J. Duke | 319a3b9 | 2007-12-01 00:00:00 +0000 | [diff] [blame^] | 1 | /* |
| 2 | * Copyright 2000-2006 Sun Microsystems, Inc. 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 |
| 6 | * under the terms of the GNU General Public License version 2 only, as |
| 7 | * published by the Free Software Foundation. Sun designates this |
| 8 | * particular file as subject to the "Classpath" exception as provided |
| 9 | * by Sun 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 | * |
| 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, |
| 22 | * CA 95054 USA or visit www.sun.com if you need additional information or |
| 23 | * have any questions. |
| 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<K,V>(-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<K,V>(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 | } |