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1: /* LinkedHashMap.java -- a class providing hashtable data structure, 2: mapping Object --> Object, with linked list traversal 3: Copyright (C) 2001, 2002, 2005 Free Software Foundation, Inc. 4: 5: This file is part of GNU Classpath. 6: 7: GNU Classpath is free software; you can redistribute it and/or modify 8: it under the terms of the GNU General Public License as published by 9: the Free Software Foundation; either version 2, or (at your option) 10: any later version. 11: 12: GNU Classpath is distributed in the hope that it will be useful, but 13: WITHOUT ANY WARRANTY; without even the implied warranty of 14: MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 15: General Public License for more details. 16: 17: You should have received a copy of the GNU General Public License 18: along with GNU Classpath; see the file COPYING. If not, write to the 19: Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 20: 02110-1301 USA. 21: 22: Linking this library statically or dynamically with other modules is 23: making a combined work based on this library. Thus, the terms and 24: conditions of the GNU General Public License cover the whole 25: combination. 26: 27: As a special exception, the copyright holders of this library give you 28: permission to link this library with independent modules to produce an 29: executable, regardless of the license terms of these independent 30: modules, and to copy and distribute the resulting executable under 31: terms of your choice, provided that you also meet, for each linked 32: independent module, the terms and conditions of the license of that 33: module. An independent module is a module which is not derived from 34: or based on this library. If you modify this library, you may extend 35: this exception to your version of the library, but you are not 36: obligated to do so. If you do not wish to do so, delete this 37: exception statement from your version. */ 38: 39: 40: package java.util; 41: 42: /** 43: * This class provides a hashtable-backed implementation of the 44: * Map interface, with predictable traversal order. 45: * <p> 46: * 47: * It uses a hash-bucket approach; that is, hash collisions are handled 48: * by linking the new node off of the pre-existing node (or list of 49: * nodes). In this manner, techniques such as linear probing (which 50: * can cause primary clustering) and rehashing (which does not fit very 51: * well with Java's method of precomputing hash codes) are avoided. In 52: * addition, this maintains a doubly-linked list which tracks either 53: * insertion or access order. 54: * <p> 55: * 56: * In insertion order, calling <code>put</code> adds the key to the end of 57: * traversal, unless the key was already in the map; changing traversal order 58: * requires removing and reinserting a key. On the other hand, in access 59: * order, all calls to <code>put</code> and <code>get</code> cause the 60: * accessed key to move to the end of the traversal list. Note that any 61: * accesses to the map's contents via its collection views and iterators do 62: * not affect the map's traversal order, since the collection views do not 63: * call <code>put</code> or <code>get</code>. 64: * <p> 65: * 66: * One of the nice features of tracking insertion order is that you can 67: * copy a hashtable, and regardless of the implementation of the original, 68: * produce the same results when iterating over the copy. This is possible 69: * without needing the overhead of <code>TreeMap</code>. 70: * <p> 71: * 72: * When using this {@link #LinkedHashMap(int, float, boolean) constructor}, 73: * you can build an access-order mapping. This can be used to implement LRU 74: * caches, for example. By overriding {@link #removeEldestEntry(Map.Entry)}, 75: * you can also control the removal of the oldest entry, and thereby do 76: * things like keep the map at a fixed size. 77: * <p> 78: * 79: * Under ideal circumstances (no collisions), LinkedHashMap offers O(1) 80: * performance on most operations (<code>containsValue()</code> is, 81: * of course, O(n)). In the worst case (all keys map to the same 82: * hash code -- very unlikely), most operations are O(n). Traversal is 83: * faster than in HashMap (proportional to the map size, and not the space 84: * allocated for the map), but other operations may be slower because of the 85: * overhead of the maintaining the traversal order list. 86: * <p> 87: * 88: * LinkedHashMap accepts the null key and null values. It is not 89: * synchronized, so if you need multi-threaded access, consider using:<br> 90: * <code>Map m = Collections.synchronizedMap(new LinkedHashMap(...));</code> 91: * <p> 92: * 93: * The iterators are <i>fail-fast</i>, meaning that any structural 94: * modification, except for <code>remove()</code> called on the iterator 95: * itself, cause the iterator to throw a 96: * {@link ConcurrentModificationException} rather than exhibit 97: * non-deterministic behavior. 98: * 99: * @author Eric Blake (ebb9@email.byu.edu) 100: * @see Object#hashCode() 101: * @see Collection 102: * @see Map 103: * @see HashMap 104: * @see TreeMap 105: * @see Hashtable 106: * @since 1.4 107: * @status updated to 1.4 108: */ 109: public class LinkedHashMap extends HashMap 110: { 111: /** 112: * Compatible with JDK 1.4. 113: */ 114: private static final long serialVersionUID = 3801124242820219131L; 115: 116: /** 117: * The oldest Entry to begin iteration at. 118: */ 119: transient LinkedHashEntry root; 120: 121: /** 122: * The iteration order of this linked hash map: <code>true</code> for 123: * access-order, <code>false</code> for insertion-order. 124: * 125: * @serial true for access order traversal 126: */ 127: final boolean accessOrder; 128: 129: /** 130: * Class to represent an entry in the hash table. Holds a single key-value 131: * pair and the doubly-linked insertion order list. 132: */ 133: class LinkedHashEntry extends HashEntry 134: { 135: /** 136: * The predecessor in the iteration list. If this entry is the root 137: * (eldest), pred points to the newest entry. 138: */ 139: LinkedHashEntry pred; 140: 141: /** The successor in the iteration list, null if this is the newest. */ 142: LinkedHashEntry succ; 143: 144: /** 145: * Simple constructor. 146: * 147: * @param key the key 148: * @param value the value 149: */ 150: LinkedHashEntry(Object key, Object value) 151: { 152: super(key, value); 153: if (root == null) 154: { 155: root = this; 156: pred = this; 157: } 158: else 159: { 160: pred = root.pred; 161: pred.succ = this; 162: root.pred = this; 163: } 164: } 165: 166: /** 167: * Called when this entry is accessed via put or get. This version does 168: * the necessary bookkeeping to keep the doubly-linked list in order, 169: * after moving this element to the newest position in access order. 170: */ 171: void access() 172: { 173: if (accessOrder && succ != null) 174: { 175: modCount++; 176: if (this == root) 177: { 178: root = succ; 179: pred.succ = this; 180: succ = null; 181: } 182: else 183: { 184: pred.succ = succ; 185: succ.pred = pred; 186: succ = null; 187: pred = root.pred; 188: pred.succ = this; 189: root.pred = this; 190: } 191: } 192: } 193: 194: /** 195: * Called when this entry is removed from the map. This version does 196: * the necessary bookkeeping to keep the doubly-linked list in order. 197: * 198: * @return the value of this key as it is removed 199: */ 200: Object cleanup() 201: { 202: if (this == root) 203: { 204: root = succ; 205: if (succ != null) 206: succ.pred = pred; 207: } 208: else if (succ == null) 209: { 210: pred.succ = null; 211: root.pred = pred; 212: } 213: else 214: { 215: pred.succ = succ; 216: succ.pred = pred; 217: } 218: return value; 219: } 220: } // class LinkedHashEntry 221: 222: /** 223: * Construct a new insertion-ordered LinkedHashMap with the default 224: * capacity (11) and the default load factor (0.75). 225: */ 226: public LinkedHashMap() 227: { 228: super(); 229: accessOrder = false; 230: } 231: 232: /** 233: * Construct a new insertion-ordered LinkedHashMap from the given Map, 234: * with initial capacity the greater of the size of <code>m</code> or 235: * the default of 11. 236: * <p> 237: * 238: * Every element in Map m will be put into this new HashMap, in the 239: * order of m's iterator. 240: * 241: * @param m a Map whose key / value pairs will be put into 242: * the new HashMap. <b>NOTE: key / value pairs 243: * are not cloned in this constructor.</b> 244: * @throws NullPointerException if m is null 245: */ 246: public LinkedHashMap(Map m) 247: { 248: super(m); 249: accessOrder = false; 250: } 251: 252: /** 253: * Construct a new insertion-ordered LinkedHashMap with a specific 254: * inital capacity and default load factor of 0.75. 255: * 256: * @param initialCapacity the initial capacity of this HashMap (>= 0) 257: * @throws IllegalArgumentException if (initialCapacity < 0) 258: */ 259: public LinkedHashMap(int initialCapacity) 260: { 261: super(initialCapacity); 262: accessOrder = false; 263: } 264: 265: /** 266: * Construct a new insertion-orderd LinkedHashMap with a specific 267: * inital capacity and load factor. 268: * 269: * @param initialCapacity the initial capacity (>= 0) 270: * @param loadFactor the load factor (> 0, not NaN) 271: * @throws IllegalArgumentException if (initialCapacity < 0) || 272: * ! (loadFactor > 0.0) 273: */ 274: public LinkedHashMap(int initialCapacity, float loadFactor) 275: { 276: super(initialCapacity, loadFactor); 277: accessOrder = false; 278: } 279: 280: /** 281: * Construct a new LinkedHashMap with a specific inital capacity, load 282: * factor, and ordering mode. 283: * 284: * @param initialCapacity the initial capacity (>=0) 285: * @param loadFactor the load factor (>0, not NaN) 286: * @param accessOrder true for access-order, false for insertion-order 287: * @throws IllegalArgumentException if (initialCapacity < 0) || 288: * ! (loadFactor > 0.0) 289: */ 290: public LinkedHashMap(int initialCapacity, float loadFactor, 291: boolean accessOrder) 292: { 293: super(initialCapacity, loadFactor); 294: this.accessOrder = accessOrder; 295: } 296: 297: /** 298: * Clears the Map so it has no keys. This is O(1). 299: */ 300: public void clear() 301: { 302: super.clear(); 303: root = null; 304: } 305: 306: /** 307: * Returns <code>true</code> if this HashMap contains a value 308: * <code>o</code>, such that <code>o.equals(value)</code>. 309: * 310: * @param value the value to search for in this HashMap 311: * @return <code>true</code> if at least one key maps to the value 312: */ 313: public boolean containsValue(Object value) 314: { 315: LinkedHashEntry e = root; 316: while (e != null) 317: { 318: if (equals(value, e.value)) 319: return true; 320: e = e.succ; 321: } 322: return false; 323: } 324: 325: /** 326: * Return the value in this Map associated with the supplied key, 327: * or <code>null</code> if the key maps to nothing. If this is an 328: * access-ordered Map and the key is found, this performs structural 329: * modification, moving the key to the newest end of the list. NOTE: 330: * Since the value could also be null, you must use containsKey to 331: * see if this key actually maps to something. 332: * 333: * @param key the key for which to fetch an associated value 334: * @return what the key maps to, if present 335: * @see #put(Object, Object) 336: * @see #containsKey(Object) 337: */ 338: public Object get(Object key) 339: { 340: int idx = hash(key); 341: HashEntry e = buckets[idx]; 342: while (e != null) 343: { 344: if (equals(key, e.key)) 345: { 346: e.access(); 347: return e.value; 348: } 349: e = e.next; 350: } 351: return null; 352: } 353: 354: /** 355: * Returns <code>true</code> if this map should remove the eldest entry. 356: * This method is invoked by all calls to <code>put</code> and 357: * <code>putAll</code> which place a new entry in the map, providing 358: * the implementer an opportunity to remove the eldest entry any time 359: * a new one is added. This can be used to save memory usage of the 360: * hashtable, as well as emulating a cache, by deleting stale entries. 361: * <p> 362: * 363: * For example, to keep the Map limited to 100 entries, override as follows: 364: * <pre> 365: * private static final int MAX_ENTRIES = 100; 366: * protected boolean removeEldestEntry(Map.Entry eldest) 367: * { 368: * return size() > MAX_ENTRIES; 369: * } 370: * </pre><p> 371: * 372: * Typically, this method does not modify the map, but just uses the 373: * return value as an indication to <code>put</code> whether to proceed. 374: * However, if you override it to modify the map, you must return false 375: * (indicating that <code>put</code> should leave the modified map alone), 376: * or you face unspecified behavior. Remember that in access-order mode, 377: * even calling <code>get</code> is a structural modification, but using 378: * the collections views (such as <code>keySet</code>) is not. 379: * <p> 380: * 381: * This method is called after the eldest entry has been inserted, so 382: * if <code>put</code> was called on a previously empty map, the eldest 383: * entry is the one you just put in! The default implementation just 384: * returns <code>false</code>, so that this map always behaves like 385: * a normal one with unbounded growth. 386: * 387: * @param eldest the eldest element which would be removed if this 388: * returns true. For an access-order map, this is the least 389: * recently accessed; for an insertion-order map, this is the 390: * earliest element inserted. 391: * @return true if <code>eldest</code> should be removed 392: */ 393: protected boolean removeEldestEntry(Map.Entry eldest) 394: { 395: return false; 396: } 397: 398: /** 399: * Helper method called by <code>put</code>, which creates and adds a 400: * new Entry, followed by performing bookkeeping (like removeEldestEntry). 401: * 402: * @param key the key of the new Entry 403: * @param value the value 404: * @param idx the index in buckets where the new Entry belongs 405: * @param callRemove whether to call the removeEldestEntry method 406: * @see #put(Object, Object) 407: * @see #removeEldestEntry(Map.Entry) 408: * @see LinkedHashEntry#LinkedHashEntry(Object, Object) 409: */ 410: void addEntry(Object key, Object value, int idx, boolean callRemove) 411: { 412: LinkedHashEntry e = new LinkedHashEntry(key, value); 413: e.next = buckets[idx]; 414: buckets[idx] = e; 415: if (callRemove && removeEldestEntry(root)) 416: remove(root.key); 417: } 418: 419: /** 420: * Helper method, called by clone() to reset the doubly-linked list. 421: * 422: * @param m the map to add entries from 423: * @see #clone() 424: */ 425: void putAllInternal(Map m) 426: { 427: root = null; 428: super.putAllInternal(m); 429: } 430: 431: /** 432: * Generates a parameterized iterator. This allows traversal to follow 433: * the doubly-linked list instead of the random bin order of HashMap. 434: * 435: * @param type {@link #KEYS}, {@link #VALUES}, or {@link #ENTRIES} 436: * @return the appropriate iterator 437: */ 438: Iterator iterator(final int type) 439: { 440: return new Iterator() 441: { 442: /** The current Entry. */ 443: LinkedHashEntry current = root; 444: 445: /** The previous Entry returned by next(). */ 446: LinkedHashEntry last; 447: 448: /** The number of known modifications to the backing Map. */ 449: int knownMod = modCount; 450: 451: /** 452: * Returns true if the Iterator has more elements. 453: * 454: * @return true if there are more elements 455: */ 456: public boolean hasNext() 457: { 458: return current != null; 459: } 460: 461: /** 462: * Returns the next element in the Iterator's sequential view. 463: * 464: * @return the next element 465: * @throws ConcurrentModificationException if the HashMap was modified 466: * @throws NoSuchElementException if there is none 467: */ 468: public Object next() 469: { 470: if (knownMod != modCount) 471: throw new ConcurrentModificationException(); 472: if (current == null) 473: throw new NoSuchElementException(); 474: last = current; 475: current = current.succ; 476: return type == VALUES ? last.value : type == KEYS ? last.key : last; 477: } 478: 479: /** 480: * Removes from the backing HashMap the last element which was fetched 481: * with the <code>next()</code> method. 482: * 483: * @throws ConcurrentModificationException if the HashMap was modified 484: * @throws IllegalStateException if called when there is no last element 485: */ 486: public void remove() 487: { 488: if (knownMod != modCount) 489: throw new ConcurrentModificationException(); 490: if (last == null) 491: throw new IllegalStateException(); 492: LinkedHashMap.this.remove(last.key); 493: last = null; 494: knownMod++; 495: } 496: }; 497: } 498: } // class LinkedHashMap