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. 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 a the array into which the elements of this collection are to be 236 * stored, if it is big enough; otherwise, a new array of the same 237 * runtime type is allocated for this purpose. 238 * @return an array containing all of the elements in this collection 239 * @throws ArrayStoreException if the runtime type of the specified array 240 * is not a supertype of the runtime type of every element in 241 * this collection 242 * @throws NullPointerException if the specified array is null 243 */ 244 <T> T[] toArray(T[] a); 245 246 // Modification Operations 247 248 /** 249 * Ensures that this collection contains the specified element (optional 250 * operation). Returns <tt>true</tt> if this collection changed as a 251 * result of the call. (Returns <tt>false</tt> if this collection does 252 * not permit duplicates and already contains the specified element.)<p> 253 * 254 * Collections that support this operation may place limitations on what 255 * elements may be added to this collection. In particular, some 256 * collections will refuse to add <tt>null</tt> elements, and others will 257 * impose restrictions on the type of elements that may be added. 258 * Collection classes should clearly specify in their documentation any 259 * restrictions on what elements may be added.<p> 260 * 261 * If a collection refuses to add a particular element for any reason 262 * other than that it already contains the element, it <i>must</i> throw 263 * an exception (rather than returning <tt>false</tt>). This preserves 264 * the invariant that a collection always contains the specified element 265 * after this call returns. 266 * 267 * @param e element whose presence in this collection is to be ensured 268 * @return <tt>true</tt> if this collection changed as a result of the 269 * call 270 * @throws UnsupportedOperationException if the <tt>add</tt> operation 271 * is not supported by this collection 272 * @throws ClassCastException if the class of the specified element 273 * prevents it from being added to this collection 274 * @throws NullPointerException if the specified element is null and this 275 * collection does not permit null elements 276 * @throws IllegalArgumentException if some property of the element 277 * prevents it from being added to this collection 278 * @throws IllegalStateException if the element cannot be added at this 279 * time due to insertion restrictions 280 */ 281 boolean add(E e); 282 283 /** 284 * Removes a single instance of the specified element from this 285 * collection, if it is present (optional operation). More formally, 286 * removes an element <tt>e</tt> such that 287 * <tt>(o==null ? e==null : o.equals(e))</tt>, if 288 * this collection contains one or more such elements. Returns 289 * <tt>true</tt> if this collection contained the specified element (or 290 * equivalently, if this collection changed as a result of the call). 291 * 292 * @param o element to be removed from this collection, if present 293 * @return <tt>true</tt> if an element was removed as a result of this call 294 * @throws ClassCastException if the type of the specified element 295 * is incompatible with this collection 296 * (<a href="#optional-restrictions">optional</a>) 297 * @throws NullPointerException if the specified element is null and this 298 * collection does not permit null elements 299 * (<a href="#optional-restrictions">optional</a>) 300 * @throws UnsupportedOperationException if the <tt>remove</tt> operation 301 * is not supported by this collection 302 */ 303 boolean remove(Object o); 304 305 306 // Bulk Operations 307 308 /** 309 * Returns <tt>true</tt> if this collection contains all of the elements 310 * in the specified collection. 311 * 312 * @param c collection to be checked for containment in this collection 313 * @return <tt>true</tt> if this collection contains all of the elements 314 * in the specified collection 315 * @throws ClassCastException if the types of one or more elements 316 * in the specified collection are incompatible with this 317 * collection 318 * (<a href="#optional-restrictions">optional</a>) 319 * @throws NullPointerException if the specified collection contains one 320 * or more null elements and this collection does not permit null 321 * elements 322 * (<a href="#optional-restrictions">optional</a>), 323 * or if the specified collection is null. 324 * @see #contains(Object) 325 */ 326 boolean containsAll(Collection<?> c); 327 328 /** 329 * Adds all of the elements in the specified collection to this collection 330 * (optional operation). The behavior of this operation is undefined if 331 * the specified collection is modified while the operation is in progress. 332 * (This implies that the behavior of this call is undefined if the 333 * specified collection is this collection, and this collection is 334 * nonempty.) 335 * 336 * @param c collection containing elements to be added to this collection 337 * @return <tt>true</tt> if this collection changed as a result of the call 338 * @throws UnsupportedOperationException if the <tt>addAll</tt> operation 339 * is not supported by this collection 340 * @throws ClassCastException if the class of an element of the specified 341 * collection prevents it from being added to this collection 342 * @throws NullPointerException if the specified collection contains a 343 * null element and this collection does not permit null elements, 344 * or if the specified collection is null 345 * @throws IllegalArgumentException if some property of an element of the 346 * specified collection prevents it from being added to this 347 * collection 348 * @throws IllegalStateException if not all the elements can be added at 349 * this time due to insertion restrictions 350 * @see #add(Object) 351 */ 352 boolean addAll(Collection<? extends E> c); 353 354 /** 355 * Removes all of this collection's elements that are also contained in the 356 * specified collection (optional operation). After this call returns, 357 * this collection will contain no elements in common with the specified 358 * collection. 359 * 360 * @param c collection containing elements to be removed from this collection 361 * @return <tt>true</tt> if this collection changed as a result of the 362 * call 363 * @throws UnsupportedOperationException if the <tt>removeAll</tt> method 364 * is not supported by this collection 365 * @throws ClassCastException if the types of one or more elements 366 * in this collection are incompatible with the specified 367 * collection 368 * (<a href="#optional-restrictions">optional</a>) 369 * @throws NullPointerException if this collection contains one or more 370 * null elements and the specified collection does not support 371 * null elements 372 * (<a href="#optional-restrictions">optional</a>), 373 * or if the specified collection is null 374 * @see #remove(Object) 375 * @see #contains(Object) 376 */ 377 boolean removeAll(Collection<?> c); 378 379 /** 380 * Removes all of the elements of this collection that satisfy the given 381 * predicate. Errors or runtime exceptions thrown by the predicate are 382 * relayed to the caller. 383 * 384 * @implSpec 385 * The default implementation traverses all elements of the collection using 386 * its {@link #iterator}. Each matching element is removed using 387 * {@link Iterator#remove()}. If the collection's iterator does not 388 * support removal then an {@code UnsupportedOperationException} will be 389 * thrown on the first matching element. 390 * 391 * @param filter a predicate which returns {@code true} for elements to be 392 * removed 393 * @return {@code true} if any elements were removed 394 * @throws NullPointerException if the specified filter is null 395 * @throws UnsupportedOperationException if the {@code remove} 396 * method is not supported by this collection's 397 * {@link #iterator} 398 * @since 1.8 399 */ 400 default boolean removeIf(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. 507 * 508 * <p>The default implementation should be overridden by subclasses that 509 * can return a more efficient spliterator. In order to 510 * preserve expected laziness behavior for the {@link #stream()} and 511 * {@link #parallelStream()}} methods, spliterators should either have the 512 * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be 513 * <em><a href="Spliterator.html#binding">late-binding</a></em>. 514 * If none of these is practical, the overriding class should describe the 515 * spliterator's documented policy of binding and structural interference, 516 * and should override the {@link #stream()} and {@link #parallelStream()} 517 * methods to create streams using a {@code Supplier} of the spliterator, 518 * as in: 519 * <pre>{@code 520 * Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics) 521 * }</pre> 522 * <p>These requirements ensure that streams produced by the 523 * {@link #stream()} and {@link #parallelStream()} methods will reflect the 524 * contents of the collection as of initiation of the terminal stream 525 * operation. 526 * 527 * @implSpec 528 * The default implementation creates a 529 * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator 530 * from the collections's {@code Iterator}. The spliterator inherits the 531 * <em>fail-fast</em> properties of the collection's iterator. 532 * 533 * @implNote 534 * The returned {@code Spliterator} additionally reports 535 * {@link Spliterator#SUBSIZED}. 536 * 537 * @return a {@code Spliterator} over the elements in this collection 538 * @since 1.8 539 */ 540 default Spliterator<E> spliterator() { 541 return Spliterators.spliterator(this, 0); 542 } 543 544 /** 545 * Returns a sequential {@code Stream} with this collection as its source. 546 * 547 * <p>This method should be overridden when the {@link #spliterator()} 548 * method cannot return a spliterator that is {@code IMMUTABLE}, 549 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()} 550 * for details.) 551 * 552 * @implSpec 553 * The default implementation creates a sequential {@code Stream} from the 554 * collection's {@code Spliterator}. 555 * 556 * @return a sequential {@code Stream} over the elements in this collection 557 * @since 1.8 558 */ 559 default Stream<E> stream() { 560 return StreamSupport.stream(spliterator()); 561 } 562 563 /** 564 * Returns a possibly parallel {@code Stream} with this collection as its 565 * source. It is allowable for this method to return a sequential stream. 566 * 567 * <p>This method should be overridden when the {@link #spliterator()} 568 * method cannot return a spliterator that is {@code IMMUTABLE}, 569 * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()} 570 * for details.) 571 * 572 * @implSpec 573 * The default implementation creates a parallel {@code Stream} from the 574 * collection's {@code Spliterator}. 575 * 576 * @return a possibly parallel {@code Stream} over the elements in this 577 * collection 578 * @since 1.8 579 */ 580 default Stream<E> parallelStream() { 581 return StreamSupport.parallelStream(spliterator()); 582 } 583 }