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 26 package java.util; 27 28 import java.util.function.Predicate; 29 import java.util.stream.Stream; 30 import java.util.stream.StreamSupport; 31 32 /** 33 * The root interface in the <i>collection hierarchy</i>. A collection 34 * represents a group of objects, known as its <i>elements</i>. Some 35 * collections allow duplicate elements and others do not. Some are ordered 36 * and others unordered. The JDK does not provide any <i>direct</i> 37 * implementations of this interface: it provides implementations of more 38 * specific subinterfaces like <tt>Set</tt> and <tt>List</tt>. This interface 39 * is typically used to pass collections around and manipulate them where 40 * maximum generality is desired. 41 * 42 * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain 43 * duplicate elements) should implement this interface directly. 44 * 45 * <p>All general-purpose <tt>Collection</tt> implementation classes (which 46 * typically implement <tt>Collection</tt> indirectly through one of its 47 * subinterfaces) should provide two "standard" constructors: a void (no 48 * arguments) constructor, which creates an empty collection, and a 49 * constructor with a single argument of type <tt>Collection</tt>, which 50 * creates a new collection with the same elements as its argument. In 51 * effect, the latter constructor allows the user to copy any collection, 52 * producing an equivalent collection of the desired implementation type. 53 * There is no way to enforce this convention (as interfaces cannot contain 54 * constructors) but all of the general-purpose <tt>Collection</tt> 55 * implementations in the Java platform libraries comply. 56 * 57 * <p>The "destructive" methods contained in this interface, that is, the 58 * methods that modify the collection on which they operate, are specified to 59 * throw <tt>UnsupportedOperationException</tt> if this collection does not 60 * support the operation. If this is the case, these methods may, but are not 61 * required to, throw an <tt>UnsupportedOperationException</tt> if the 62 * invocation would have no effect on the collection. For example, invoking 63 * the {@link #addAll(Collection)} method on an unmodifiable collection may, 64 * but is not required to, throw the exception if the collection to be added 65 * is empty. 66 * 67 * <p><a name="optional-restrictions"> 68 * Some collection implementations have restrictions on the elements that 69 * they may contain.</a> For example, some implementations prohibit null elements, 70 * and some have restrictions on the types of their elements. Attempting to 71 * add an ineligible element throws an unchecked exception, typically 72 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>. Attempting 73 * to query the presence of an ineligible element may throw an exception, 74 * or it may simply return false; some implementations will exhibit the former 75 * behavior and some will exhibit the latter. More generally, attempting an 76 * operation on an ineligible element whose completion would not result in 77 * the insertion of an ineligible element into the collection may throw an 78 * exception or it may succeed, at the option of the implementation. 79 * Such exceptions are marked as "optional" in the specification for this 80 * interface. 81 * 82 * <p>It is up to each collection to determine its own synchronization 83 * policy. In the absence of a stronger guarantee by the 84 * implementation, undefined behavior may result from the invocation 85 * of any method on a collection that is being mutated by another 86 * thread; this includes direct invocations, passing the collection to 87 * a method that might perform invocations, and using an existing 88 * iterator to examine the collection. 89 * 90 * <p>Many methods in Collections Framework interfaces are defined in 91 * terms of the {@link Object#equals(Object) equals} method. For example, 92 * the specification for the {@link #contains(Object) contains(Object o)} 93 * method says: "returns <tt>true</tt> if and only if this collection 94 * contains at least one element <tt>e</tt> such that 95 * <tt>(o==null ? e==null : o.equals(e))</tt>." This specification should 96 * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt> 97 * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be 98 * invoked for any element <tt>e</tt>. Implementations are free to implement 99 * optimizations whereby the <tt>equals</tt> invocation is avoided, for 100 * example, by first comparing the hash codes of the two elements. (The 101 * {@link Object#hashCode()} specification guarantees that two objects with 102 * unequal hash codes cannot be equal.) More generally, implementations of 103 * the various Collections Framework interfaces are free to take advantage of 104 * the specified behavior of underlying {@link Object} methods wherever the 105 * implementor deems it appropriate. 106 * 107 * <p>Some collection operations which perform recursive traversal of the 108 * collection may fail with an exception for self-referential instances where 109 * the collection directly or indirectly contains itself. This includes the 110 * {@code clone()}, {@code equals()}, {@code hashCode()} and {@code toString()} 111 * methods. Implementations may optionally handle the self-referential scenario, 112 * however most current implementations do not do so. 113 * 114 * <p>This interface is a member of the 115 * <a href="{@docRoot}openjdk-redirect.html?v=8&path=/technotes/guides/collections/index.html"> 116 * Java Collections Framework</a>. 117 * 118 * @implSpec 119 * The default method implementations (inherited or otherwise) do not apply any 120 * synchronization protocol. If a {@code Collection} implementation has a 121 * specific synchronization protocol, then it must override default 122 * implementations to apply that protocol. 123 * 124 * @param <E> the type of elements in this collection 125 * 126 * @author Josh Bloch 127 * @author Neal Gafter 128 * @see Set 129 * @see List 130 * @see Map 131 * @see SortedSet 132 * @see SortedMap 133 * @see HashSet 134 * @see TreeSet 135 * @see ArrayList 136 * @see LinkedList 137 * @see Vector 138 * @see Collections 139 * @see Arrays 140 * @see AbstractCollection 141 * @since 1.2 142 */ 143 144 public interface Collection<E> extends Iterable<E> { 145 // Query Operations 146 147 /** 148 * Returns the number of elements in this collection. If this collection 149 * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns 150 * <tt>Integer.MAX_VALUE</tt>. 151 * 152 * @return the number of elements in this collection 153 */ 154 int size(); 155 156 /** 157 * Returns <tt>true</tt> if this collection contains no elements. 158 * 159 * @return <tt>true</tt> if this collection contains no elements 160 */ 161 boolean isEmpty(); 162 163 /** 164 * Returns <tt>true</tt> if this collection contains the specified element. 165 * More formally, returns <tt>true</tt> if and only if this collection 166 * contains at least one element <tt>e</tt> such that 167 * <tt>(o==null ? e==null : o.equals(e))</tt>. 168 * 169 * @param o element whose presence in this collection is to be tested 170 * @return <tt>true</tt> if this collection contains the specified 171 * element 172 * @throws ClassCastException if the type of the specified element 173 * is incompatible with this collection 174 * (<a href="#optional-restrictions">optional</a>) 175 * @throws NullPointerException if the specified element is null and this 176 * collection does not permit null elements 177 * (<a href="#optional-restrictions">optional</a>) 178 */ 179 boolean contains(Object o); 180 181 /** 182 * Returns an iterator over the elements in this collection. There are no 183 * guarantees concerning the order in which the elements are returned 184 * (unless this collection is an instance of some class that provides a 185 * guarantee). 186 * 187 * @return an <tt>Iterator</tt> over the elements in this collection 188 */ 189 Iterator<E> iterator(); 190 191 /** 192 * Returns an array containing all of the elements in this collection. 193 * If this collection makes any guarantees as to what order its elements 194 * are returned by its iterator, this method must return the elements in 195 * the same order. 196 * 197 * <p>The returned array will be "safe" in that no references to it are 198 * maintained by this collection. (In other words, this method must 199 * allocate a new array even if this collection is backed by an array). 200 * The caller is thus free to modify the returned array. 201 * 202 * <p>This method acts as bridge between array-based and collection-based 203 * APIs. 204 * 205 * @return an array containing all of the elements in this collection 206 */ 207 Object[] toArray(); 208 209 /** 210 * Returns an array containing all of the elements in this collection; 211 * the runtime type of the returned array is that of the specified array. 212 * If the collection fits in the specified array, it is returned therein. 213 * Otherwise, a new array is allocated with the runtime type of the 214 * specified array and the size of this collection. 215 * 216 * <p>If this collection fits in the specified array with room to spare 217 * (i.e., the array has more elements than this collection), the element 218 * in the array immediately following the end of the collection is set to 219 * <tt>null</tt>. (This is useful in determining the length of this 220 * collection <i>only</i> if the caller knows that this collection does 221 * not contain any <tt>null</tt> elements.) 222 * 223 * <p>If this collection makes any guarantees as to what order its elements 224 * are returned by its iterator, this method must return the elements in 225 * the same order. 226 * 227 * <p>Like the {@link #toArray()} method, this method acts as bridge between 228 * array-based and collection-based APIs. Further, this method allows 229 * precise control over the runtime type of the output array, and may, 230 * under certain circumstances, be used to save allocation costs. 231 * 232 * <p>Suppose <tt>x</tt> is a collection known to contain only strings. 233 * The following code can be used to dump the collection into a newly 234 * allocated array of <tt>String</tt>: 235 * 236 * <pre> 237 * String[] y = x.toArray(new String[0]);</pre> 238 * 239 * Note that <tt>toArray(new Object[0])</tt> is identical in function to 240 * <tt>toArray()</tt>. 241 * 242 * @param <T> the runtime type of the array to contain the collection 243 * @param a the array into which the elements of this collection are to be 244 * stored, if it is big enough; otherwise, a new array of the same 245 * runtime type is allocated for this purpose. 246 * @return an array containing all of the elements in this collection 247 * @throws ArrayStoreException if the runtime type of the specified array 248 * is not a supertype of the runtime type of every element in 249 * this collection 250 * @throws NullPointerException if the specified array is null 251 */ 252 <T> T[] toArray(T[] a); 253 254 // Modification Operations 255 256 /** 257 * Ensures that this collection contains the specified element (optional 258 * operation). Returns <tt>true</tt> if this collection changed as a 259 * result of the call. (Returns <tt>false</tt> if this collection does 260 * not permit duplicates and already contains the specified element.)<p> 261 * 262 * Collections that support this operation may place limitations on what 263 * elements may be added to this collection. In particular, some 264 * collections will refuse to add <tt>null</tt> elements, and others will 265 * impose restrictions on the type of elements that may be added. 266 * Collection classes should clearly specify in their documentation any 267 * restrictions on what elements may be added.<p> 268 * 269 * If a collection refuses to add a particular element for any reason 270 * other than that it already contains the element, it <i>must</i> throw 271 * an exception (rather than returning <tt>false</tt>). This preserves 272 * the invariant that a collection always contains the specified element 273 * after this call returns. 274 * 275 * @param e element whose presence in this collection is to be ensured 276 * @return <tt>true</tt> if this collection changed as a result of the 277 * call 278 * @throws UnsupportedOperationException if the <tt>add</tt> operation 279 * is not supported by this collection 280 * @throws ClassCastException if the class of the specified element 281 * prevents it from being added to this collection 282 * @throws NullPointerException if the specified element is null and this 283 * collection does not permit null elements 284 * @throws IllegalArgumentException if some property of the element 285 * prevents it from being added to this collection 286 * @throws IllegalStateException if the element cannot be added at this 287 * time due to insertion restrictions 288 */ 289 boolean add(E e); 290 291 /** 292 * Removes a single instance of the specified element from this 293 * collection, if it is present (optional operation). More formally, 294 * removes an element <tt>e</tt> such that 295 * <tt>(o==null ? e==null : o.equals(e))</tt>, if 296 * this collection contains one or more such elements. Returns 297 * <tt>true</tt> if this collection contained the specified element (or 298 * equivalently, if this collection changed as a result of the call). 299 * 300 * @param o element to be removed from this collection, if present 301 * @return <tt>true</tt> if an element was removed as a result of this call 302 * @throws ClassCastException if the type of the specified element 303 * is incompatible with this collection 304 * (<a href="#optional-restrictions">optional</a>) 305 * @throws NullPointerException if the specified element is null and this 306 * collection does not permit null elements 307 * (<a href="#optional-restrictions">optional</a>) 308 * @throws UnsupportedOperationException if the <tt>remove</tt> operation 309 * is not supported by this collection 310 */ 311 boolean remove(Object o); 312 313 314 // Bulk Operations 315 316 /** 317 * Returns <tt>true</tt> if this collection contains all of the elements 318 * in the specified collection. 319 * 320 * @param c collection to be checked for containment in this collection 321 * @return <tt>true</tt> if this collection contains all of the elements 322 * in the specified collection 323 * @throws ClassCastException if the types of one or more elements 324 * in the specified collection are incompatible with this 325 * collection 326 * (<a href="#optional-restrictions">optional</a>) 327 * @throws NullPointerException if the specified collection contains one 328 * or more null elements and this collection does not permit null 329 * elements 330 * (<a href="#optional-restrictions">optional</a>), 331 * or if the specified collection is null. 332 * @see #contains(Object) 333 */ 334 boolean containsAll(Collection<?> c); 335 336 /** 337 * Adds all of the elements in the specified collection to this collection 338 * (optional operation). The behavior of this operation is undefined if 339 * the specified collection is modified while the operation is in progress. 340 * (This implies that the behavior of this call is undefined if the 341 * specified collection is this collection, and this collection is 342 * nonempty.) 343 * 344 * @param c collection containing elements to be added to this collection 345 * @return <tt>true</tt> if this collection changed as a result of the call 346 * @throws UnsupportedOperationException if the <tt>addAll</tt> operation 347 * is not supported by this collection 348 * @throws ClassCastException if the class of an element of the specified 349 * collection prevents it from being added to this collection 350 * @throws NullPointerException if the specified collection contains a 351 * null element and this collection does not permit null elements, 352 * or if the specified collection is null 353 * @throws IllegalArgumentException if some property of an element of the 354 * specified collection prevents it from being added to this 355 * collection 356 * @throws IllegalStateException if not all the elements can be added at 357 * this time due to insertion restrictions 358 * @see #add(Object) 359 */ 360 boolean addAll(Collection<? extends E> c); 361 362 /** 363 * Removes all of this collection's elements that are also contained in the 364 * specified collection (optional operation). After this call returns, 365 * this collection will contain no elements in common with the specified 366 * collection. 367 * 368 * @param c collection containing elements to be removed from this collection 369 * @return <tt>true</tt> if this collection changed as a result of the 370 * call 371 * @throws UnsupportedOperationException if the <tt>removeAll</tt> method 372 * is not supported by this collection 373 * @throws ClassCastException if the types of one or more elements 374 * in this collection are incompatible with the specified 375 * collection 376 * (<a href="#optional-restrictions">optional</a>) 377 * @throws NullPointerException if this collection contains one or more 378 * null elements and the specified collection does not support 379 * null elements 380 * (<a href="#optional-restrictions">optional</a>), 381 * or if the specified collection is null 382 * @see #remove(Object) 383 * @see #contains(Object) 384 */ 385 boolean removeAll(Collection<?> c); 386 387 /** 388 * Removes all of the elements of this collection that satisfy the given 389 * predicate. Errors or runtime exceptions thrown during iteration or by 390 * the predicate are relayed to the caller. 391 * 392 * @implSpec 393 * The default implementation traverses all elements of the collection using 394 * its {@link #iterator}. Each matching element is removed using 395 * {@link Iterator#remove()}. If the collection's iterator does not 396 * support removal then an {@code UnsupportedOperationException} will be 397 * thrown on the first matching element. 398 * 399 * @param filter a predicate which returns {@code true} for elements to be 400 * removed 401 * @return {@code true} if any elements were removed 402 * @throws NullPointerException if the specified filter is null 403 * @throws UnsupportedOperationException if elements cannot be removed 404 * from this collection. Implementations may throw this exception if a 405 * matching element cannot be removed or if, in general, removal is not 406 * supported. 407 * @since 1.8 408 */ 409 default boolean removeIf(Predicate<? super E> filter) { 410 Objects.requireNonNull(filter); 411 boolean removed = false; 412 final Iterator<E> each = iterator(); 413 while (each.hasNext()) { 414 if (filter.test(each.next())) { 415 each.remove(); 416 removed = true; 417 } 418 } 419 return removed; 420 } 421 422 /** 423 * Retains only the elements in this collection that are contained in the 424 * specified collection (optional operation). In other words, removes from 425 * this collection all of its elements that are not contained in the 426 * specified collection. 427 * 428 * @param c collection containing elements to be retained in this collection 429 * @return <tt>true</tt> if this collection changed as a result of the call 430 * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation 431 * is not supported by this collection 432 * @throws ClassCastException if the types of one or more elements 433 * in this collection are incompatible with the specified 434 * collection 435 * (<a href="#optional-restrictions">optional</a>) 436 * @throws NullPointerException if this collection contains one or more 437 * null elements and the specified collection does not permit null 438 * elements 439 * (<a href="#optional-restrictions">optional</a>), 440 * or if the specified collection is null 441 * @see #remove(Object) 442 * @see #contains(Object) 443 */ 444 boolean retainAll(Collection<?> c); 445 446 /** 447 * Removes all of the elements from this collection (optional operation). 448 * The collection will be empty after this method returns. 449 * 450 * @throws UnsupportedOperationException if the <tt>clear</tt> operation 451 * is not supported by this collection 452 */ 453 void clear(); 454 455 456 // Comparison and hashing 457 458 /** 459 * Compares the specified object with this collection for equality. <p> 460 * 461 * While the <tt>Collection</tt> interface adds no stipulations to the 462 * general contract for the <tt>Object.equals</tt>, programmers who 463 * implement the <tt>Collection</tt> interface "directly" (in other words, 464 * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt> 465 * or a <tt>List</tt>) must exercise care if they choose to override the 466 * <tt>Object.equals</tt>. It is not necessary to do so, and the simplest 467 * course of action is to rely on <tt>Object</tt>'s implementation, but 468 * the implementor may wish to implement a "value comparison" in place of 469 * the default "reference comparison." (The <tt>List</tt> and 470 * <tt>Set</tt> interfaces mandate such value comparisons.)<p> 471 * 472 * The general contract for the <tt>Object.equals</tt> method states that 473 * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and 474 * only if <tt>b.equals(a)</tt>). The contracts for <tt>List.equals</tt> 475 * and <tt>Set.equals</tt> state that lists are only equal to other lists, 476 * and sets to other sets. Thus, a custom <tt>equals</tt> method for a 477 * collection class that implements neither the <tt>List</tt> nor 478 * <tt>Set</tt> interface must return <tt>false</tt> when this collection 479 * is compared to any list or set. (By the same logic, it is not possible 480 * to write a class that correctly implements both the <tt>Set</tt> and 481 * <tt>List</tt> interfaces.) 482 * 483 * @param o object to be compared for equality with this collection 484 * @return <tt>true</tt> if the specified object is equal to this 485 * collection 486 * 487 * @see Object#equals(Object) 488 * @see Set#equals(Object) 489 * @see List#equals(Object) 490 */ 491 boolean equals(Object o); 492 493 /** 494 * Returns the hash code value for this collection. While the 495 * <tt>Collection</tt> interface adds no stipulations to the general 496 * contract for the <tt>Object.hashCode</tt> method, programmers should 497 * take note that any class that overrides the <tt>Object.equals</tt> 498 * method must also override the <tt>Object.hashCode</tt> method in order 499 * to satisfy the general contract for the <tt>Object.hashCode</tt> method. 500 * In particular, <tt>c1.equals(c2)</tt> implies that 501 * <tt>c1.hashCode()==c2.hashCode()</tt>. 502 * 503 * @return the hash code value for this collection 504 * 505 * @see Object#hashCode() 506 * @see Object#equals(Object) 507 */ 508 int hashCode(); 509 510 /** 511 * Creates a {@link Spliterator} over the elements in this collection. 512 * 513 * Implementations should document characteristic values reported by the 514 * spliterator. Such characteristic values are not required to be reported 515 * if the spliterator reports {@link Spliterator#SIZED} and this collection 516 * contains no elements. 517 * 518 * <p>The default implementation should be overridden by subclasses that 519 * can return a more efficient spliterator. In order to 520 * preserve expected laziness behavior for the {@link #stream()} and 521 * {@link #parallelStream()}} methods, spliterators should either have the 522 * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be 523 * <em><a href="Spliterator.html#binding">late-binding</a></em>. 524 * If none of these is practical, the overriding class should describe the 525 * spliterator's documented policy of binding and structural interference, 526 * and should override the {@link #stream()} and {@link #parallelStream()} 527 * methods to create streams using a {@code Supplier} of the spliterator, 528 * as in: 529 * <pre>{@code 530 * Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics) 531 * }</pre> 532 * <p>These requirements ensure that streams produced by the 533 * {@link #stream()} and {@link #parallelStream()} methods will reflect the 534 * contents of the collection as of initiation of the terminal stream 535 * operation. 536 * 537 * @implSpec 538 * The default implementation creates a 539 * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator 540 * from the collections's {@code Iterator}. The spliterator inherits the 541 * <em>fail-fast</em> properties of the collection's iterator. 542 * <p> 543 * The created {@code Spliterator} reports {@link Spliterator#SIZED}. 544 * 545 * @implNote 546 * The created {@code Spliterator} additionally reports 547 * {@link Spliterator#SUBSIZED}. 548 * 549 * <p>If a spliterator covers no elements then the reporting of additional 550 * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED}, 551 * does not aid clients to control, specialize or simplify computation. 552 * However, this does enable shared use of an immutable and empty 553 * spliterator instance (see {@link Spliterators#emptySpliterator()}) for 554 * empty collections, and enables clients to determine if such a spliterator 555 * covers no elements. 556 * 557 * @return a {@code Spliterator} over the elements in this collection 558 * @since 1.8 559 */ 560 @Override 561 default Spliterator<E> spliterator() { 562 return Spliterators.spliterator(this, 0); 563 } 564 565 /** 566 * Returns a sequential {@code Stream} with this collection as its source. 567 * 568 * <p>This method should be overridden when the {@link #spliterator()} 569 * method cannot return a spliterator that is {@code IMMUTABLE}, 570 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()} 571 * for details.) 572 * 573 * @implSpec 574 * The default implementation creates a sequential {@code Stream} from the 575 * collection's {@code Spliterator}. 576 * 577 * @return a sequential {@code Stream} over the elements in this collection 578 * @since 1.8 579 */ 580 default Stream<E> stream() { 581 return StreamSupport.stream(spliterator(), false); 582 } 583 584 /** 585 * Returns a possibly parallel {@code Stream} with this collection as its 586 * source. It is allowable for this method to return a sequential stream. 587 * 588 * <p>This method should be overridden when the {@link #spliterator()} 589 * method cannot return a spliterator that is {@code IMMUTABLE}, 590 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()} 591 * for details.) 592 * 593 * @implSpec 594 * The default implementation creates a parallel {@code Stream} from the 595 * collection's {@code Spliterator}. 596 * 597 * @return a possibly parallel {@code Stream} over the elements in this 598 * collection 599 * @since 1.8 600 */ 601 default Stream<E> parallelStream() { 602 return StreamSupport.stream(spliterator(), true); 603 } 604 } 605
