Collection.java revision 5c35c7ebda68cc39b6bdee20f678a150336ebd1d
1/* 2 * Copyright (c) 1997, 2013, 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 28import java.util.function.Predicate; 29 30/** 31 * The root interface in the <i>collection hierarchy</i>. A collection 32 * represents a group of objects, known as its <i>elements</i>. Some 33 * collections allow duplicate elements and others do not. Some are ordered 34 * and others unordered. The JDK does not provide any <i>direct</i> 35 * implementations of this interface: it provides implementations of more 36 * specific subinterfaces like <tt>Set</tt> and <tt>List</tt>. This interface 37 * is typically used to pass collections around and manipulate them where 38 * maximum generality is desired. 39 * 40 * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain 41 * duplicate elements) should implement this interface directly. 42 * 43 * <p>All general-purpose <tt>Collection</tt> implementation classes (which 44 * typically implement <tt>Collection</tt> indirectly through one of its 45 * subinterfaces) should provide two "standard" constructors: a void (no 46 * arguments) constructor, which creates an empty collection, and a 47 * constructor with a single argument of type <tt>Collection</tt>, which 48 * creates a new collection with the same elements as its argument. In 49 * effect, the latter constructor allows the user to copy any collection, 50 * producing an equivalent collection of the desired implementation type. 51 * There is no way to enforce this convention (as interfaces cannot contain 52 * constructors) but all of the general-purpose <tt>Collection</tt> 53 * implementations in the Java platform libraries comply. 54 * 55 * <p>The "destructive" methods contained in this interface, that is, the 56 * methods that modify the collection on which they operate, are specified to 57 * throw <tt>UnsupportedOperationException</tt> if this collection does not 58 * support the operation. If this is the case, these methods may, but are not 59 * required to, throw an <tt>UnsupportedOperationException</tt> if the 60 * invocation would have no effect on the collection. For example, invoking 61 * the {@link #addAll(Collection)} method on an unmodifiable collection may, 62 * but is not required to, throw the exception if the collection to be added 63 * is empty. 64 * 65 * <p><a name="optional-restrictions"> 66 * Some collection implementations have restrictions on the elements that 67 * they may contain.</a> For example, some implementations prohibit null elements, 68 * and some have restrictions on the types of their elements. Attempting to 69 * add an ineligible element throws an unchecked exception, typically 70 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>. Attempting 71 * to query the presence of an ineligible element may throw an exception, 72 * or it may simply return false; some implementations will exhibit the former 73 * behavior and some will exhibit the latter. More generally, attempting an 74 * operation on an ineligible element whose completion would not result in 75 * the insertion of an ineligible element into the collection may throw an 76 * exception or it may succeed, at the option of the implementation. 77 * Such exceptions are marked as "optional" in the specification for this 78 * interface. 79 * 80 * <p>It is up to each collection to determine its own synchronization 81 * policy. In the absence of a stronger guarantee by the 82 * implementation, undefined behavior may result from the invocation 83 * of any method on a collection that is being mutated by another 84 * thread; this includes direct invocations, passing the collection to 85 * a method that might perform invocations, and using an existing 86 * iterator to examine the collection. 87 * 88 * <p>Many methods in Collections Framework interfaces are defined in 89 * terms of the {@link Object#equals(Object) equals} method. For example, 90 * the specification for the {@link #contains(Object) contains(Object o)} 91 * method says: "returns <tt>true</tt> if and only if this collection 92 * contains at least one element <tt>e</tt> such that 93 * <tt>(o==null ? e==null : o.equals(e))</tt>." This specification should 94 * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt> 95 * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be 96 * invoked for any element <tt>e</tt>. Implementations are free to implement 97 * optimizations whereby the <tt>equals</tt> invocation is avoided, for 98 * example, by first comparing the hash codes of the two elements. (The 99 * {@link Object#hashCode()} specification guarantees that two objects with 100 * unequal hash codes cannot be equal.) More generally, implementations of 101 * the various Collections Framework interfaces are free to take advantage of 102 * the specified behavior of underlying {@link Object} methods wherever the 103 * implementor deems it appropriate. 104 * 105 * <p>This interface is a member of the 106 * <a href="{@docRoot}/../technotes/guides/collections/index.html"> 107 * Java Collections Framework</a>. 108 * 109 * @param <E> the type of elements in this collection 110 * 111 * @author Josh Bloch 112 * @author Neal Gafter 113 * @see Set 114 * @see List 115 * @see Map 116 * @see SortedSet 117 * @see SortedMap 118 * @see HashSet 119 * @see TreeSet 120 * @see ArrayList 121 * @see LinkedList 122 * @see Vector 123 * @see Collections 124 * @see Arrays 125 * @see AbstractCollection 126 * @since 1.2 127 */ 128 129public interface Collection<E> extends Iterable<E> { 130 // Query Operations 131 132 /** 133 * Returns the number of elements in this collection. If this collection 134 * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns 135 * <tt>Integer.MAX_VALUE</tt>. 136 * 137 * @return the number of elements in this collection 138 */ 139 int size(); 140 141 /** 142 * Returns <tt>true</tt> if this collection contains no elements. 143 * 144 * @return <tt>true</tt> if this collection contains no elements 145 */ 146 boolean isEmpty(); 147 148 /** 149 * Returns <tt>true</tt> if this collection contains the specified element. 150 * More formally, returns <tt>true</tt> if and only if this collection 151 * contains at least one element <tt>e</tt> such that 152 * <tt>(o==null ? e==null : o.equals(e))</tt>. 153 * 154 * @param o element whose presence in this collection is to be tested 155 * @return <tt>true</tt> if this collection contains the specified 156 * element 157 * @throws ClassCastException if the type of the specified element 158 * is incompatible with this collection 159 * (<a href="#optional-restrictions">optional</a>) 160 * @throws NullPointerException if the specified element is null and this 161 * collection does not permit null elements 162 * (<a href="#optional-restrictions">optional</a>) 163 */ 164 boolean contains(Object o); 165 166 /** 167 * Returns an iterator over the elements in this collection. There are no 168 * guarantees concerning the order in which the elements are returned 169 * (unless this collection is an instance of some class that provides a 170 * guarantee). 171 * 172 * @return an <tt>Iterator</tt> over the elements in this collection 173 */ 174 Iterator<E> iterator(); 175 176 /** 177 * Returns an array containing all of the elements in this collection. 178 * If this collection makes any guarantees as to what order its elements 179 * are returned by its iterator, this method must return the elements in 180 * the same order. 181 * 182 * <p>The returned array will be "safe" in that no references to it are 183 * maintained by this collection. (In other words, this method must 184 * allocate a new array even if this collection is backed by an array). 185 * The caller is thus free to modify the returned array. 186 * 187 * <p>This method acts as bridge between array-based and collection-based 188 * APIs. 189 * 190 * @return an array containing all of the elements in this collection 191 */ 192 Object[] toArray(); 193 194 /** 195 * Returns an array containing all of the elements in this collection; 196 * the runtime type of the returned array is that of the specified array. 197 * If the collection fits in the specified array, it is returned therein. 198 * Otherwise, a new array is allocated with the runtime type of the 199 * specified array and the size of this collection. 200 * 201 * <p>If this collection fits in the specified array with room to spare 202 * (i.e., the array has more elements than this collection), the element 203 * in the array immediately following the end of the collection is set to 204 * <tt>null</tt>. (This is useful in determining the length of this 205 * collection <i>only</i> if the caller knows that this collection does 206 * not contain any <tt>null</tt> elements.) 207 * 208 * <p>If this collection makes any guarantees as to what order its elements 209 * are returned by its iterator, this method must return the elements in 210 * the same order. 211 * 212 * <p>Like the {@link #toArray()} method, this method acts as bridge between 213 * array-based and collection-based APIs. Further, this method allows 214 * precise control over the runtime type of the output array, and may, 215 * under certain circumstances, be used to save allocation costs. 216 * 217 * <p>Suppose <tt>x</tt> is a collection known to contain only strings. 218 * The following code can be used to dump the collection into a newly 219 * allocated array of <tt>String</tt>: 220 * 221 * <pre> 222 * String[] y = x.toArray(new String[0]);</pre> 223 * 224 * Note that <tt>toArray(new Object[0])</tt> is identical in function to 225 * <tt>toArray()</tt>. 226 * 227 * @param a the array into which the elements of this collection are to be 228 * stored, if it is big enough; otherwise, a new array of the same 229 * runtime type is allocated for this purpose. 230 * @return an array containing all of the elements in this collection 231 * @throws ArrayStoreException if the runtime type of the specified array 232 * is not a supertype of the runtime type of every element in 233 * this collection 234 * @throws NullPointerException if the specified array is null 235 */ 236 <T> T[] toArray(T[] a); 237 238 // Modification Operations 239 240 /** 241 * Ensures that this collection contains the specified element (optional 242 * operation). Returns <tt>true</tt> if this collection changed as a 243 * result of the call. (Returns <tt>false</tt> if this collection does 244 * not permit duplicates and already contains the specified element.)<p> 245 * 246 * Collections that support this operation may place limitations on what 247 * elements may be added to this collection. In particular, some 248 * collections will refuse to add <tt>null</tt> elements, and others will 249 * impose restrictions on the type of elements that may be added. 250 * Collection classes should clearly specify in their documentation any 251 * restrictions on what elements may be added.<p> 252 * 253 * If a collection refuses to add a particular element for any reason 254 * other than that it already contains the element, it <i>must</i> throw 255 * an exception (rather than returning <tt>false</tt>). This preserves 256 * the invariant that a collection always contains the specified element 257 * after this call returns. 258 * 259 * @param e element whose presence in this collection is to be ensured 260 * @return <tt>true</tt> if this collection changed as a result of the 261 * call 262 * @throws UnsupportedOperationException if the <tt>add</tt> operation 263 * is not supported by this collection 264 * @throws ClassCastException if the class of the specified element 265 * prevents it from being added to this collection 266 * @throws NullPointerException if the specified element is null and this 267 * collection does not permit null elements 268 * @throws IllegalArgumentException if some property of the element 269 * prevents it from being added to this collection 270 * @throws IllegalStateException if the element cannot be added at this 271 * time due to insertion restrictions 272 */ 273 boolean add(E e); 274 275 /** 276 * Removes a single instance of the specified element from this 277 * collection, if it is present (optional operation). More formally, 278 * removes an element <tt>e</tt> such that 279 * <tt>(o==null ? e==null : o.equals(e))</tt>, if 280 * this collection contains one or more such elements. Returns 281 * <tt>true</tt> if this collection contained the specified element (or 282 * equivalently, if this collection changed as a result of the call). 283 * 284 * @param o element to be removed from this collection, if present 285 * @return <tt>true</tt> if an element was removed as a result of this call 286 * @throws ClassCastException if the type of the specified element 287 * is incompatible with this collection 288 * (<a href="#optional-restrictions">optional</a>) 289 * @throws NullPointerException if the specified element is null and this 290 * collection does not permit null elements 291 * (<a href="#optional-restrictions">optional</a>) 292 * @throws UnsupportedOperationException if the <tt>remove</tt> operation 293 * is not supported by this collection 294 */ 295 boolean remove(Object o); 296 297 298 // Bulk Operations 299 300 /** 301 * Returns <tt>true</tt> if this collection contains all of the elements 302 * in the specified collection. 303 * 304 * @param c collection to be checked for containment in this collection 305 * @return <tt>true</tt> if this collection contains all of the elements 306 * in the specified collection 307 * @throws ClassCastException if the types of one or more elements 308 * in the specified collection are incompatible with this 309 * collection 310 * (<a href="#optional-restrictions">optional</a>) 311 * @throws NullPointerException if the specified collection contains one 312 * or more null elements and this collection does not permit null 313 * elements 314 * (<a href="#optional-restrictions">optional</a>), 315 * or if the specified collection is null. 316 * @see #contains(Object) 317 */ 318 boolean containsAll(Collection<?> c); 319 320 /** 321 * Adds all of the elements in the specified collection to this collection 322 * (optional operation). The behavior of this operation is undefined if 323 * the specified collection is modified while the operation is in progress. 324 * (This implies that the behavior of this call is undefined if the 325 * specified collection is this collection, and this collection is 326 * nonempty.) 327 * 328 * @param c collection containing elements to be added to this collection 329 * @return <tt>true</tt> if this collection changed as a result of the call 330 * @throws UnsupportedOperationException if the <tt>addAll</tt> operation 331 * is not supported by this collection 332 * @throws ClassCastException if the class of an element of the specified 333 * collection prevents it from being added to this collection 334 * @throws NullPointerException if the specified collection contains a 335 * null element and this collection does not permit null elements, 336 * or if the specified collection is null 337 * @throws IllegalArgumentException if some property of an element of the 338 * specified collection prevents it from being added to this 339 * collection 340 * @throws IllegalStateException if not all the elements can be added at 341 * this time due to insertion restrictions 342 * @see #add(Object) 343 */ 344 boolean addAll(Collection<? extends E> c); 345 346 /** 347 * Removes all of this collection's elements that are also contained in the 348 * specified collection (optional operation). After this call returns, 349 * this collection will contain no elements in common with the specified 350 * collection. 351 * 352 * @param c collection containing elements to be removed from this collection 353 * @return <tt>true</tt> if this collection changed as a result of the 354 * call 355 * @throws UnsupportedOperationException if the <tt>removeAll</tt> method 356 * is not supported by this collection 357 * @throws ClassCastException if the types of one or more elements 358 * in this collection are incompatible with the specified 359 * collection 360 * (<a href="#optional-restrictions">optional</a>) 361 * @throws NullPointerException if this collection contains one or more 362 * null elements and the specified collection does not support 363 * null elements 364 * (<a href="#optional-restrictions">optional</a>), 365 * or if the specified collection is null 366 * @see #remove(Object) 367 * @see #contains(Object) 368 */ 369 boolean removeAll(Collection<?> c); 370 371 372 /** 373 * Removes all of the elements of this collection that satisfy the given 374 * predicate. Errors or runtime exceptions thrown during iteration or by 375 * the predicate are relayed to the caller. 376 * 377 * @implSpec 378 * The default implementation traverses all elements of the collection using 379 * its {@link #iterator}. Each matching element is removed using 380 * {@link Iterator#remove()}. If the collection's iterator does not 381 * support removal then an {@code UnsupportedOperationException} will be 382 * thrown on the first matching element. 383 * 384 * @param filter a predicate which returns {@code true} for elements to be 385 * removed 386 * @return {@code true} if any elements were removed 387 * @throws NullPointerException if the specified filter is null 388 * @throws UnsupportedOperationException if elements cannot be removed 389 * from this collection. Implementations may throw this exception if a 390 * matching element cannot be removed or if, in general, removal is not 391 * supported. 392 * @since 1.8 393 */ 394 default boolean removeIf(Predicate<? super E> filter) { 395 Objects.requireNonNull(filter); 396 boolean removed = false; 397 final Iterator<E> each = iterator(); 398 while (each.hasNext()) { 399 if (filter.test(each.next())) { 400 each.remove(); 401 removed = true; 402 } 403 } 404 return removed; 405 } 406 407 /** 408 * Retains only the elements in this collection that are contained in the 409 * specified collection (optional operation). In other words, removes from 410 * this collection all of its elements that are not contained in the 411 * specified collection. 412 * 413 * @param c collection containing elements to be retained in this collection 414 * @return <tt>true</tt> if this collection changed as a result of the call 415 * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation 416 * is not supported by this collection 417 * @throws ClassCastException if the types of one or more elements 418 * in this collection are incompatible with the specified 419 * collection 420 * (<a href="#optional-restrictions">optional</a>) 421 * @throws NullPointerException if this collection contains one or more 422 * null elements and the specified collection does not permit null 423 * elements 424 * (<a href="#optional-restrictions">optional</a>), 425 * or if the specified collection is null 426 * @see #remove(Object) 427 * @see #contains(Object) 428 */ 429 boolean retainAll(Collection<?> c); 430 431 /** 432 * Removes all of the elements from this collection (optional operation). 433 * The collection will be empty after this method returns. 434 * 435 * @throws UnsupportedOperationException if the <tt>clear</tt> operation 436 * is not supported by this collection 437 */ 438 void clear(); 439 440 441 // Comparison and hashing 442 443 /** 444 * Compares the specified object with this collection for equality. <p> 445 * 446 * While the <tt>Collection</tt> interface adds no stipulations to the 447 * general contract for the <tt>Object.equals</tt>, programmers who 448 * implement the <tt>Collection</tt> interface "directly" (in other words, 449 * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt> 450 * or a <tt>List</tt>) must exercise care if they choose to override the 451 * <tt>Object.equals</tt>. It is not necessary to do so, and the simplest 452 * course of action is to rely on <tt>Object</tt>'s implementation, but 453 * the implementor may wish to implement a "value comparison" in place of 454 * the default "reference comparison." (The <tt>List</tt> and 455 * <tt>Set</tt> interfaces mandate such value comparisons.)<p> 456 * 457 * The general contract for the <tt>Object.equals</tt> method states that 458 * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and 459 * only if <tt>b.equals(a)</tt>). The contracts for <tt>List.equals</tt> 460 * and <tt>Set.equals</tt> state that lists are only equal to other lists, 461 * and sets to other sets. Thus, a custom <tt>equals</tt> method for a 462 * collection class that implements neither the <tt>List</tt> nor 463 * <tt>Set</tt> interface must return <tt>false</tt> when this collection 464 * is compared to any list or set. (By the same logic, it is not possible 465 * to write a class that correctly implements both the <tt>Set</tt> and 466 * <tt>List</tt> interfaces.) 467 * 468 * @param o object to be compared for equality with this collection 469 * @return <tt>true</tt> if the specified object is equal to this 470 * collection 471 * 472 * @see Object#equals(Object) 473 * @see Set#equals(Object) 474 * @see List#equals(Object) 475 */ 476 boolean equals(Object o); 477 478 /** 479 * Returns the hash code value for this collection. While the 480 * <tt>Collection</tt> interface adds no stipulations to the general 481 * contract for the <tt>Object.hashCode</tt> method, programmers should 482 * take note that any class that overrides the <tt>Object.equals</tt> 483 * method must also override the <tt>Object.hashCode</tt> method in order 484 * to satisfy the general contract for the <tt>Object.hashCode</tt> method. 485 * In particular, <tt>c1.equals(c2)</tt> implies that 486 * <tt>c1.hashCode()==c2.hashCode()</tt>. 487 * 488 * @return the hash code value for this collection 489 * 490 * @see Object#hashCode() 491 * @see Object#equals(Object) 492 */ 493 int hashCode(); 494 495 // TODO: Restore links to #stream and #parallelStream. 496 /** 497 * Creates a {@link Spliterator} over the elements in this collection. 498 * 499 * Implementations should document characteristic values reported by the 500 * spliterator. Such characteristic values are not required to be reported 501 * if the spliterator reports {@link Spliterator#SIZED} and this collection 502 * contains no elements. 503 * 504 * <p>The default implementation should be overridden by subclasses that 505 * can return a more efficient spliterator. In order to 506 * preserve expected laziness behavior for the {@code stream} and 507 * {@code parallelStream()}} methods, spliterators should either have the 508 * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be 509 * <em><a href="Spliterator.html#binding">late-binding</a></em>. 510 * If none of these is practical, the overriding class should describe the 511 * spliterator's documented policy of binding and structural interference, 512 * and should override the {@code stream} and {@code parallelStream} 513 * methods to create streams using a {@code Supplier} of the spliterator, 514 * as in: 515 * <pre>{@code 516 * Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics) 517 * }</pre> 518 * <p>These requirements ensure that streams produced by the 519 * {@code stream} and {@code parallelStream} methods will reflect the 520 * contents of the collection as of initiation of the terminal stream 521 * operation. 522 * 523 * @implSpec 524 * The default implementation creates a 525 * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator 526 * from the collections's {@code Iterator}. The spliterator inherits the 527 * <em>fail-fast</em> properties of the collection's iterator. 528 * <p> 529 * The created {@code Spliterator} reports {@link Spliterator#SIZED}. 530 * 531 * @implNote 532 * The created {@code Spliterator} additionally reports 533 * {@link Spliterator#SUBSIZED}. 534 * 535 * <p>If a spliterator covers no elements then the reporting of additional 536 * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED}, 537 * does not aid clients to control, specialize or simplify computation. 538 * However, this does enable shared use of an immutable and empty 539 * spliterator instance (see {@link Spliterators#emptySpliterator()}) for 540 * empty collections, and enables clients to determine if such a spliterator 541 * covers no elements. 542 * 543 * @return a {@code Spliterator} over the elements in this collection 544 * @since 1.8 545 */ 546 @Override 547 default Spliterator<E> spliterator() { 548 return Spliterators.spliterator(this, 0); 549 } 550} 551