1 /* 2 * Copyright (c) 1997, 2018, 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.IntFunction; 29 import java.util.function.Predicate; 30 import java.util.stream.Stream; 31 import java.util.stream.StreamSupport; 32 33 /** 34 * The root interface in the <i>collection hierarchy</i>. A collection 35 * represents a group of objects, known as its <i>elements</i>. Some 36 * collections allow duplicate elements and others do not. Some are ordered 37 * and others unordered. The JDK does not provide any <i>direct</i> 38 * implementations of this interface: it provides implementations of more 39 * specific subinterfaces like {@code Set} and {@code List}. This interface 40 * is typically used to pass collections around and manipulate them where 41 * maximum generality is desired. 42 * 43 * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain 44 * duplicate elements) should implement this interface directly. 45 * 46 * <p>All general-purpose {@code Collection} implementation classes (which 47 * typically implement {@code Collection} indirectly through one of its 48 * subinterfaces) should provide two "standard" constructors: a void (no 49 * arguments) constructor, which creates an empty collection, and a 50 * constructor with a single argument of type {@code Collection}, which 51 * creates a new collection with the same elements as its argument. In 52 * effect, the latter constructor allows the user to copy any collection, 53 * producing an equivalent collection of the desired implementation type. 54 * There is no way to enforce this convention (as interfaces cannot contain 55 * constructors) but all of the general-purpose {@code Collection} 56 * implementations in the Java platform libraries comply. 57 * 58 * <p>Certain methods are specified to be 59 * <i>optional</i>. If a collection implementation doesn't implement a 60 * particular operation, it should define the corresponding method to throw 61 * {@code UnsupportedOperationException}. Such methods are marked "optional 62 * operation" in method specifications of the collections interfaces. 63 * 64 * <p><a id="optional-restrictions"></a>Some collection implementations 65 * have restrictions on the elements that they may contain. 66 * For example, some implementations prohibit null elements, 67 * and some have restrictions on the types of their elements. Attempting to 68 * add an ineligible element throws an unchecked exception, typically 69 * {@code NullPointerException} or {@code ClassCastException}. Attempting 70 * to query the presence of an ineligible element may throw an exception, 71 * or it may simply return false; some implementations will exhibit the former 72 * behavior and some will exhibit the latter. More generally, attempting an 73 * operation on an ineligible element whose completion would not result in 74 * the insertion of an ineligible element into the collection may throw an 75 * exception or it may succeed, at the option of the implementation. 76 * Such exceptions are marked as "optional" in the specification for this 77 * interface. 78 * 79 * <p>It is up to each collection to determine its own synchronization 80 * policy. In the absence of a stronger guarantee by the 81 * implementation, undefined behavior may result from the invocation 82 * of any method on a collection that is being mutated by another 83 * thread; this includes direct invocations, passing the collection to 84 * a method that might perform invocations, and using an existing 85 * iterator to examine the collection. 86 * 87 * <p>Many methods in Collections Framework interfaces are defined in 88 * terms of the {@link Object#equals(Object) equals} method. For example, 89 * the specification for the {@link #contains(Object) contains(Object o)} 90 * method says: "returns {@code true} if and only if this collection 91 * contains at least one element {@code e} such that 92 * {@code (o==null ? e==null : o.equals(e))}." This specification should 93 * <i>not</i> be construed to imply that invoking {@code Collection.contains} 94 * with a non-null argument {@code o} will cause {@code o.equals(e)} to be 95 * invoked for any element {@code e}. Implementations are free to implement 96 * optimizations whereby the {@code equals} invocation is avoided, for 97 * example, by first comparing the hash codes of the two elements. (The 98 * {@link Object#hashCode()} specification guarantees that two objects with 99 * unequal hash codes cannot be equal.) More generally, implementations of 100 * the various Collections Framework interfaces are free to take advantage of 101 * the specified behavior of underlying {@link Object} methods wherever the 102 * implementor deems it appropriate. 103 * 104 * <p>Some collection operations which perform recursive traversal of the 105 * collection may fail with an exception for self-referential instances where 106 * the collection directly or indirectly contains itself. This includes the 107 * {@code clone()}, {@code equals()}, {@code hashCode()} and {@code toString()} 108 * methods. Implementations may optionally handle the self-referential scenario, 109 * however most current implementations do not do so. 110 * 111 * <h2><a id="view">View Collections</a></h2> 112 * 113 * <p>Most collections manage storage for elements they contain. By contrast, <i>view 114 * collections</i> themselves do not store elements, but instead they rely on a 115 * backing collection to store the actual elements. Operations that are not handled 116 * by the view collection itself are delegated to the backing collection. Examples of 117 * view collections include the wrapper collections returned by methods such as 118 * {@link Collections#checkedCollection Collections.checkedCollection}, 119 * {@link Collections#synchronizedCollection Collections.synchronizedCollection}, and 120 * {@link Collections#unmodifiableCollection Collections.unmodifiableCollection}. 121 * Other examples of view collections include collections that provide a 122 * different representation of the same elements, for example, as 123 * provided by {@link List#subList List.subList}, 124 * {@link NavigableSet#subSet NavigableSet.subSet}, or 125 * {@link Map#entrySet Map.entrySet}. 126 * Any changes made to the backing collection are visible in the view collection. 127 * Correspondingly, any changes made to the view collection — if changes 128 * are permitted — are written through to the backing collection. 129 * Although they technically aren't collections, instances of 130 * {@link Iterator} and {@link ListIterator} can also allow modifications 131 * to be written through to the backing collection, and in some cases, 132 * modifications to the backing collection will be visible to the Iterator 133 * during iteration. 134 * 135 * <h2><a id="unmodifiable">Unmodifiable Collections</a></h2> 136 * 137 * <p>Certain methods of this interface are considered "destructive" and are called 138 * "mutator" methods in that they modify the group of objects contained within 139 * the collection on which they operate. They can be specified to throw 140 * {@code UnsupportedOperationException} if this collection implementation 141 * does not support the operation. Such methods should (but are not required 142 * to) throw an {@code UnsupportedOperationException} if the invocation would 143 * have no effect on the collection. For example, consider a collection that 144 * does not support the {@link #add add} operation. What will happen if the 145 * {@link #addAll addAll} method is invoked on this collection, with an empty 146 * collection as the argument? The addition of zero elements has no effect, 147 * so it is permissible for this collection simply to do nothing and not to throw 148 * an exception. However, it is recommended that such cases throw an exception 149 * unconditionally, as throwing only in certain cases can lead to 150 * programming errors. 151 * 152 * <p>An <i>unmodifiable collection</i> is a collection, all of whose 153 * mutator methods (as defined above) are specified to throw 154 * {@code UnsupportedOperationException}. Such a collection thus cannot be 155 * modified by calling any methods on it. For a collection to be properly 156 * unmodifiable, any view collections derived from it must also be unmodifiable. 157 * For example, if a List is unmodifiable, the List returned by 158 * {@link List#subList List.subList} is also unmodifiable. 159 * 160 * <p>An unmodifiable collection is not necessarily immutable. If the 161 * contained elements are mutable, the entire collection is clearly 162 * mutable, even though it might be unmodifiable. For example, consider 163 * two unmodifiable lists containing mutable elements. The result of calling 164 * {@code list1.equals(list2)} might differ from one call to the next if 165 * the elements had been mutated, even though both lists are unmodifiable. 166 * However, if an unmodifiable collection contains all immutable elements, 167 * it can be considered effectively immutable. 168 * 169 * <h2><a id="unmodview">Unmodifiable View Collections</a></h2> 170 * 171 * <p>An <i>unmodifiable view collection</i> is a collection that is unmodifiable 172 * and that is also a view onto a backing collection. Its mutator methods throw 173 * {@code UnsupportedOperationException}, as described above, while 174 * reading and querying methods are delegated to the backing collection. 175 * The effect is to provide read-only access to the backing collection. 176 * This is useful for a component to provide users with read access to 177 * an internal collection, while preventing them from modifying such 178 * collections unexpectedly. Examples of unmodifiable view collections 179 * are those returned by the 180 * {@link Collections#unmodifiableCollection Collections.unmodifiableCollection}, 181 * {@link Collections#unmodifiableList Collections.unmodifiableList}, and 182 * related methods. 183 * 184 * <p>Note that changes to the backing collection might still be possible, 185 * and if they occur, they are visible through the unmodifiable view. Thus, 186 * an unmodifiable view collection is not necessarily immutable. However, 187 * if the backing collection of an unmodifiable view is effectively immutable, 188 * or if the only reference to the backing collection is through an 189 * unmodifiable view, the view can be considered effectively immutable. 190 * 191 * <p>This interface is a member of the 192 * <a href="{@docRoot}/../technotes/guides/collections/index.html"> 193 * Java Collections Framework</a>. 194 * 195 * @implSpec 196 * The default method implementations (inherited or otherwise) do not apply any 197 * synchronization protocol. If a {@code Collection} implementation has a 198 * specific synchronization protocol, then it must override default 199 * implementations to apply that protocol. 200 * 201 * @param <E> the type of elements in this collection 202 * 203 * @author Josh Bloch 204 * @author Neal Gafter 205 * @see Set 206 * @see List 207 * @see Map 208 * @see SortedSet 209 * @see SortedMap 210 * @see HashSet 211 * @see TreeSet 212 * @see ArrayList 213 * @see LinkedList 214 * @see Vector 215 * @see Collections 216 * @see Arrays 217 * @see AbstractCollection 218 * @since 1.2 219 */ 220 221 public interface Collection<E> extends Iterable<E> { 222 // Query Operations 223 224 /** 225 * Returns the number of elements in this collection. If this collection 226 * contains more than {@code Integer.MAX_VALUE} elements, returns 227 * {@code Integer.MAX_VALUE}. 228 * 229 * @return the number of elements in this collection 230 */ size()231 int size(); 232 233 /** 234 * Returns {@code true} if this collection contains no elements. 235 * 236 * @return {@code true} if this collection contains no elements 237 */ isEmpty()238 boolean isEmpty(); 239 240 /** 241 * Returns {@code true} if this collection contains the specified element. 242 * More formally, returns {@code true} if and only if this collection 243 * contains at least one element {@code e} such that 244 * {@code Objects.equals(o, e)}. 245 * 246 * @param o element whose presence in this collection is to be tested 247 * @return {@code true} if this collection contains the specified 248 * element 249 * @throws ClassCastException if the type of the specified element 250 * is incompatible with this collection 251 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) 252 * @throws NullPointerException if the specified element is null and this 253 * collection does not permit null elements 254 * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>) 255 */ contains(Object o)256 boolean contains(Object o); 257 258 /** 259 * Returns an iterator over the elements in this collection. There are no 260 * guarantees concerning the order in which the elements are returned 261 * (unless this collection is an instance of some class that provides a 262 * guarantee). 263 * 264 * @return an {@code Iterator} over the elements in this collection 265 */ iterator()266 Iterator<E> iterator(); 267 268 /** 269 * Returns an array containing all of the elements in this collection. 270 * If this collection makes any guarantees as to what order its elements 271 * are returned by its iterator, this method must return the elements in 272 * the same order. The returned array's {@linkplain Class#getComponentType 273 * runtime component type} is {@code Object}. 274 * 275 * <p>The returned array will be "safe" in that no references to it are 276 * maintained by this collection. (In other words, this method must 277 * allocate a new array even if this collection is backed by an array). 278 * The caller is thus free to modify the returned array. 279 * 280 * @apiNote 281 * This method acts as a bridge between array-based and collection-based APIs. 282 * It returns an array whose runtime type is {@code Object[]}. 283 * Use {@link #toArray(Object[]) toArray(T[])} to reuse an existing 284 * array, or use {@link #toArray(IntFunction)} to control the runtime type 285 * of the array. 286 * 287 * @return an array, whose {@linkplain Class#getComponentType runtime component 288 * type} is {@code Object}, containing all of the elements in this collection 289 */ toArray()290 Object[] toArray(); 291 292 /** 293 * Returns an array containing all of the elements in this collection; 294 * the runtime type of the returned array is that of the specified array. 295 * If the collection fits in the specified array, it is returned therein. 296 * Otherwise, a new array is allocated with the runtime type of the 297 * specified array and the size of this collection. 298 * 299 * <p>If this collection fits in the specified array with room to spare 300 * (i.e., the array has more elements than this collection), the element 301 * in the array immediately following the end of the collection is set to 302 * {@code null}. (This is useful in determining the length of this 303 * collection <i>only</i> if the caller knows that this collection does 304 * not contain any {@code null} elements.) 305 * 306 * <p>If this collection makes any guarantees as to what order its elements 307 * are returned by its iterator, this method must return the elements in 308 * the same order. 309 * 310 * @apiNote 311 * This method acts as a bridge between array-based and collection-based APIs. 312 * It allows an existing array to be reused under certain circumstances. 313 * Use {@link #toArray()} to create an array whose runtime type is {@code Object[]}, 314 * or use {@link #toArray(IntFunction)} to control the runtime type of 315 * the array. 316 * 317 * <p>Suppose {@code x} is a collection known to contain only strings. 318 * The following code can be used to dump the collection into a previously 319 * allocated {@code String} array: 320 * 321 * <pre> 322 * String[] y = new String[SIZE]; 323 * ... 324 * y = x.toArray(y);</pre> 325 * 326 * <p>The return value is reassigned to the variable {@code y}, because a 327 * new array will be allocated and returned if the collection {@code x} has 328 * too many elements to fit into the existing array {@code y}. 329 * 330 * <p>Note that {@code toArray(new Object[0])} is identical in function to 331 * {@code toArray()}. 332 * 333 * @param <T> the component type of the array to contain the collection 334 * @param a the array into which the elements of this collection are to be 335 * stored, if it is big enough; otherwise, a new array of the same 336 * runtime type is allocated for this purpose. 337 * @return an array containing all of the elements in this collection 338 * @throws ArrayStoreException if the runtime type of any element in this 339 * collection is not assignable to the {@linkplain Class#getComponentType 340 * runtime component type} of the specified array 341 * @throws NullPointerException if the specified array is null 342 */ toArray(T[] a)343 <T> T[] toArray(T[] a); 344 345 /** 346 * Returns an array containing all of the elements in this collection, 347 * using the provided {@code generator} function to allocate the returned array. 348 * 349 * <p>If this collection makes any guarantees as to what order its elements 350 * are returned by its iterator, this method must return the elements in 351 * the same order. 352 * 353 * @apiNote 354 * This method acts as a bridge between array-based and collection-based APIs. 355 * It allows creation of an array of a particular runtime type. Use 356 * {@link #toArray()} to create an array whose runtime type is {@code Object[]}, 357 * or use {@link #toArray(Object[]) toArray(T[])} to reuse an existing array. 358 * 359 * <p>Suppose {@code x} is a collection known to contain only strings. 360 * The following code can be used to dump the collection into a newly 361 * allocated array of {@code String}: 362 * 363 * <pre> 364 * String[] y = x.toArray(String[]::new);</pre> 365 * 366 * @implSpec 367 * The default implementation calls the generator function with zero 368 * and then passes the resulting array to {@link #toArray(Object[]) toArray(T[])}. 369 * 370 * @param <T> the component type of the array to contain the collection 371 * @param generator a function which produces a new array of the desired 372 * type and the provided length 373 * @return an array containing all of the elements in this collection 374 * @throws ArrayStoreException if the runtime type of any element in this 375 * collection is not assignable to the {@linkplain Class#getComponentType 376 * runtime component type} of the generated array 377 * @throws NullPointerException if the generator function is null 378 * @since 11 379 */ toArray(IntFunction<T[]> generator)380 default <T> T[] toArray(IntFunction<T[]> generator) { 381 return toArray(generator.apply(0)); 382 } 383 384 // Modification Operations 385 386 /** 387 * Ensures that this collection contains the specified element (optional 388 * operation). Returns {@code true} if this collection changed as a 389 * result of the call. (Returns {@code false} if this collection does 390 * not permit duplicates and already contains the specified element.)<p> 391 * 392 * Collections that support this operation may place limitations on what 393 * elements may be added to this collection. In particular, some 394 * collections will refuse to add {@code null} elements, and others will 395 * impose restrictions on the type of elements that may be added. 396 * Collection classes should clearly specify in their documentation any 397 * restrictions on what elements may be added.<p> 398 * 399 * If a collection refuses to add a particular element for any reason 400 * other than that it already contains the element, it <i>must</i> throw 401 * an exception (rather than returning {@code false}). This preserves 402 * the invariant that a collection always contains the specified element 403 * after this call returns. 404 * 405 * @param e element whose presence in this collection is to be ensured 406 * @return {@code true} if this collection changed as a result of the 407 * call 408 * @throws UnsupportedOperationException if the {@code add} operation 409 * is not supported by this collection 410 * @throws ClassCastException if the class of the specified element 411 * prevents it from being added to this collection 412 * @throws NullPointerException if the specified element is null and this 413 * collection does not permit null elements 414 * @throws IllegalArgumentException if some property of the element 415 * prevents it from being added to this collection 416 * @throws IllegalStateException if the element cannot be added at this 417 * time due to insertion restrictions 418 */ add(E e)419 boolean add(E e); 420 421 /** 422 * Removes a single instance of the specified element from this 423 * collection, if it is present (optional operation). More formally, 424 * removes an element {@code e} such that 425 * {@code Objects.equals(o, e)}, if 426 * this collection contains one or more such elements. Returns 427 * {@code true} if this collection contained the specified element (or 428 * equivalently, if this collection changed as a result of the call). 429 * 430 * @param o element to be removed from this collection, if present 431 * @return {@code true} if an element was removed as a result of this call 432 * @throws ClassCastException if the type of the specified element 433 * is incompatible with this collection 434 * (<a href="#optional-restrictions">optional</a>) 435 * @throws NullPointerException if the specified element is null and this 436 * collection does not permit null elements 437 * (<a href="#optional-restrictions">optional</a>) 438 * @throws UnsupportedOperationException if the <tt>remove</tt> operation 439 * is not supported by this collection 440 */ remove(Object o)441 boolean remove(Object o); 442 443 444 // Bulk Operations 445 446 /** 447 * Returns {@code true} if this collection contains all of the elements 448 * in the specified collection. 449 * 450 * @param c collection to be checked for containment in this collection 451 * @return {@code true} if this collection contains all of the elements 452 * in the specified collection 453 * @throws ClassCastException if the types of one or more elements 454 * in the specified collection are incompatible with this 455 * collection 456 * (<a href="#optional-restrictions">optional</a>) 457 * @throws NullPointerException if the specified collection contains one 458 * or more null elements and this collection does not permit null 459 * elements 460 * (<a href="#optional-restrictions">optional</a>), 461 * or if the specified collection is null. 462 * @see #contains(Object) 463 */ containsAll(Collection<?> c)464 boolean containsAll(Collection<?> c); 465 466 /** 467 * Adds all of the elements in the specified collection to this collection 468 * (optional operation). The behavior of this operation is undefined if 469 * the specified collection is modified while the operation is in progress. 470 * (This implies that the behavior of this call is undefined if the 471 * specified collection is this collection, and this collection is 472 * nonempty.) 473 * 474 * @param c collection containing elements to be added to this collection 475 * @return {@code true} if this collection changed as a result of the call 476 * @throws UnsupportedOperationException if the {@code addAll} operation 477 * is not supported by this collection 478 * @throws ClassCastException if the class of an element of the specified 479 * collection prevents it from being added to this collection 480 * @throws NullPointerException if the specified collection contains a 481 * null element and this collection does not permit null elements, 482 * or if the specified collection is null 483 * @throws IllegalArgumentException if some property of an element of the 484 * specified collection prevents it from being added to this 485 * collection 486 * @throws IllegalStateException if not all the elements can be added at 487 * this time due to insertion restrictions 488 * @see #add(Object) 489 */ addAll(Collection<? extends E> c)490 boolean addAll(Collection<? extends E> c); 491 492 /** 493 * Removes all of this collection's elements that are also contained in the 494 * specified collection (optional operation). After this call returns, 495 * this collection will contain no elements in common with the specified 496 * collection. 497 * 498 * @param c collection containing elements to be removed from this collection 499 * @return {@code true} if this collection changed as a result of the 500 * call 501 * @throws UnsupportedOperationException if the {@code removeAll} method 502 * is not supported by this collection 503 * @throws ClassCastException if the types of one or more elements 504 * in this collection are incompatible with the specified 505 * collection 506 * (<a href="#optional-restrictions">optional</a>) 507 * @throws NullPointerException if this collection contains one or more 508 * null elements and the specified collection does not support 509 * null elements 510 * (<a href="#optional-restrictions">optional</a>), 511 * or if the specified collection is null 512 * @see #remove(Object) 513 * @see #contains(Object) 514 */ removeAll(Collection<?> c)515 boolean removeAll(Collection<?> c); 516 517 /** 518 * Removes all of the elements of this collection that satisfy the given 519 * predicate. Errors or runtime exceptions thrown during iteration or by 520 * the predicate are relayed to the caller. 521 * 522 * @implSpec 523 * The default implementation traverses all elements of the collection using 524 * its {@link #iterator}. Each matching element is removed using 525 * {@link Iterator#remove()}. If the collection's iterator does not 526 * support removal then an {@code UnsupportedOperationException} will be 527 * thrown on the first matching element. 528 * 529 * @param filter a predicate which returns {@code true} for elements to be 530 * removed 531 * @return {@code true} if any elements were removed 532 * @throws NullPointerException if the specified filter is null 533 * @throws UnsupportedOperationException if elements cannot be removed 534 * from this collection. Implementations may throw this exception if a 535 * matching element cannot be removed or if, in general, removal is not 536 * supported. 537 * @since 1.8 538 */ removeIf(Predicate<? super E> filter)539 default boolean removeIf(Predicate<? super E> filter) { 540 Objects.requireNonNull(filter); 541 boolean removed = false; 542 final Iterator<E> each = iterator(); 543 while (each.hasNext()) { 544 if (filter.test(each.next())) { 545 each.remove(); 546 removed = true; 547 } 548 } 549 return removed; 550 } 551 552 /** 553 * Retains only the elements in this collection that are contained in the 554 * specified collection (optional operation). In other words, removes from 555 * this collection all of its elements that are not contained in the 556 * specified collection. 557 * 558 * @param c collection containing elements to be retained in this collection 559 * @return {@code true} if this collection changed as a result of the call 560 * @throws UnsupportedOperationException if the {@code retainAll} operation 561 * is not supported by this collection 562 * @throws ClassCastException if the types of one or more elements 563 * in this collection are incompatible with the specified 564 * collection 565 * (<a href="#optional-restrictions">optional</a>) 566 * @throws NullPointerException if this collection contains one or more 567 * null elements and the specified collection does not permit null 568 * elements 569 * (<a href="#optional-restrictions">optional</a>), 570 * or if the specified collection is null 571 * @see #remove(Object) 572 * @see #contains(Object) 573 */ retainAll(Collection<?> c)574 boolean retainAll(Collection<?> c); 575 576 /** 577 * Removes all of the elements from this collection (optional operation). 578 * The collection will be empty after this method returns. 579 * 580 * @throws UnsupportedOperationException if the {@code clear} operation 581 * is not supported by this collection 582 */ clear()583 void clear(); 584 585 586 // Comparison and hashing 587 588 /** 589 * Compares the specified object with this collection for equality. <p> 590 * 591 * While the {@code Collection} interface adds no stipulations to the 592 * general contract for the {@code Object.equals}, programmers who 593 * implement the {@code Collection} interface "directly" (in other words, 594 * create a class that is a {@code Collection} but is not a {@code Set} 595 * or a {@code List}) must exercise care if they choose to override the 596 * {@code Object.equals}. It is not necessary to do so, and the simplest 597 * course of action is to rely on {@code Object}'s implementation, but 598 * the implementor may wish to implement a "value comparison" in place of 599 * the default "reference comparison." (The {@code List} and 600 * {@code Set} interfaces mandate such value comparisons.)<p> 601 * 602 * The general contract for the {@code Object.equals} method states that 603 * equals must be symmetric (in other words, {@code a.equals(b)} if and 604 * only if {@code b.equals(a)}). The contracts for {@code List.equals} 605 * and {@code Set.equals} state that lists are only equal to other lists, 606 * and sets to other sets. Thus, a custom {@code equals} method for a 607 * collection class that implements neither the {@code List} nor 608 * {@code Set} interface must return {@code false} when this collection 609 * is compared to any list or set. (By the same logic, it is not possible 610 * to write a class that correctly implements both the {@code Set} and 611 * {@code List} interfaces.) 612 * 613 * @param o object to be compared for equality with this collection 614 * @return {@code true} if the specified object is equal to this 615 * collection 616 * 617 * @see Object#equals(Object) 618 * @see Set#equals(Object) 619 * @see List#equals(Object) 620 */ equals(Object o)621 boolean equals(Object o); 622 623 /** 624 * Returns the hash code value for this collection. While the 625 * {@code Collection} interface adds no stipulations to the general 626 * contract for the {@code Object.hashCode} method, programmers should 627 * take note that any class that overrides the {@code Object.equals} 628 * method must also override the {@code Object.hashCode} method in order 629 * to satisfy the general contract for the {@code Object.hashCode} method. 630 * In particular, {@code c1.equals(c2)} implies that 631 * {@code c1.hashCode()==c2.hashCode()}. 632 * 633 * @return the hash code value for this collection 634 * 635 * @see Object#hashCode() 636 * @see Object#equals(Object) 637 */ hashCode()638 int hashCode(); 639 640 /** 641 * Creates a {@link Spliterator} over the elements in this collection. 642 * 643 * Implementations should document characteristic values reported by the 644 * spliterator. Such characteristic values are not required to be reported 645 * if the spliterator reports {@link Spliterator#SIZED} and this collection 646 * contains no elements. 647 * 648 * <p>The default implementation should be overridden by subclasses that 649 * can return a more efficient spliterator. In order to 650 * preserve expected laziness behavior for the {@link #stream()} and 651 * {@link #parallelStream()} methods, spliterators should either have the 652 * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be 653 * <em><a href="Spliterator.html#binding">late-binding</a></em>. 654 * If none of these is practical, the overriding class should describe the 655 * spliterator's documented policy of binding and structural interference, 656 * and should override the {@link #stream()} and {@link #parallelStream()} 657 * methods to create streams using a {@code Supplier} of the spliterator, 658 * as in: 659 * <pre>{@code 660 * Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics) 661 * }</pre> 662 * <p>These requirements ensure that streams produced by the 663 * {@link #stream()} and {@link #parallelStream()} methods will reflect the 664 * contents of the collection as of initiation of the terminal stream 665 * operation. 666 * 667 * @implSpec 668 * The default implementation creates a 669 * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator 670 * from the collection's {@code Iterator}. The spliterator inherits the 671 * <em>fail-fast</em> properties of the collection's iterator. 672 * <p> 673 * The created {@code Spliterator} reports {@link Spliterator#SIZED}. 674 * 675 * @implNote 676 * The created {@code Spliterator} additionally reports 677 * {@link Spliterator#SUBSIZED}. 678 * 679 * <p>If a spliterator covers no elements then the reporting of additional 680 * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED}, 681 * does not aid clients to control, specialize or simplify computation. 682 * However, this does enable shared use of an immutable and empty 683 * spliterator instance (see {@link Spliterators#emptySpliterator()}) for 684 * empty collections, and enables clients to determine if such a spliterator 685 * covers no elements. 686 * 687 * @return a {@code Spliterator} over the elements in this collection 688 * @since 1.8 689 */ 690 @Override spliterator()691 default Spliterator<E> spliterator() { 692 return Spliterators.spliterator(this, 0); 693 } 694 695 /** 696 * Returns a sequential {@code Stream} with this collection as its source. 697 * 698 * <p>This method should be overridden when the {@link #spliterator()} 699 * method cannot return a spliterator that is {@code IMMUTABLE}, 700 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()} 701 * for details.) 702 * 703 * @implSpec 704 * The default implementation creates a sequential {@code Stream} from the 705 * collection's {@code Spliterator}. 706 * 707 * @return a sequential {@code Stream} over the elements in this collection 708 * @since 1.8 709 */ stream()710 default Stream<E> stream() { 711 return StreamSupport.stream(spliterator(), false); 712 } 713 714 /** 715 * Returns a possibly parallel {@code Stream} with this collection as its 716 * source. It is allowable for this method to return a sequential stream. 717 * 718 * <p>This method should be overridden when the {@link #spliterator()} 719 * method cannot return a spliterator that is {@code IMMUTABLE}, 720 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()} 721 * for details.) 722 * 723 * @implSpec 724 * The default implementation creates a parallel {@code Stream} from the 725 * collection's {@code Spliterator}. 726 * 727 * @return a possibly parallel {@code Stream} over the elements in this 728 * collection 729 * @since 1.8 730 */ parallelStream()731 default Stream<E> parallelStream() { 732 return StreamSupport.stream(spliterator(), true); 733 } 734 } 735