps the key to {@code null}. 313 * The {@link #containsKey containsKey} operation may be used to 314 * distinguish these two cases. 315 */ 316 public V get(Object key) { 317 LinkedHashMapEntry<K,V> e = (LinkedHashMapEntry<K,V>)getEntry(key); 318 if (e == null) 319 return null; 320 e.recordAccess(this); 321 return e.value; 322 } 323 324 /** 325 * Removes all of the mappings from this map. 326 * The map will be empty after this call returns. 327 */ 328 public void clear() { 329 super.clear(); 330 header.before = header.after = header; 331 } 332 333 /** 334 * LinkedHashMap entry. 335 */ 336 private static class LinkedHashMapEntry<K,V> extends HashMapEntry<K,V> { 337 // These fields comprise the doubly linked list used for iteration. 338 LinkedHashMapEntry<K,V> before, after; 339 340 LinkedHashMapEntry(int hash, K key, V value, HashMapEntry<K,V> next) { 341 super(hash, key, value, next); 342 } 343 344 /** 345 * Removes this entry from the linked list. 346 */ 347 private void remove() { 348 before.after = after; 349 after.before = before; 350 } 351 352 /** 353 * Inserts this entry before the specified existing entry in the list. 354 */ 355 private void addBefore(LinkedHashMapEntry<K,V> existingEntry) { 356 after = existingEntry; 357 before = existingEntry.before; 358 before.after = this; 359 after.before = this; 360 } 361 362 /** 363 * This method is invoked by the superclass whenever the value 364 * of a pre-existing entry is read by Map.get or modified by Map.set. 365 * If the enclosing Map is access-ordered, it moves the entry 366 * to the end of the list; otherwise, it does nothing. 367 */ 368 void recordAccess(HashMap<K,V> m) { 369 LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m; 370 if (lm.accessOrder) { 371 lm.modCount++; 372 remove(); 373 addBefore(lm.header); 374 } 375 } 376 377 void recordRemoval(HashMap<K,V> m) { 378 remove(); 379 } 380 } 381 382 private abstract class LinkedHashIterator<T> implements Iterator<T> { 383 LinkedHashMapEntry<K,V> nextEntry = header.after; 384 LinkedHashMapEntry<K,V> lastReturned = null; 385 386 /** 387 * The modCount value that the iterator believes that the backing 388 * List should have. If this expectation is violated, the iterator 389 * has detected concurrent modification. 390 */ 391 int expectedModCount = modCount; 392 393 public boolean hasNext() { 394 return nextEntry != header; 395 } 396 397 public void remove() { 398 if (lastReturned == null) 399 throw new IllegalStateException(); 400 if (modCount != expectedModCount) 401 throw new ConcurrentModificationException(); 402 403 LinkedHashMap.this.remove(lastReturned.key); 404 lastReturned = null; 405 expectedModCount = modCount; 406 } 407 408 Entry<K,V> nextEntry() { 409 if (modCount != expectedModCount) 410 throw new ConcurrentModificationException(); 411 if (nextEntry == header) 412 throw new NoSuchElementException(); 413 414 LinkedHashMapEntry<K,V> e = lastReturned = nextEntry; 415 nextEntry = e.after; 416 return e; 417 } 418 } 419 420 private class KeyIterator extends LinkedHashIterator<K> { 421 public K next() { return nextEntry().getKey(); } 422 } 423 424 private class ValueIterator extends LinkedHashIterator<V> { 425 public V next() { return nextEntry().getValue(); } 426 } 427 428 private class EntryIterator extends LinkedHashIterator<Map.Entry<K,V>> { 429 public Map.Entry<K,V> next() { return nextEntry(); } 430 } 431 432 // These Overrides alter the behavior of superclass view iterator() methods 433 Iterator<K> newKeyIterator() { return new KeyIterator(); } 434 Iterator<V> newValueIterator() { return new ValueIterator(); } 435 Iterator<Map.Entry<K,V>> newEntryIterator() { return new EntryIterator(); } 436 437 /** 438 * This override alters behavior of superclass put method. It causes newly 439 * allocated entry to get inserted at the end of the linked list and 440 * removes the eldest entry if appropriate. 441 */ 442 void addEntry(int hash, K key, V value, int bucketIndex) { 443 // Previous Android releases called removeEldestEntry() before actually 444 // inserting a value but after increasing the size. 445 // The RI is documented to call it afterwards. 446 // **** THIS CHANGE WILL BE REVERTED IN A FUTURE ANDROID RELEASE **** 447 448 // Remove eldest entry if instructed 449 LinkedHashMapEntry<K,V> eldest = header.after; 450 if (eldest != header) { 451 boolean removeEldest; 452 size++; 453 try { 454 removeEldest = removeEldestEntry(eldest); 455 } finally { 456 size--; 457 } 458 if (removeEldest) { 459 removeEntryForKey(eldest.key); 460 } 461 } 462 463 super.addEntry(hash, key, value, bucketIndex); 464 } 465 466 /** 467 * Returns the eldest entry in the map, or {@code null} if the map is empty. 468 * 469 * Android-added. 470 * 471 * @hide 472 */ 473 public Map.Entry<K, V> eldest() { 474 Entry<K, V> eldest = header.after; 475 return eldest != header ? eldest : null; 476 } 477 478 /** 479 * This override differs from addEntry in that it doesn't resize the 480 * table or remove the eldest entry. 481 */ 482 void createEntry(int hash, K key, V value, int bucketIndex) { 483 HashMapEntry<K,V> old = table[bucketIndex]; 484 LinkedHashMapEntry<K,V> e = new LinkedHashMapEntry<>(hash, key, value, old); 485 table[bucketIndex] = e; 486 e.addBefore(header); 487 size++; 488 } 489 490 // Intentionally make this not JavaDoc, as the we don't conform to 491 // the behaviour documented here (we call removeEldestEntry before 492 // inserting the new value to be consistent with previous Android 493 // releases). 494 // **** THIS CHANGE WILL BE REVERTED IN A FUTURE ANDROID RELEASE **** 495 /* 496 * Returns <tt>true</tt> if this map should remove its eldest entry. 497 * This method is invoked by <tt>put</tt> and <tt>putAll</tt> after 498 * inserting a new entry into the map. It provides the implementor 499 * with the opportunity to remove the eldest entry each time a new one 500 * is added. This is useful if the map represents a cache: it allows 501 * the map to reduce memory consumption by deleting stale entries. 502 * 503 * <p>Sample use: this override will allow the map to grow up to 100 504 * entries and then delete the eldest entry each time a new entry is 505 * added, maintaining a steady state of 100 entries. 506 * <pre> 507 * private static final int MAX_ENTRIES = 100; 508 * 509 * protected boolean removeEldestEntry(Map.Entry eldest) { 510 * return size() > MAX_ENTRIES; 511 * } 512 * </pre> 513 * 514 * <p>This method typically does not modify the map in any way, 515 * instead allowing the map to modify itself as directed by its 516 * return value. It <i>is</i> permitted for this method to modify 517 * the map directly, but if it does so, it <i>must</i> return 518 * <tt>false</tt> (indicating that the map should not attempt any 519 * further modification). The effects of returning <tt>true</tt> 520 * after modifying the map from within this method are unspecified. 521 * 522 * <p>This implementation merely returns <tt>false</tt> (so that this 523 * map acts like a normal map - the eldest element is never removed). 524 * 525 * @param eldest The least recently inserted entry in the map, or if 526 * this is an access-ordered map, the least recently accessed 527 * entry. This is the entry that will be removed it this 528 * method returns <tt>true</tt>. If the map was empty prior 529 * to the <tt>put</tt> or <tt>putAll</tt> invocation resulting 530 * in this invocation, this will be the entry that was just 531 * inserted; in other words, if the map contains a single 532 * entry, the eldest entry is also the newest. 533 * @return <tt>true</tt> if the eldest entry should be removed 534 * from the map; <tt>false</tt> if it should be retained. 535 */ 536 protected boolean removeEldestEntry(Map.Entry<K,V> eldest) { 537 return false; 538 } 539 540 // Map overrides 541 public void forEach(BiConsumer<? super K, ? super V> action) { 542 if (action == null) 543 throw new NullPointerException(); 544 int mc = modCount; 545 // Android modified - breaks from the loop when modCount != mc 546 for (LinkedHashMapEntry<K,V> e = header.after; modCount == mc && e != header; e = e.after) 547 action.accept(e.key, e.value); 548 if (modCount != mc) 549 throw new ConcurrentModificationException(); 550 } 551} 552