LinkedHashMap.java revision 983b2c6ff9ea6d35adf7ab6398dccf870b7e180a
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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 * 17 * You should have received a copy of the GNU General Public License version 18 * 2 along with this work; if not, write to the Free Software Foundation, 19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 20 * 21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 22 * or visit www.oracle.com if you need additional information or have any 23 * questions. 24 */ 25 26package java.util; 27import 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 147public 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 * Transfers all entries to new table array. This method is called 247 * by superclass resize. It is overridden for performance, as it is 248 * faster to iterate using our linked list. 249 */ 250 @Override 251 void transfer(HashMap.Entry[] newTable, boolean rehash) { 252 int newCapacity = newTable.length; 253 for (Entry<K,V> e = header.after; e != header; e = e.after) { 254 if (rehash) 255 e.hash = (e.key == null) ? 0 : hash(e.key); 256 int index = indexFor(e.hash, newCapacity); 257 e.next = newTable[index]; 258 newTable[index] = e; 259 } 260 } 261 262 263 /** 264 * Returns <tt>true</tt> if this map maps one or more keys to the 265 * specified value. 266 * 267 * @param value value whose presence in this map is to be tested 268 * @return <tt>true</tt> if this map maps one or more keys to the 269 * specified value 270 */ 271 public boolean containsValue(Object value) { 272 // Overridden to take advantage of faster iterator 273 if (value==null) { 274 for (Entry e = header.after; e != header; e = e.after) 275 if (e.value==null) 276 return true; 277 } else { 278 for (Entry e = header.after; e != header; e = e.after) 279 if (value.equals(e.value)) 280 return true; 281 } 282 return false; 283 } 284 285 /** 286 * Returns the value to which the specified key is mapped, 287 * or {@code null} if this map contains no mapping for the key. 288 * 289 * <p>More formally, if this map contains a mapping from a key 290 * {@code k} to a value {@code v} such that {@code (key==null ? k==null : 291 * key.equals(k))}, then this method returns {@code v}; otherwise 292 * it returns {@code null}. (There can be at most one such mapping.) 293 * 294 * <p>A return value of {@code null} does not <i>necessarily</i> 295 * indicate that the map contains no mapping for the key; it's also 296 * possible that the map explicitly maps the key to {@code null}. 297 * The {@link #containsKey containsKey} operation may be used to 298 * distinguish these two cases. 299 */ 300 public V get(Object key) { 301 Entry<K,V> e = (Entry<K,V>)getEntry(key); 302 if (e == null) 303 return null; 304 e.recordAccess(this); 305 return e.value; 306 } 307 308 /** 309 * Removes all of the mappings from this map. 310 * The map will be empty after this call returns. 311 */ 312 public void clear() { 313 super.clear(); 314 header.before = header.after = header; 315 } 316 317 /** 318 * LinkedHashMap entry. 319 */ 320 // Android-changed: make public. 321 public static class Entry<K,V> extends HashMap.Entry<K,V> { 322 // These fields comprise the doubly linked list used for iteration. 323 Entry<K,V> before, after; 324 325 Entry(int hash, K key, V value, HashMap.Entry<K,V> next) { 326 super(hash, key, value, next); 327 } 328 329 /** 330 * Removes this entry from the linked list. 331 */ 332 private void remove() { 333 before.after = after; 334 after.before = before; 335 } 336 337 /** 338 * Inserts this entry before the specified existing entry in the list. 339 */ 340 private void addBefore(Entry<K,V> existingEntry) { 341 after = existingEntry; 342 before = existingEntry.before; 343 before.after = this; 344 after.before = this; 345 } 346 347 /** 348 * This method is invoked by the superclass whenever the value 349 * of a pre-existing entry is read by Map.get or modified by Map.set. 350 * If the enclosing Map is access-ordered, it moves the entry 351 * to the end of the list; otherwise, it does nothing. 352 */ 353 void recordAccess(HashMap<K,V> m) { 354 LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m; 355 if (lm.accessOrder) { 356 lm.modCount++; 357 remove(); 358 addBefore(lm.header); 359 } 360 } 361 362 void recordRemoval(HashMap<K,V> m) { 363 remove(); 364 } 365 } 366 367 private abstract class LinkedHashIterator<T> implements Iterator<T> { 368 Entry<K,V> nextEntry = header.after; 369 Entry<K,V> lastReturned = null; 370 371 /** 372 * The modCount value that the iterator believes that the backing 373 * List should have. If this expectation is violated, the iterator 374 * has detected concurrent modification. 375 */ 376 int expectedModCount = modCount; 377 378 public boolean hasNext() { 379 return nextEntry != header; 380 } 381 382 public void remove() { 383 if (lastReturned == null) 384 throw new IllegalStateException(); 385 if (modCount != expectedModCount) 386 throw new ConcurrentModificationException(); 387 388 LinkedHashMap.this.remove(lastReturned.key); 389 lastReturned = null; 390 expectedModCount = modCount; 391 } 392 393 Entry<K,V> nextEntry() { 394 if (modCount != expectedModCount) 395 throw new ConcurrentModificationException(); 396 if (nextEntry == header) 397 throw new NoSuchElementException(); 398 399 Entry<K,V> e = lastReturned = nextEntry; 400 nextEntry = e.after; 401 return e; 402 } 403 } 404 405 private class KeyIterator extends LinkedHashIterator<K> { 406 public K next() { return nextEntry().getKey(); } 407 } 408 409 private class ValueIterator extends LinkedHashIterator<V> { 410 public V next() { return nextEntry().value; } 411 } 412 413 private class EntryIterator extends LinkedHashIterator<Map.Entry<K,V>> { 414 public Map.Entry<K,V> next() { return nextEntry(); } 415 } 416 417 // These Overrides alter the behavior of superclass view iterator() methods 418 Iterator<K> newKeyIterator() { return new KeyIterator(); } 419 Iterator<V> newValueIterator() { return new ValueIterator(); } 420 Iterator<Map.Entry<K,V>> newEntryIterator() { return new EntryIterator(); } 421 422 /** 423 * This override alters behavior of superclass put method. It causes newly 424 * allocated entry to get inserted at the end of the linked list and 425 * removes the eldest entry if appropriate. 426 */ 427 void addEntry(int hash, K key, V value, int bucketIndex) { 428 super.addEntry(hash, key, value, bucketIndex); 429 430 // Remove eldest entry if instructed 431 Entry<K,V> eldest = header.after; 432 if (removeEldestEntry(eldest)) { 433 removeEntryForKey(eldest.key); 434 } 435 } 436 437 /** 438 * Returns the eldest entry in the map, or {@code null} if the map is empty. 439 * 440 * Android-added. 441 * 442 * @hide 443 */ 444 public Entry<K, V> eldest() { 445 Entry<K, V> eldest = header.after; 446 return eldest != header ? eldest : null; 447 } 448 449 /** 450 * This override differs from addEntry in that it doesn't resize the 451 * table or remove the eldest entry. 452 */ 453 void createEntry(int hash, K key, V value, int bucketIndex) { 454 HashMap.Entry<K,V> old = table[bucketIndex]; 455 Entry<K,V> e = new Entry<>(hash, key, value, old); 456 table[bucketIndex] = e; 457 e.addBefore(header); 458 size++; 459 } 460 461 /** 462 * Returns <tt>true</tt> if this map should remove its eldest entry. 463 * This method is invoked by <tt>put</tt> and <tt>putAll</tt> after 464 * inserting a new entry into the map. It provides the implementor 465 * with the opportunity to remove the eldest entry each time a new one 466 * is added. This is useful if the map represents a cache: it allows 467 * the map to reduce memory consumption by deleting stale entries. 468 * 469 * <p>Sample use: this override will allow the map to grow up to 100 470 * entries and then delete the eldest entry each time a new entry is 471 * added, maintaining a steady state of 100 entries. 472 * <pre> 473 * private static final int MAX_ENTRIES = 100; 474 * 475 * protected boolean removeEldestEntry(Map.Entry eldest) { 476 * return size() > MAX_ENTRIES; 477 * } 478 * </pre> 479 * 480 * <p>This method typically does not modify the map in any way, 481 * instead allowing the map to modify itself as directed by its 482 * return value. It <i>is</i> permitted for this method to modify 483 * the map directly, but if it does so, it <i>must</i> return 484 * <tt>false</tt> (indicating that the map should not attempt any 485 * further modification). The effects of returning <tt>true</tt> 486 * after modifying the map from within this method are unspecified. 487 * 488 * <p>This implementation merely returns <tt>false</tt> (so that this 489 * map acts like a normal map - the eldest element is never removed). 490 * 491 * @param eldest The least recently inserted entry in the map, or if 492 * this is an access-ordered map, the least recently accessed 493 * entry. This is the entry that will be removed it this 494 * method returns <tt>true</tt>. If the map was empty prior 495 * to the <tt>put</tt> or <tt>putAll</tt> invocation resulting 496 * in this invocation, this will be the entry that was just 497 * inserted; in other words, if the map contains a single 498 * entry, the eldest entry is also the newest. 499 * @return <tt>true</tt> if the eldest entry should be removed 500 * from the map; <tt>false</tt> if it should be retained. 501 */ 502 protected boolean removeEldestEntry(Map.Entry<K,V> eldest) { 503 return false; 504 } 505} 506