1 /* 2 * Copyright (c) 1997, 2017, 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 {@code Set} and {@code List}. 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 {@code Collection} implementation classes (which 46 * typically implement {@code Collection} 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 {@code Collection}, 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 {@code Collection} 55 * implementations in the Java platform libraries comply. 56 * 57 * <p>Certain methods are specified to be 58 * <i>optional</i>. If a collection implementation doesn't implement a 59 * particular operation, it should define the corresponding method to throw 60 * {@code UnsupportedOperationException}. Such methods are marked "optional 61 * operation" in method specifications of the collections interfaces. 62 * 63 * <p><a id="optional-restrictions"></a>Some collection implementations 64 * have restrictions on the elements that they may contain. 65 * 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 * {@code NullPointerException} or {@code ClassCastException}. 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 {@code true} if and only if this collection 90 * contains at least one element {@code e} such that 91 * {@code (o==null ? e==null : o.equals(e))}." This specification should 92 * <i>not</i> be construed to imply that invoking {@code Collection.contains} 93 * with a non-null argument {@code o} will cause {@code o.equals(e)} to be 94 * invoked for any element {@code e}. Implementations are free to implement 95 * optimizations whereby the {@code equals} 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>Some collection operations which perform recursive traversal of the 104 * collection may fail with an exception for self-referential instances where 105 * the collection directly or indirectly contains itself. This includes the 106 * {@code clone()}, {@code equals()}, {@code hashCode()} and {@code toString()} 107 * methods. Implementations may optionally handle the self-referential scenario, 108 * however most current implementations do not do so. 109 * 110 * <h2><a id="view">View Collections</a></h2> 111 * 112 * <p>Most collections manage storage for elements they contain. By contrast, <i>view 113 * collections</i> themselves do not store elements, but instead they rely on a 114 * backing collection to store the actual elements. Operations that are not handled 115 * by the view collection itself are delegated to the backing collection. Examples of 116 * view collections include the wrapper collections returned by methods such as 117 * {@link Collections#checkedCollection Collections.checkedCollection}, 118 * {@link Collections#synchronizedCollection Collections.synchronizedCollection}, and 119 * {@link Collections#unmodifiableCollection Collections.unmodifiableCollection}. 120 * Other examples of view collections include collections that provide a 121 * different representation of the same elements, for example, as 122 * provided by {@link List#subList List.subList}, 123 * {@link NavigableSet#subSet NavigableSet.subSet}, or 124 * {@link Map#entrySet Map.entrySet}. 125 * Any changes made to the backing collection are visible in the view collection. 126 * Correspondingly, any changes made to the view collection — if changes 127 * are permitted — are written through to the backing collection. 128 * Although they technically aren't collections, instances of 129 * {@link Iterator} and {@link ListIterator} can also allow modifications 130 * to be written through to the backing collection, and in some cases, 131 * modifications to the backing collection will be visible to the Iterator 132 * during iteration. 133 * 134 * <h2><a id="unmodifiable">Unmodifiable Collections</a></h2> 135 * 136 * <p>Certain methods of this interface are considered "destructive" and are called 137 * "mutator" methods in that they modify the group of objects contained within 138 * the collection on which they operate. They can be specified to throw 139 * {@code UnsupportedOperationException} if this collection implementation 140 * does not support the operation. Such methods should (but are not required 141 * to) throw an {@code UnsupportedOperationException} if the invocation would 142 * have no effect on the collection. For example, consider a collection that 143 * does not support the {@link #add add} operation. What will happen if the 144 * {@link #addAll addAll} method is invoked on this collection, with an empty 145 * collection as the argument? The addition of zero elements has no effect, 146 * so it is permissible for this collection simply to do nothing and not to throw 147 * an exception. However, it is recommended that such cases throw an exception 148 * unconditionally, as throwing only in certain cases can lead to 149 * programming errors. 150 * 151 * <p>An <i>unmodifiable collection</i> is a collection, all of whose 152 * mutator methods (as defined above) are specified to throw 153 * {@code UnsupportedOperationException}. Such a collection thus cannot be 154 * modified by calling any methods on it. For a collection to be properly 155 * unmodifiable, any view collections derived from it must also be unmodifiable. 156 * For example, if a List is unmodifiable, the List returned by 157 * {@link List#subList List.subList} is also unmodifiable. 158 * 159 * <p>An unmodifiable collection is not necessarily immutable. If the 160 * contained elements are mutable, the entire collection is clearly 161 * mutable, even though it might be unmodifiable. For example, consider 162 * two unmodifiable lists containing mutable elements. The result of calling 163 * {@code list1.equals(list2)} might differ from one call to the next if 164 * the elements had been mutated, even though both lists are unmodifiable. 165 * However, if an unmodifiable collection contains all immutable elements, 166 * it can be considered effectively immutable. 167 * 168 * <h2><a id="unmodview">Unmodifiable View Collections</a></h2> 169 * 170 * <p>An <i>unmodifiable view collection</i> is a collection that is unmodifiable 171 * and that is also a view onto a backing collection. Its mutator methods throw 172 * {@code UnsupportedOperationException}, as described above, while 173 * reading and querying methods are delegated to the backing collection. 174 * The effect is to provide read-only access to the backing collection. 175 * This is useful for a component to provide users with read access to 176 * an internal collection, while preventing them from modifying such 177 * collections unexpectedly. Examples of unmodifiable view collections 178 * are those returned by the 179 * {@link Collections#unmodifiableCollection Collections.unmodifiableCollection}, 180 * {@link Collections#unmodifiableList Collections.unmodifiableList}, and 181 * related methods. 182 * 183 * <p>Note that changes to the backing collection might still be possible, 184 * and if they occur, they are visible through the unmodifiable view. Thus, 185 * an unmodifiable view collection is not necessarily immutable. However, 186 * if the backing collection of an unmodifiable view is effectively immutable, 187 * or if the only reference to the backing collection is through an 188 * unmodifiable view, the view can be considered effectively immutable. 189 * 190 * <p>This interface is a member of the 191 * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework"> 192 * Java Collections Framework</a>. 193 * 194 * @implSpec 195 * The default method implementations (inherited or otherwise) do not apply any 196 * synchronization protocol. If a {@code Collection} implementation has a 197 * specific synchronization protocol, then it must override default 198 * implementations to apply that protocol. 199 * 200 * @param <E> the type of elements in this collection 201 * 202 * @author Josh Bloch 203 * @author Neal Gafter 204 * @see Set 205 * @see List 206 * @see Map 207 * @see SortedSet 208 * @see SortedMap 209 * @see HashSet 210 * @see TreeSet 211 * @see ArrayList 212 * @see LinkedList 213 * @see Vector 214 * @see Collections 215 * @see Arrays 216 * @see AbstractCollection 217 * @since 1.2 218 */ 219 220 public interface Collection<E> extends Iterable<E> { 221 // Query Operations 222 223 /** 224 * Returns the number of elements in this collection. If this collection 225 * contains more than {@code Integer.MAX_VALUE} elements, returns 226 * {@code Integer.MAX_VALUE}. 227 * 228 * @return the number of elements in this collection 229 */ 230 int size(); 231 232 /** 233 * Returns {@code true} if this collection contains no elements. 234 * 235 * @return {@code true} if this collection contains no elements 236 */ 237 boolean isEmpty(); 238 239 /** 240 * Returns {@code true} if this collection contains the specified element. 241 * More formally, returns {@code true} if and only if this collection 242 * contains at least one element {@code e} such that 243 * {@code Objects.equals(o, e)}. 244 * 245 * @param o element whose presence in this collection is to be tested 246 * @return {@code true} if this collection contains the specified 247 * element 248 * @throws ClassCastException if the type of the specified element 249 * is incompatible with this collection 250 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) 251 * @throws NullPointerException if the specified element is null and this 252 * collection does not permit null elements 253 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) 254 */ 255 boolean contains(Object o); 256 257 /** 258 * Returns an iterator over the elements in this collection. There are no 259 * guarantees concerning the order in which the elements are returned 260 * (unless this collection is an instance of some class that provides a 261 * guarantee). 262 * 263 * @return an {@code Iterator} over the elements in this collection 264 */ 265 Iterator<E> iterator(); 266 267 /** 268 * Returns an array containing all of the elements in this collection. 269 * If this collection makes any guarantees as to what order its elements 270 * are returned by its iterator, this method must return the elements in 271 * the same order. The returned array's {@linkplain Class#getComponentType 272 * runtime component type} is {@code Object}. 273 * 274 * <p>The returned array will be "safe" in that no references to it are 275 * maintained by this collection. (In other words, this method must 276 * allocate a new array even if this collection is backed by an array). 277 * The caller is thus free to modify the returned array. 278 * 279 * <p>This method acts as bridge between array-based and collection-based 280 * APIs. 281 * 282 * @return an array, whose {@linkplain Class#getComponentType runtime component 283 * type} is {@code Object}, containing all of the elements in this collection 284 */ 285 Object[] toArray(); 286 287 /** 288 * Returns an array containing all of the elements in this collection; 289 * the runtime type of the returned array is that of the specified array. 290 * If the collection fits in the specified array, it is returned therein. 291 * Otherwise, a new array is allocated with the runtime type of the 292 * specified array and the size of this collection. 293 * 294 * <p>If this collection fits in the specified array with room to spare 295 * (i.e., the array has more elements than this collection), the element 296 * in the array immediately following the end of the collection is set to 297 * {@code null}. (This is useful in determining the length of this 298 * collection <i>only</i> if the caller knows that this collection does 299 * not contain any {@code null} elements.) 300 * 301 * <p>If this collection makes any guarantees as to what order its elements 302 * are returned by its iterator, this method must return the elements in 303 * the same order. 304 * 305 * <p>Like the {@link #toArray()} method, this method acts as bridge between 306 * array-based and collection-based APIs. Further, this method allows 307 * precise control over the runtime type of the output array, and may, 308 * under certain circumstances, be used to save allocation costs. 309 * 310 * <p>Suppose {@code x} is a collection known to contain only strings. 311 * The following code can be used to dump the collection into a newly 312 * allocated array of {@code String}: 313 * 314 * <pre> 315 * String[] y = x.toArray(new String[0]);</pre> 316 * 317 * Note that {@code toArray(new Object[0])} is identical in function to 318 * {@code toArray()}. 319 * 320 * @param <T> the component type of the array to contain the collection 321 * @param a the array into which the elements of this collection are to be 322 * stored, if it is big enough; otherwise, a new array of the same 323 * runtime type is allocated for this purpose. 324 * @return an array containing all of the elements in this collection 325 * @throws ArrayStoreException if the runtime type of any element in this 326 * collection is not assignable to the {@linkplain Class#getComponentType 327 * runtime component type} of the specified array 328 * @throws NullPointerException if the specified array is null 329 */ 330 <T> T[] toArray(T[] a); 331 332 // Modification Operations 333 334 /** 335 * Ensures that this collection contains the specified element (optional 336 * operation). Returns {@code true} if this collection changed as a 337 * result of the call. (Returns {@code false} if this collection does 338 * not permit duplicates and already contains the specified element.)<p> 339 * 340 * Collections that support this operation may place limitations on what 341 * elements may be added to this collection. In particular, some 342 * collections will refuse to add {@code null} elements, and others will 343 * impose restrictions on the type of elements that may be added. 344 * Collection classes should clearly specify in their documentation any 345 * restrictions on what elements may be added.<p> 346 * 347 * If a collection refuses to add a particular element for any reason 348 * other than that it already contains the element, it <i>must</i> throw 349 * an exception (rather than returning {@code false}). This preserves 350 * the invariant that a collection always contains the specified element 351 * after this call returns. 352 * 353 * @param e element whose presence in this collection is to be ensured 354 * @return {@code true} if this collection changed as a result of the 355 * call 356 * @throws UnsupportedOperationException if the {@code add} operation 357 * is not supported by this collection 358 * @throws ClassCastException if the class of the specified element 359 * prevents it from being added to this collection 360 * @throws NullPointerException if the specified element is null and this 361 * collection does not permit null elements 362 * @throws IllegalArgumentException if some property of the element 363 * prevents it from being added to this collection 364 * @throws IllegalStateException if the element cannot be added at this 365 * time due to insertion restrictions 366 */ 367 boolean add(E e); 368 369 /** 370 * Removes a single instance of the specified element from this 371 * collection, if it is present (optional operation). More formally, 372 * removes an element {@code e} such that 373 * {@code Objects.equals(o, e)}, if 374 * this collection contains one or more such elements. Returns 375 * {@code true} if this collection contained the specified element (or 376 * equivalently, if this collection changed as a result of the call). 377 * 378 * @param o element to be removed from this collection, if present 379 * @return {@code true} if an element was removed as a result of this call 380 * @throws ClassCastException if the type of the specified element 381 * is incompatible with this collection 382 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) 383 * @throws NullPointerException if the specified element is null and this 384 * collection does not permit null elements 385 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) 386 * @throws UnsupportedOperationException if the {@code remove} operation 387 * is not supported by this collection 388 */ 389 boolean remove(Object o); 390 391 392 // Bulk Operations 393 394 /** 395 * Returns {@code true} if this collection contains all of the elements 396 * in the specified collection. 397 * 398 * @param c collection to be checked for containment in this collection 399 * @return {@code true} if this collection contains all of the elements 400 * in the specified collection 401 * @throws ClassCastException if the types of one or more elements 402 * in the specified collection are incompatible with this 403 * collection 404 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) 405 * @throws NullPointerException if the specified collection contains one 406 * or more null elements and this collection does not permit null 407 * elements 408 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>), 409 * or if the specified collection is null. 410 * @see #contains(Object) 411 */ 412 boolean containsAll(Collection<?> c); 413 414 /** 415 * Adds all of the elements in the specified collection to this collection 416 * (optional operation). The behavior of this operation is undefined if 417 * the specified collection is modified while the operation is in progress. 418 * (This implies that the behavior of this call is undefined if the 419 * specified collection is this collection, and this collection is 420 * nonempty.) 421 * 422 * @param c collection containing elements to be added to this collection 423 * @return {@code true} if this collection changed as a result of the call 424 * @throws UnsupportedOperationException if the {@code addAll} operation 425 * is not supported by this collection 426 * @throws ClassCastException if the class of an element of the specified 427 * collection prevents it from being added to this collection 428 * @throws NullPointerException if the specified collection contains a 429 * null element and this collection does not permit null elements, 430 * or if the specified collection is null 431 * @throws IllegalArgumentException if some property of an element of the 432 * specified collection prevents it from being added to this 433 * collection 434 * @throws IllegalStateException if not all the elements can be added at 435 * this time due to insertion restrictions 436 * @see #add(Object) 437 */ 438 boolean addAll(Collection<? extends E> c); 439 440 /** 441 * Removes all of this collection's elements that are also contained in the 442 * specified collection (optional operation). After this call returns, 443 * this collection will contain no elements in common with the specified 444 * collection. 445 * 446 * @param c collection containing elements to be removed from this collection 447 * @return {@code true} if this collection changed as a result of the 448 * call 449 * @throws UnsupportedOperationException if the {@code removeAll} method 450 * is not supported by this collection 451 * @throws ClassCastException if the types of one or more elements 452 * in this collection are incompatible with the specified 453 * collection 454 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) 455 * @throws NullPointerException if this collection contains one or more 456 * null elements and the specified collection does not support 457 * null elements 458 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>), 459 * or if the specified collection is null 460 * @see #remove(Object) 461 * @see #contains(Object) 462 */ 463 boolean removeAll(Collection<?> c); 464 465 /** 466 * Removes all of the elements of this collection that satisfy the given 467 * predicate. Errors or runtime exceptions thrown during iteration or by 468 * the predicate are relayed to the caller. 469 * 470 * @implSpec 471 * The default implementation traverses all elements of the collection using 472 * its {@link #iterator}. Each matching element is removed using 473 * {@link Iterator#remove()}. If the collection's iterator does not 474 * support removal then an {@code UnsupportedOperationException} will be 475 * thrown on the first matching element. 476 * 477 * @param filter a predicate which returns {@code true} for elements to be 478 * removed 479 * @return {@code true} if any elements were removed 480 * @throws NullPointerException if the specified filter is null 481 * @throws UnsupportedOperationException if elements cannot be removed 482 * from this collection. Implementations may throw this exception if a 483 * matching element cannot be removed or if, in general, removal is not 484 * supported. 485 * @since 1.8 486 */ 487 default boolean removeIf(Predicate<? super E> filter) { 488 Objects.requireNonNull(filter); 489 boolean removed = false; 490 final Iterator<E> each = iterator(); 491 while (each.hasNext()) { 492 if (filter.test(each.next())) { 493 each.remove(); 494 removed = true; 495 } 496 } 497 return removed; 498 } 499 500 /** 501 * Retains only the elements in this collection that are contained in the 502 * specified collection (optional operation). In other words, removes from 503 * this collection all of its elements that are not contained in the 504 * specified collection. 505 * 506 * @param c collection containing elements to be retained in this collection 507 * @return {@code true} if this collection changed as a result of the call 508 * @throws UnsupportedOperationException if the {@code retainAll} operation 509 * is not supported by this collection 510 * @throws ClassCastException if the types of one or more elements 511 * in this collection are incompatible with the specified 512 * collection 513 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) 514 * @throws NullPointerException if this collection contains one or more 515 * null elements and the specified collection does not permit null 516 * elements 517 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>), 518 * or if the specified collection is null 519 * @see #remove(Object) 520 * @see #contains(Object) 521 */ 522 boolean retainAll(Collection<?> c); 523 524 /** 525 * Removes all of the elements from this collection (optional operation). 526 * The collection will be empty after this method returns. 527 * 528 * @throws UnsupportedOperationException if the {@code clear} operation 529 * is not supported by this collection 530 */ 531 void clear(); 532 533 534 // Comparison and hashing 535 536 /** 537 * Compares the specified object with this collection for equality. <p> 538 * 539 * While the {@code Collection} interface adds no stipulations to the 540 * general contract for the {@code Object.equals}, programmers who 541 * implement the {@code Collection} interface "directly" (in other words, 542 * create a class that is a {@code Collection} but is not a {@code Set} 543 * or a {@code List}) must exercise care if they choose to override the 544 * {@code Object.equals}. It is not necessary to do so, and the simplest 545 * course of action is to rely on {@code Object}'s implementation, but 546 * the implementor may wish to implement a "value comparison" in place of 547 * the default "reference comparison." (The {@code List} and 548 * {@code Set} interfaces mandate such value comparisons.)<p> 549 * 550 * The general contract for the {@code Object.equals} method states that 551 * equals must be symmetric (in other words, {@code a.equals(b)} if and 552 * only if {@code b.equals(a)}). The contracts for {@code List.equals} 553 * and {@code Set.equals} state that lists are only equal to other lists, 554 * and sets to other sets. Thus, a custom {@code equals} method for a 555 * collection class that implements neither the {@code List} nor 556 * {@code Set} interface must return {@code false} when this collection 557 * is compared to any list or set. (By the same logic, it is not possible 558 * to write a class that correctly implements both the {@code Set} and 559 * {@code List} interfaces.) 560 * 561 * @param o object to be compared for equality with this collection 562 * @return {@code true} if the specified object is equal to this 563 * collection 564 * 565 * @see Object#equals(Object) 566 * @see Set#equals(Object) 567 * @see List#equals(Object) 568 */ 569 boolean equals(Object o); 570 571 /** 572 * Returns the hash code value for this collection. While the 573 * {@code Collection} interface adds no stipulations to the general 574 * contract for the {@code Object.hashCode} method, programmers should 575 * take note that any class that overrides the {@code Object.equals} 576 * method must also override the {@code Object.hashCode} method in order 577 * to satisfy the general contract for the {@code Object.hashCode} method. 578 * In particular, {@code c1.equals(c2)} implies that 579 * {@code c1.hashCode()==c2.hashCode()}. 580 * 581 * @return the hash code value for this collection 582 * 583 * @see Object#hashCode() 584 * @see Object#equals(Object) 585 */ 586 int hashCode(); 587 588 /** 589 * Creates a {@link Spliterator} over the elements in this collection. 590 * 591 * Implementations should document characteristic values reported by the 592 * spliterator. Such characteristic values are not required to be reported 593 * if the spliterator reports {@link Spliterator#SIZED} and this collection 594 * contains no elements. 595 * 596 * <p>The default implementation should be overridden by subclasses that 597 * can return a more efficient spliterator. In order to 598 * preserve expected laziness behavior for the {@link #stream()} and 599 * {@link #parallelStream()} methods, spliterators should either have the 600 * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be 601 * <em><a href="Spliterator.html#binding">late-binding</a></em>. 602 * If none of these is practical, the overriding class should describe the 603 * spliterator's documented policy of binding and structural interference, 604 * and should override the {@link #stream()} and {@link #parallelStream()} 605 * methods to create streams using a {@code Supplier} of the spliterator, 606 * as in: 607 * <pre>{@code 608 * Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics) 609 * }</pre> 610 * <p>These requirements ensure that streams produced by the 611 * {@link #stream()} and {@link #parallelStream()} methods will reflect the 612 * contents of the collection as of initiation of the terminal stream 613 * operation. 614 * 615 * @implSpec 616 * The default implementation creates a 617 * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator 618 * from the collection's {@code Iterator}. The spliterator inherits the 619 * <em>fail-fast</em> properties of the collection's iterator. 620 * <p> 621 * The created {@code Spliterator} reports {@link Spliterator#SIZED}. 622 * 623 * @implNote 624 * The created {@code Spliterator} additionally reports 625 * {@link Spliterator#SUBSIZED}. 626 * 627 * <p>If a spliterator covers no elements then the reporting of additional 628 * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED}, 629 * does not aid clients to control, specialize or simplify computation. 630 * However, this does enable shared use of an immutable and empty 631 * spliterator instance (see {@link Spliterators#emptySpliterator()}) for 632 * empty collections, and enables clients to determine if such a spliterator 633 * covers no elements. 634 * 635 * @return a {@code Spliterator} over the elements in this collection 636 * @since 1.8 637 */ 638 @Override 639 default Spliterator<E> spliterator() { 640 return Spliterators.spliterator(this, 0); 641 } 642 643 /** 644 * Returns a sequential {@code Stream} with this collection as its source. 645 * 646 * <p>This method should be overridden when the {@link #spliterator()} 647 * method cannot return a spliterator that is {@code IMMUTABLE}, 648 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()} 649 * for details.) 650 * 651 * @implSpec 652 * The default implementation creates a sequential {@code Stream} from the 653 * collection's {@code Spliterator}. 654 * 655 * @return a sequential {@code Stream} over the elements in this collection 656 * @since 1.8 657 */ 658 default Stream<E> stream() { 659 return StreamSupport.stream(spliterator(), false); 660 } 661 662 /** 663 * Returns a possibly parallel {@code Stream} with this collection as its 664 * source. It is allowable for this method to return a sequential stream. 665 * 666 * <p>This method should be overridden when the {@link #spliterator()} 667 * method cannot return a spliterator that is {@code IMMUTABLE}, 668 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()} 669 * for details.) 670 * 671 * @implSpec 672 * The default implementation creates a parallel {@code Stream} from the 673 * collection's {@code Spliterator}. 674 * 675 * @return a possibly parallel {@code Stream} over the elements in this 676 * collection 677 * @since 1.8 678 */ 679 default Stream<E> parallelStream() { 680 return StreamSupport.stream(spliterator(), true); 681 } 682 }