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