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