LinkedHashMap.java revision 135ea6cd4f7ebe2f225351a1928d8a62bce5cb00
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.BiFunction; 31import java.util.function.Consumer; 32import java.util.function.BiConsumer; 33 34/** 35 * <p>Hash table and linked list implementation of the <tt>Map</tt> interface, 36 * with predictable iteration order. This implementation differs from 37 * <tt>HashMap</tt> in that it maintains a doubly-linked list running through 38 * all of its entries. This linked list defines the iteration ordering, 39 * which is normally the order in which keys were inserted into the map 40 * (<i>insertion-order</i>). Note that insertion order is not affected 41 * if a key is <i>re-inserted</i> into the map. (A key <tt>k</tt> is 42 * reinserted into a map <tt>m</tt> if <tt>m.put(k, v)</tt> is invoked when 43 * <tt>m.containsKey(k)</tt> would return <tt>true</tt> immediately prior to 44 * the invocation.) 45 * 46 * <p>This implementation spares its clients from the unspecified, generally 47 * chaotic ordering provided by {@link HashMap} (and {@link Hashtable}), 48 * without incurring the increased cost associated with {@link TreeMap}. It 49 * can be used to produce a copy of a map that has the same order as the 50 * original, regardless of the original map's implementation: 51 * <pre> 52 * void foo(Map m) { 53 * Map copy = new LinkedHashMap(m); 54 * ... 55 * } 56 * </pre> 57 * This technique is particularly useful if a module takes a map on input, 58 * copies it, and later returns results whose order is determined by that of 59 * the copy. (Clients generally appreciate having things returned in the same 60 * order they were presented.) 61 * 62 * <p>A special {@link #LinkedHashMap(int,float,boolean) constructor} is 63 * provided to create a linked hash map whose order of iteration is the order 64 * in which its entries were last accessed, from least-recently accessed to 65 * most-recently (<i>access-order</i>). This kind of map is well-suited to 66 * building LRU caches. Invoking the {@code put}, {@code putIfAbsent}, 67 * {@code get}, {@code getOrDefault}, {@code compute}, {@code computeIfAbsent}, 68 * {@code computeIfPresent}, or {@code merge} methods results 69 * in an access to the corresponding entry (assuming it exists after the 70 * invocation completes). The {@code replace} methods only result in an access 71 * of the entry if the value is replaced. The {@code putAll} method generates one 72 * entry access for each mapping in the specified map, in the order that 73 * key-value mappings are provided by the specified map's entry set iterator. 74 * <i>No other methods generate entry accesses.</i> In particular, operations 75 * on collection-views do <i>not</i> affect the order of iteration of the 76 * backing map. * 77 * <p>The {@link #removeEldestEntry(Map.Entry)} method may be overridden to 78 * impose a policy for removing stale mappings automatically when new mappings 79 * are added to the map. 80 * 81 * <p>This class provides all of the optional <tt>Map</tt> operations, and 82 * permits null elements. Like <tt>HashMap</tt>, it provides constant-time 83 * performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and 84 * <tt>remove</tt>), assuming the hash function disperses elements 85 * properly among the buckets. Performance is likely to be just slightly 86 * below that of <tt>HashMap</tt>, due to the added expense of maintaining the 87 * linked list, with one exception: Iteration over the collection-views 88 * of a <tt>LinkedHashMap</tt> requires time proportional to the <i>size</i> 89 * of the map, regardless of its capacity. Iteration over a <tt>HashMap</tt> 90 * is likely to be more expensive, requiring time proportional to its 91 * <i>capacity</i>. 92 * 93 * <p>A linked hash map has two parameters that affect its performance: 94 * <i>initial capacity</i> and <i>load factor</i>. They are defined precisely 95 * as for <tt>HashMap</tt>. Note, however, that the penalty for choosing an 96 * excessively high value for initial capacity is less severe for this class 97 * than for <tt>HashMap</tt>, as iteration times for this class are unaffected 98 * by capacity. 99 * 100 * <p><strong>Note that this implementation is not synchronized.</strong> 101 * If multiple threads access a linked hash map concurrently, and at least 102 * one of the threads modifies the map structurally, it <em>must</em> be 103 * synchronized externally. This is typically accomplished by 104 * synchronizing on some object that naturally encapsulates the map. 105 * 106 * If no such object exists, the map should be "wrapped" using the 107 * {@link Collections#synchronizedMap Collections.synchronizedMap} 108 * method. This is best done at creation time, to prevent accidental 109 * unsynchronized access to the map:<pre> 110 * Map m = Collections.synchronizedMap(new LinkedHashMap(...));</pre> 111 * 112 * A structural modification is any operation that adds or deletes one or more 113 * mappings or, in the case of access-ordered linked hash maps, affects 114 * iteration order. In insertion-ordered linked hash maps, merely changing 115 * the value associated with a key that is already contained in the map is not 116 * a structural modification. <strong>In access-ordered linked hash maps, 117 * merely querying the map with <tt>get</tt> is a structural 118 * modification.</strong>) 119 * 120 * <p>The iterators returned by the <tt>iterator</tt> method of the collections 121 * returned by all of this class's collection view methods are 122 * <em>fail-fast</em>: if the map is structurally modified at any time after 123 * the iterator is created, in any way except through the iterator's own 124 * <tt>remove</tt> method, the iterator will throw a {@link 125 * ConcurrentModificationException}. Thus, in the face of concurrent 126 * modification, the iterator fails quickly and cleanly, rather than risking 127 * arbitrary, non-deterministic behavior at an undetermined time in the future. 128 * 129 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed 130 * as it is, generally speaking, impossible to make any hard guarantees in the 131 * presence of unsynchronized concurrent modification. Fail-fast iterators 132 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis. 133 * Therefore, it would be wrong to write a program that depended on this 134 * exception for its correctness: <i>the fail-fast behavior of iterators 135 * should be used only to detect bugs.</i> 136 * 137 * <p>The spliterators returned by the spliterator method of the collections 138 * returned by all of this class's collection view methods are 139 * <em><a href="Spliterator.html#binding">late-binding</a></em>, 140 * <em>fail-fast</em>, and additionally report {@link Spliterator#ORDERED}. 141 * 142 * <p>This class is a member of the 143 * <a href="{@docRoot}/../technotes/guides/collections/index.html"> 144 * Java Collections Framework</a>. 145 * 146 * @implNote 147 * The spliterators returned by the spliterator method of the collections 148 * returned by all of this class's collection view methods are created from 149 * the iterators of the corresponding collections. 150 * 151 * @param <K> the type of keys maintained by this map 152 * @param <V> the type of mapped values 153 * 154 * @author Josh Bloch 155 * @see Object#hashCode() 156 * @see Collection 157 * @see Map 158 * @see HashMap 159 * @see TreeMap 160 * @see Hashtable 161 * @since 1.4 162 */ 163 164public class LinkedHashMap<K,V> 165 extends HashMap<K,V> 166 implements Map<K,V> 167{ 168 169 private static final long serialVersionUID = 3801124242820219131L; 170 171 /** 172 * The head of the doubly linked list. 173 */ 174 private transient LinkedHashMapEntry<K,V> header; 175 176 /** 177 * The iteration ordering method for this linked hash map: <tt>true</tt> 178 * for access-order, <tt>false</tt> for insertion-order. 179 * 180 * @serial 181 */ 182 private final boolean accessOrder; 183 184 /** 185 * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance 186 * with the specified initial capacity and load factor. 187 * 188 * @param initialCapacity the initial capacity 189 * @param loadFactor the load factor 190 * @throws IllegalArgumentException if the initial capacity is negative 191 * or the load factor is nonpositive 192 */ 193 public LinkedHashMap(int initialCapacity, float loadFactor) { 194 super(initialCapacity, loadFactor); 195 accessOrder = false; 196 } 197 198 /** 199 * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance 200 * with the specified initial capacity and a default load factor (0.75). 201 * 202 * @param initialCapacity the initial capacity 203 * @throws IllegalArgumentException if the initial capacity is negative 204 */ 205 public LinkedHashMap(int initialCapacity) { 206 super(initialCapacity); 207 accessOrder = false; 208 } 209 210 /** 211 * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance 212 * with the default initial capacity (16) and load factor (0.75). 213 */ 214 public LinkedHashMap() { 215 super(); 216 accessOrder = false; 217 } 218 219 /** 220 * Constructs an insertion-ordered <tt>LinkedHashMap</tt> instance with 221 * the same mappings as the specified map. The <tt>LinkedHashMap</tt> 222 * instance is created with a default load factor (0.75) and an initial 223 * capacity sufficient to hold the mappings in the specified map. 224 * 225 * @param m the map whose mappings are to be placed in this map 226 * @throws NullPointerException if the specified map is null 227 */ 228 public LinkedHashMap(Map<? extends K, ? extends V> m) { 229 super(m); 230 accessOrder = false; 231 } 232 233 /** 234 * Constructs an empty <tt>LinkedHashMap</tt> instance with the 235 * specified initial capacity, load factor and ordering mode. 236 * 237 * @param initialCapacity the initial capacity 238 * @param loadFactor the load factor 239 * @param accessOrder the ordering mode - <tt>true</tt> for 240 * access-order, <tt>false</tt> for insertion-order 241 * @throws IllegalArgumentException if the initial capacity is negative 242 * or the load factor is nonpositive 243 */ 244 public LinkedHashMap(int initialCapacity, 245 float loadFactor, 246 boolean accessOrder) { 247 super(initialCapacity, loadFactor); 248 this.accessOrder = accessOrder; 249 } 250 251 /** 252 * Called by superclass constructors and pseudoconstructors (clone, 253 * readObject) before any entries are inserted into the map. Initializes 254 * the chain. 255 */ 256 @Override 257 void init() { 258 header = new LinkedHashMapEntry<>(-1, null, null, null); 259 header.before = header.after = header; 260 } 261 262 /** 263 * Transfers all entries to new table array. This method is called 264 * by superclass resize. It is overridden for performance, as it is 265 * faster to iterate using our linked list. 266 */ 267 @Override 268 void transfer(HashMapEntry[] newTable, boolean rehash) { 269 int newCapacity = newTable.length; 270 for (LinkedHashMapEntry<K,V> e = header.after; e != header; e = e.after) { 271 if (rehash) 272 e.hash = (e.key == null) ? 0 : hash(e.key); 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 if (value==null) { 291 for (LinkedHashMapEntry e = header.after; e != header; e = e.after) 292 if (e.value==null) 293 return true; 294 } else { 295 for (LinkedHashMapEntry e = header.after; e != header; e = e.after) 296 if (value.equals(e.value)) 297 return true; 298 } 299 return false; 300 } 301 302 /** 303 * Returns the value to which the specified key is mapped, 304 * or {@code null} if this map contains no mapping for the key. 305 * 306 * <p>More formally, if this map contains a mapping from a key 307 * {@code k} to a value {@code v} such that {@code (key==null ? k==null : 308 * key.equals(k))}, then this method returns {@code v}; otherwise 309 * it returns {@code null}. (There can be at most one such mapping.) 310 * 311 * <p>A return value of {@code null} does not <i>necessarily</i> 312 * indicate that the map contains no mapping for the key; it's also 313 * possible that the map explicitly maps the key to {@code null}. 314 * The {@link #containsKey containsKey} operation may be used to 315 * distinguish these two cases. 316 */ 317 public V get(Object key) { 318 LinkedHashMapEntry<K,V> e = (LinkedHashMapEntry<K,V>)getEntry(key); 319 if (e == null) 320 return null; 321 e.recordAccess(this); 322 return e.value; 323 } 324 325 /** 326 * Removes all of the mappings from this map. 327 * The map will be empty after this call returns. 328 */ 329 public void clear() { 330 super.clear(); 331 header.before = header.after = header; 332 } 333 334 /** 335 * LinkedHashMap entry. 336 */ 337 private static class LinkedHashMapEntry<K,V> extends HashMapEntry<K,V> { 338 // These fields comprise the doubly linked list used for iteration. 339 LinkedHashMapEntry<K,V> before, after; 340 341 LinkedHashMapEntry(int hash, K key, V value, HashMapEntry<K,V> next) { 342 super(hash, key, value, next); 343 } 344 345 /** 346 * Removes this entry from the linked list. 347 */ 348 private void remove() { 349 before.after = after; 350 after.before = before; 351 } 352 353 /** 354 * Inserts this entry before the specified existing entry in the list. 355 */ 356 private void addBefore(LinkedHashMapEntry<K,V> existingEntry) { 357 after = existingEntry; 358 before = existingEntry.before; 359 before.after = this; 360 after.before = this; 361 } 362 363 /** 364 * This method is invoked by the superclass whenever the value 365 * of a pre-existing entry is read by Map.get or modified by Map.set. 366 * If the enclosing Map is access-ordered, it moves the entry 367 * to the end of the list; otherwise, it does nothing. 368 */ 369 void recordAccess(HashMap<K,V> m) { 370 LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m; 371 if (lm.accessOrder) { 372 lm.modCount++; 373 remove(); 374 addBefore(lm.header); 375 } 376 } 377 378 void recordRemoval(HashMap<K,V> m) { 379 remove(); 380 } 381 } 382 383 private abstract class LinkedHashIterator<T> implements Iterator<T> { 384 LinkedHashMapEntry<K,V> nextEntry = header.after; 385 LinkedHashMapEntry<K,V> lastReturned = null; 386 387 /** 388 * The modCount value that the iterator believes that the backing 389 * List should have. If this expectation is violated, the iterator 390 * has detected concurrent modification. 391 */ 392 int expectedModCount = modCount; 393 394 public boolean hasNext() { 395 return nextEntry != header; 396 } 397 398 public void remove() { 399 if (lastReturned == null) 400 throw new IllegalStateException(); 401 if (modCount != expectedModCount) 402 throw new ConcurrentModificationException(); 403 404 LinkedHashMap.this.remove(lastReturned.key); 405 lastReturned = null; 406 expectedModCount = modCount; 407 } 408 409 Entry<K,V> nextEntry() { 410 if (modCount != expectedModCount) 411 throw new ConcurrentModificationException(); 412 if (nextEntry == header) 413 throw new NoSuchElementException(); 414 415 LinkedHashMapEntry<K,V> e = lastReturned = nextEntry; 416 nextEntry = e.after; 417 return e; 418 } 419 } 420 421 private class KeyIterator extends LinkedHashIterator<K> { 422 public K next() { return nextEntry().getKey(); } 423 } 424 425 private class ValueIterator extends LinkedHashIterator<V> { 426 public V next() { return nextEntry().getValue(); } 427 } 428 429 private class EntryIterator extends LinkedHashIterator<Map.Entry<K,V>> { 430 public Map.Entry<K,V> next() { return nextEntry(); } 431 } 432 433 // These Overrides alter the behavior of superclass view iterator() methods 434 Iterator<K> newKeyIterator() { return new KeyIterator(); } 435 Iterator<V> newValueIterator() { return new ValueIterator(); } 436 Iterator<Map.Entry<K,V>> newEntryIterator() { return new EntryIterator(); } 437 438 /** 439 * This override alters behavior of superclass put method. It causes newly 440 * allocated entry to get inserted at the end of the linked list and 441 * removes the eldest entry if appropriate. 442 */ 443 void addEntry(int hash, K key, V value, int bucketIndex) { 444 // Previous Android releases called removeEldestEntry() before actually 445 // inserting a value but after increasing the size. 446 // The RI is documented to call it afterwards. 447 // **** THIS CHANGE WILL BE REVERTED IN A FUTURE ANDROID RELEASE **** 448 449 // Remove eldest entry if instructed 450 LinkedHashMapEntry<K,V> eldest = header.after; 451 if (eldest != header) { 452 boolean removeEldest; 453 size++; 454 try { 455 removeEldest = removeEldestEntry(eldest); 456 } finally { 457 size--; 458 } 459 if (removeEldest) { 460 removeEntryForKey(eldest.key); 461 } 462 } 463 464 super.addEntry(hash, key, value, bucketIndex); 465 } 466 467 /** 468 * Returns the eldest entry in the map, or {@code null} if the map is empty. 469 * 470 * Android-added. 471 * 472 * @hide 473 */ 474 public Map.Entry<K, V> eldest() { 475 Entry<K, V> eldest = header.after; 476 return eldest != header ? eldest : null; 477 } 478 479 /** 480 * This override differs from addEntry in that it doesn't resize the 481 * table or remove the eldest entry. 482 */ 483 void createEntry(int hash, K key, V value, int bucketIndex) { 484 HashMapEntry<K,V> old = table[bucketIndex]; 485 LinkedHashMapEntry<K,V> e = new LinkedHashMapEntry<>(hash, key, value, old); 486 table[bucketIndex] = e; 487 e.addBefore(header); 488 size++; 489 } 490 491 // Intentionally make this not JavaDoc, as the we don't conform to 492 // the behaviour documented here (we call removeEldestEntry before 493 // inserting the new value to be consistent with previous Android 494 // releases). 495 // **** THIS CHANGE WILL BE REVERTED IN A FUTURE ANDROID RELEASE **** 496 /* 497 * Returns <tt>true</tt> if this map should remove its eldest entry. 498 * This method is invoked by <tt>put</tt> and <tt>putAll</tt> after 499 * inserting a new entry into the map. It provides the implementor 500 * with the opportunity to remove the eldest entry each time a new one 501 * is added. This is useful if the map represents a cache: it allows 502 * the map to reduce memory consumption by deleting stale entries. 503 * 504 * <p>Sample use: this override will allow the map to grow up to 100 505 * entries and then delete the eldest entry each time a new entry is 506 * added, maintaining a steady state of 100 entries. 507 * <pre> 508 * private static final int MAX_ENTRIES = 100; 509 * 510 * protected boolean removeEldestEntry(Map.Entry eldest) { 511 * return size() > MAX_ENTRIES; 512 * } 513 * </pre> 514 * 515 * <p>This method typically does not modify the map in any way, 516 * instead allowing the map to modify itself as directed by its 517 * return value. It <i>is</i> permitted for this method to modify 518 * the map directly, but if it does so, it <i>must</i> return 519 * <tt>false</tt> (indicating that the map should not attempt any 520 * further modification). The effects of returning <tt>true</tt> 521 * after modifying the map from within this method are unspecified. 522 * 523 * <p>This implementation merely returns <tt>false</tt> (so that this 524 * map acts like a normal map - the eldest element is never removed). 525 * 526 * @param eldest The least recently inserted entry in the map, or if 527 * this is an access-ordered map, the least recently accessed 528 * entry. This is the entry that will be removed it this 529 * method returns <tt>true</tt>. If the map was empty prior 530 * to the <tt>put</tt> or <tt>putAll</tt> invocation resulting 531 * in this invocation, this will be the entry that was just 532 * inserted; in other words, if the map contains a single 533 * entry, the eldest entry is also the newest. 534 * @return <tt>true</tt> if the eldest entry should be removed 535 * from the map; <tt>false</tt> if it should be retained. 536 */ 537 protected boolean removeEldestEntry(Map.Entry<K,V> eldest) { 538 return false; 539 } 540 541 // Map overrides 542 public void forEach(BiConsumer<? super K, ? super V> action) { 543 if (action == null) 544 throw new NullPointerException(); 545 int mc = modCount; 546 // Android modified - breaks from the loop when modCount != mc 547 for (LinkedHashMapEntry<K,V> e = header.after; modCount == mc && e != header; e = e.after) 548 action.accept(e.key, e.value); 549 if (modCount != mc) 550 throw new ConcurrentModificationException(); 551 } 552 553 public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) { 554 if (function == null) 555 throw new NullPointerException(); 556 int mc = modCount; 557 // Android modified - breaks from the loop when modCount != mc 558 for (LinkedHashMapEntry<K,V> e = header.after; modCount == mc && e != header; e = e.after) 559 e.value = function.apply(e.key, e.value); 560 if (modCount != mc) 561 throw new ConcurrentModificationException(); 562 } 563} 564