Collection.java revision 51b1b6997fd3f980076b8081f7f1165ccc2a4008
1/* 2 * Copyright (c) 1997, 2010, Oracle and/or its affiliates. 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. Oracle designates this 8 * particular file as subject to the "Classpath" exception as provided 9 * by Oracle 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 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; 27 28/** 29 * The root interface in the <i>collection hierarchy</i>. A collection 30 * represents a group of objects, known as its <i>elements</i>. Some 31 * collections allow duplicate elements and others do not. Some are ordered 32 * and others unordered. The JDK does not provide any <i>direct</i> 33 * implementations of this interface: it provides implementations of more 34 * specific subinterfaces like <tt>Set</tt> and <tt>List</tt>. This interface 35 * is typically used to pass collections around and manipulate them where 36 * maximum generality is desired. 37 * 38 * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain 39 * duplicate elements) should implement this interface directly. 40 * 41 * <p>All general-purpose <tt>Collection</tt> implementation classes (which 42 * typically implement <tt>Collection</tt> indirectly through one of its 43 * subinterfaces) should provide two "standard" constructors: a void (no 44 * arguments) constructor, which creates an empty collection, and a 45 * constructor with a single argument of type <tt>Collection</tt>, which 46 * creates a new collection with the same elements as its argument. In 47 * effect, the latter constructor allows the user to copy any collection, 48 * producing an equivalent collection of the desired implementation type. 49 * There is no way to enforce this convention (as interfaces cannot contain 50 * constructors) but all of the general-purpose <tt>Collection</tt> 51 * implementations in the Java platform libraries comply. 52 * 53 * <p>The "destructive" methods contained in this interface, that is, the 54 * methods that modify the collection on which they operate, are specified to 55 * throw <tt>UnsupportedOperationException</tt> if this collection does not 56 * support the operation. If this is the case, these methods may, but are not 57 * required to, throw an <tt>UnsupportedOperationException</tt> if the 58 * invocation would have no effect on the collection. For example, invoking 59 * the {@link #addAll(Collection)} method on an unmodifiable collection may, 60 * but is not required to, throw the exception if the collection to be added 61 * is empty. 62 * 63 * <p><a name="optional-restrictions"/> 64 * Some collection implementations have restrictions on the elements that 65 * they may contain. For example, some implementations prohibit null elements, 66 * and some have restrictions on the types of their elements. Attempting to 67 * add an ineligible element throws an unchecked exception, typically 68 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>. Attempting 69 * to query the presence of an ineligible element may throw an exception, 70 * or it may simply return false; some implementations will exhibit the former 71 * behavior and some will exhibit the latter. More generally, attempting an 72 * operation on an ineligible element whose completion would not result in 73 * the insertion of an ineligible element into the collection may throw an 74 * exception or it may succeed, at the option of the implementation. 75 * Such exceptions are marked as "optional" in the specification for this 76 * interface. 77 * 78 * <p>It is up to each collection to determine its own synchronization 79 * policy. In the absence of a stronger guarantee by the 80 * implementation, undefined behavior may result from the invocation 81 * of any method on a collection that is being mutated by another 82 * thread; this includes direct invocations, passing the collection to 83 * a method that might perform invocations, and using an existing 84 * iterator to examine the collection. 85 * 86 * <p>Many methods in Collections Framework interfaces are defined in 87 * terms of the {@link Object#equals(Object) equals} method. For example, 88 * the specification for the {@link #contains(Object) contains(Object o)} 89 * method says: "returns <tt>true</tt> if and only if this collection 90 * contains at least one element <tt>e</tt> such that 91 * <tt>(o==null ? e==null : o.equals(e))</tt>." This specification should 92 * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt> 93 * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be 94 * invoked for any element <tt>e</tt>. Implementations are free to implement 95 * optimizations whereby the <tt>equals</tt> invocation is avoided, for 96 * example, by first comparing the hash codes of the two elements. (The 97 * {@link Object#hashCode()} specification guarantees that two objects with 98 * unequal hash codes cannot be equal.) More generally, implementations of 99 * the various Collections Framework interfaces are free to take advantage of 100 * the specified behavior of underlying {@link Object} methods wherever the 101 * implementor deems it appropriate. 102 * 103 * <p>This interface is a member of the 104 * <a href="{@docRoot}/../technotes/guides/collections/index.html"> 105 * Java Collections Framework</a>. 106 * 107 * @param <E> the type of elements in this collection 108 * 109 * @author Josh Bloch 110 * @author Neal Gafter 111 * @see Set 112 * @see List 113 * @see Map 114 * @see SortedSet 115 * @see SortedMap 116 * @see HashSet 117 * @see TreeSet 118 * @see ArrayList 119 * @see LinkedList 120 * @see Vector 121 * @see Collections 122 * @see Arrays 123 * @see AbstractCollection 124 * @since 1.2 125 */ 126 127public interface Collection<E> extends Iterable<E> { 128 // Query Operations 129 130 /** 131 * Returns the number of elements in this collection. If this collection 132 * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns 133 * <tt>Integer.MAX_VALUE</tt>. 134 * 135 * @return the number of elements in this collection 136 */ 137 int size(); 138 139 /** 140 * Returns <tt>true</tt> if this collection contains no elements. 141 * 142 * @return <tt>true</tt> if this collection contains no elements 143 */ 144 boolean isEmpty(); 145 146 /** 147 * Returns <tt>true</tt> if this collection contains the specified element. 148 * More formally, returns <tt>true</tt> if and only if this collection 149 * contains at least one element <tt>e</tt> such that 150 * <tt>(o==null ? e==null : o.equals(e))</tt>. 151 * 152 * @param o element whose presence in this collection is to be tested 153 * @return <tt>true</tt> if this collection contains the specified 154 * element 155 * @throws ClassCastException if the type of the specified element 156 * is incompatible with this collection 157 * (<a href="#optional-restrictions">optional</a>) 158 * @throws NullPointerException if the specified element is null and this 159 * collection does not permit null elements 160 * (<a href="#optional-restrictions">optional</a>) 161 */ 162 boolean contains(Object o); 163 164 /** 165 * Returns an iterator over the elements in this collection. There are no 166 * guarantees concerning the order in which the elements are returned 167 * (unless this collection is an instance of some class that provides a 168 * guarantee). 169 * 170 * @return an <tt>Iterator</tt> over the elements in this collection 171 */ 172 Iterator<E> iterator(); 173 174 /** 175 * Returns an array containing all of the elements in this collection. 176 * If this collection makes any guarantees as to what order its elements 177 * are returned by its iterator, this method must return the elements in 178 * the same order. 179 * 180 * <p>The returned array will be "safe" in that no references to it are 181 * maintained by this collection. (In other words, this method must 182 * allocate a new array even if this collection is backed by an array). 183 * The caller is thus free to modify the returned array. 184 * 185 * <p>This method acts as bridge between array-based and collection-based 186 * APIs. 187 * 188 * @return an array containing all of the elements in this collection 189 */ 190 Object[] toArray(); 191 192 /** 193 * Returns an array containing all of the elements in this collection; 194 * the runtime type of the returned array is that of the specified array. 195 * If the collection fits in the specified array, it is returned therein. 196 * Otherwise, a new array is allocated with the runtime type of the 197 * specified array and the size of this collection. 198 * 199 * <p>If this collection fits in the specified array with room to spare 200 * (i.e., the array has more elements than this collection), the element 201 * in the array immediately following the end of the collection is set to 202 * <tt>null</tt>. (This is useful in determining the length of this 203 * collection <i>only</i> if the caller knows that this collection does 204 * not contain any <tt>null</tt> elements.) 205 * 206 * <p>If this collection makes any guarantees as to what order its elements 207 * are returned by its iterator, this method must return the elements in 208 * the same order. 209 * 210 * <p>Like the {@link #toArray()} method, this method acts as bridge between 211 * array-based and collection-based APIs. Further, this method allows 212 * precise control over the runtime type of the output array, and may, 213 * under certain circumstances, be used to save allocation costs. 214 * 215 * <p>Suppose <tt>x</tt> is a collection known to contain only strings. 216 * The following code can be used to dump the collection into a newly 217 * allocated array of <tt>String</tt>: 218 * 219 * <pre> 220 * String[] y = x.toArray(new String[0]);</pre> 221 * 222 * Note that <tt>toArray(new Object[0])</tt> is identical in function to 223 * <tt>toArray()</tt>. 224 * 225 * @param a the array into which the elements of this collection are to be 226 * stored, if it is big enough; otherwise, a new array of the same 227 * runtime type is allocated for this purpose. 228 * @return an array containing all of the elements in this collection 229 * @throws ArrayStoreException if the runtime type of the specified array 230 * is not a supertype of the runtime type of every element in 231 * this collection 232 * @throws NullPointerException if the specified array is null 233 */ 234 <T> T[] toArray(T[] a); 235 236 // Modification Operations 237 238 /** 239 * Ensures that this collection contains the specified element (optional 240 * operation). Returns <tt>true</tt> if this collection changed as a 241 * result of the call. (Returns <tt>false</tt> if this collection does 242 * not permit duplicates and already contains the specified element.)<p> 243 * 244 * Collections that support this operation may place limitations on what 245 * elements may be added to this collection. In particular, some 246 * collections will refuse to add <tt>null</tt> elements, and others will 247 * impose restrictions on the type of elements that may be added. 248 * Collection classes should clearly specify in their documentation any 249 * restrictions on what elements may be added.<p> 250 * 251 * If a collection refuses to add a particular element for any reason 252 * other than that it already contains the element, it <i>must</i> throw 253 * an exception (rather than returning <tt>false</tt>). This preserves 254 * the invariant that a collection always contains the specified element 255 * after this call returns. 256 * 257 * @param e element whose presence in this collection is to be ensured 258 * @return <tt>true</tt> if this collection changed as a result of the 259 * call 260 * @throws UnsupportedOperationException if the <tt>add</tt> operation 261 * is not supported by this collection 262 * @throws ClassCastException if the class of the specified element 263 * prevents it from being added to this collection 264 * @throws NullPointerException if the specified element is null and this 265 * collection does not permit null elements 266 * @throws IllegalArgumentException if some property of the element 267 * prevents it from being added to this collection 268 * @throws IllegalStateException if the element cannot be added at this 269 * time due to insertion restrictions 270 */ 271 boolean add(E e); 272 273 /** 274 * Removes a single instance of the specified element from this 275 * collection, if it is present (optional operation). More formally, 276 * removes an element <tt>e</tt> such that 277 * <tt>(o==null ? e==null : o.equals(e))</tt>, if 278 * this collection contains one or more such elements. Returns 279 * <tt>true</tt> if this collection contained the specified element (or 280 * equivalently, if this collection changed as a result of the call). 281 * 282 * @param o element to be removed from this collection, if present 283 * @return <tt>true</tt> if an element was removed as a result of this call 284 * @throws ClassCastException if the type of the specified element 285 * is incompatible with this collection 286 * (<a href="#optional-restrictions">optional</a>) 287 * @throws NullPointerException if the specified element is null and this 288 * collection does not permit null elements 289 * (<a href="#optional-restrictions">optional</a>) 290 * @throws UnsupportedOperationException if the <tt>remove</tt> operation 291 * is not supported by this collection 292 */ 293 boolean remove(Object o); 294 295 296 // Bulk Operations 297 298 /** 299 * Returns <tt>true</tt> if this collection contains all of the elements 300 * in the specified collection. 301 * 302 * @param c collection to be checked for containment in this collection 303 * @return <tt>true</tt> if this collection contains all of the elements 304 * in the specified collection 305 * @throws ClassCastException if the types of one or more elements 306 * in the specified collection are incompatible with this 307 * collection 308 * (<a href="#optional-restrictions">optional</a>) 309 * @throws NullPointerException if the specified collection contains one 310 * or more null elements and this collection does not permit null 311 * elements 312 * (<a href="#optional-restrictions">optional</a>), 313 * or if the specified collection is null. 314 * @see #contains(Object) 315 */ 316 boolean containsAll(Collection<?> c); 317 318 /** 319 * Adds all of the elements in the specified collection to this collection 320 * (optional operation). The behavior of this operation is undefined if 321 * the specified collection is modified while the operation is in progress. 322 * (This implies that the behavior of this call is undefined if the 323 * specified collection is this collection, and this collection is 324 * nonempty.) 325 * 326 * @param c collection containing elements to be added to this collection 327 * @return <tt>true</tt> if this collection changed as a result of the call 328 * @throws UnsupportedOperationException if the <tt>addAll</tt> operation 329 * is not supported by this collection 330 * @throws ClassCastException if the class of an element of the specified 331 * collection prevents it from being added to this collection 332 * @throws NullPointerException if the specified collection contains a 333 * null element and this collection does not permit null elements, 334 * or if the specified collection is null 335 * @throws IllegalArgumentException if some property of an element of the 336 * specified collection prevents it from being added to this 337 * collection 338 * @throws IllegalStateException if not all the elements can be added at 339 * this time due to insertion restrictions 340 * @see #add(Object) 341 */ 342 boolean addAll(Collection<? extends E> c); 343 344 /** 345 * Removes all of this collection's elements that are also contained in the 346 * specified collection (optional operation). After this call returns, 347 * this collection will contain no elements in common with the specified 348 * collection. 349 * 350 * @param c collection containing elements to be removed from this collection 351 * @return <tt>true</tt> if this collection changed as a result of the 352 * call 353 * @throws UnsupportedOperationException if the <tt>removeAll</tt> method 354 * is not supported by this collection 355 * @throws ClassCastException if the types of one or more elements 356 * in this collection are incompatible with the specified 357 * collection 358 * (<a href="#optional-restrictions">optional</a>) 359 * @throws NullPointerException if this collection contains one or more 360 * null elements and the specified collection does not support 361 * null elements 362 * (<a href="#optional-restrictions">optional</a>), 363 * or if the specified collection is null 364 * @see #remove(Object) 365 * @see #contains(Object) 366 */ 367 boolean removeAll(Collection<?> c); 368 369 /** 370 * Retains only the elements in this collection that are contained in the 371 * specified collection (optional operation). In other words, removes from 372 * this collection all of its elements that are not contained in the 373 * specified collection. 374 * 375 * @param c collection containing elements to be retained in this collection 376 * @return <tt>true</tt> if this collection changed as a result of the call 377 * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation 378 * is not supported by this collection 379 * @throws ClassCastException if the types of one or more elements 380 * in this collection are incompatible with the specified 381 * collection 382 * (<a href="#optional-restrictions">optional</a>) 383 * @throws NullPointerException if this collection contains one or more 384 * null elements and the specified collection does not permit null 385 * elements 386 * (<a href="#optional-restrictions">optional</a>), 387 * or if the specified collection is null 388 * @see #remove(Object) 389 * @see #contains(Object) 390 */ 391 boolean retainAll(Collection<?> c); 392 393 /** 394 * Removes all of the elements from this collection (optional operation). 395 * The collection will be empty after this method returns. 396 * 397 * @throws UnsupportedOperationException if the <tt>clear</tt> operation 398 * is not supported by this collection 399 */ 400 void clear(); 401 402 403 // Comparison and hashing 404 405 /** 406 * Compares the specified object with this collection for equality. <p> 407 * 408 * While the <tt>Collection</tt> interface adds no stipulations to the 409 * general contract for the <tt>Object.equals</tt>, programmers who 410 * implement the <tt>Collection</tt> interface "directly" (in other words, 411 * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt> 412 * or a <tt>List</tt>) must exercise care if they choose to override the 413 * <tt>Object.equals</tt>. It is not necessary to do so, and the simplest 414 * course of action is to rely on <tt>Object</tt>'s implementation, but 415 * the implementor may wish to implement a "value comparison" in place of 416 * the default "reference comparison." (The <tt>List</tt> and 417 * <tt>Set</tt> interfaces mandate such value comparisons.)<p> 418 * 419 * The general contract for the <tt>Object.equals</tt> method states that 420 * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and 421 * only if <tt>b.equals(a)</tt>). The contracts for <tt>List.equals</tt> 422 * and <tt>Set.equals</tt> state that lists are only equal to other lists, 423 * and sets to other sets. Thus, a custom <tt>equals</tt> method for a 424 * collection class that implements neither the <tt>List</tt> nor 425 * <tt>Set</tt> interface must return <tt>false</tt> when this collection 426 * is compared to any list or set. (By the same logic, it is not possible 427 * to write a class that correctly implements both the <tt>Set</tt> and 428 * <tt>List</tt> interfaces.) 429 * 430 * @param o object to be compared for equality with this collection 431 * @return <tt>true</tt> if the specified object is equal to this 432 * collection 433 * 434 * @see Object#equals(Object) 435 * @see Set#equals(Object) 436 * @see List#equals(Object) 437 */ 438 boolean equals(Object o); 439 440 /** 441 * Returns the hash code value for this collection. While the 442 * <tt>Collection</tt> interface adds no stipulations to the general 443 * contract for the <tt>Object.hashCode</tt> method, programmers should 444 * take note that any class that overrides the <tt>Object.equals</tt> 445 * method must also override the <tt>Object.hashCode</tt> method in order 446 * to satisfy the general contract for the <tt>Object.hashCode</tt> method. 447 * In particular, <tt>c1.equals(c2)</tt> implies that 448 * <tt>c1.hashCode()==c2.hashCode()</tt>. 449 * 450 * @return the hash code value for this collection 451 * 452 * @see Object#hashCode() 453 * @see Object#equals(Object) 454 */ 455 int hashCode(); 456} 457