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&nbsp;?&nbsp;e==null&nbsp;:&nbsp;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&nbsp;?&nbsp;e==null&nbsp;:&nbsp;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 by the predicate are
 383      * 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 the {@code remove}
 397      *         method is not supported by this collection's
 398      *         {@link #iterator}
 399      * @since 1.8
 400      */
 401     default boolean removeIf(Predicate<? super E> filter) {
 402         Objects.requireNonNull(filter);
 403         boolean removed = false;
 404         final Iterator<E> each = iterator();
 405         while (each.hasNext()) {
 406             if (filter.test(each.next())) {
 407                 each.remove();
 408                 removed = true;
 409             }
 410         }
 411         return removed;
 412     }
 413 
 414     /**
 415      * Retains only the elements in this collection that are contained in the
 416      * specified collection (optional operation).  In other words, removes from
 417      * this collection all of its elements that are not contained in the
 418      * specified collection.
 419      *
 420      * @param c collection containing elements to be retained in this collection
 421      * @return <tt>true</tt> if this collection changed as a result of the call
 422      * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
 423      *         is not supported by this collection
 424      * @throws ClassCastException if the types of one or more elements
 425      *         in this collection are incompatible with the specified
 426      *         collection
 427      *         (<a href="#optional-restrictions">optional</a>)
 428      * @throws NullPointerException if this collection contains one or more
 429      *         null elements and the specified collection does not permit null
 430      *         elements
 431      *         (<a href="#optional-restrictions">optional</a>),
 432      *         or if the specified collection is null
 433      * @see #remove(Object)
 434      * @see #contains(Object)
 435      */
 436     boolean retainAll(Collection<?> c);
 437 
 438     /**
 439      * Removes all of the elements from this collection (optional operation).
 440      * The collection will be empty after this method returns.
 441      *
 442      * @throws UnsupportedOperationException if the <tt>clear</tt> operation
 443      *         is not supported by this collection
 444      */
 445     void clear();
 446 
 447 
 448     // Comparison and hashing
 449 
 450     /**
 451      * Compares the specified object with this collection for equality. <p>
 452      *
 453      * While the <tt>Collection</tt> interface adds no stipulations to the
 454      * general contract for the <tt>Object.equals</tt>, programmers who
 455      * implement the <tt>Collection</tt> interface "directly" (in other words,
 456      * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
 457      * or a <tt>List</tt>) must exercise care if they choose to override the
 458      * <tt>Object.equals</tt>.  It is not necessary to do so, and the simplest
 459      * course of action is to rely on <tt>Object</tt>'s implementation, but
 460      * the implementor may wish to implement a "value comparison" in place of
 461      * the default "reference comparison."  (The <tt>List</tt> and
 462      * <tt>Set</tt> interfaces mandate such value comparisons.)<p>
 463      *
 464      * The general contract for the <tt>Object.equals</tt> method states that
 465      * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
 466      * only if <tt>b.equals(a)</tt>).  The contracts for <tt>List.equals</tt>
 467      * and <tt>Set.equals</tt> state that lists are only equal to other lists,
 468      * and sets to other sets.  Thus, a custom <tt>equals</tt> method for a
 469      * collection class that implements neither the <tt>List</tt> nor
 470      * <tt>Set</tt> interface must return <tt>false</tt> when this collection
 471      * is compared to any list or set.  (By the same logic, it is not possible
 472      * to write a class that correctly implements both the <tt>Set</tt> and
 473      * <tt>List</tt> interfaces.)
 474      *
 475      * @param o object to be compared for equality with this collection
 476      * @return <tt>true</tt> if the specified object is equal to this
 477      * collection
 478      *
 479      * @see Object#equals(Object)
 480      * @see Set#equals(Object)
 481      * @see List#equals(Object)
 482      */
 483     boolean equals(Object o);
 484 
 485     /**
 486      * Returns the hash code value for this collection.  While the
 487      * <tt>Collection</tt> interface adds no stipulations to the general
 488      * contract for the <tt>Object.hashCode</tt> method, programmers should
 489      * take note that any class that overrides the <tt>Object.equals</tt>
 490      * method must also override the <tt>Object.hashCode</tt> method in order
 491      * to satisfy the general contract for the <tt>Object.hashCode</tt> method.
 492      * In particular, <tt>c1.equals(c2)</tt> implies that
 493      * <tt>c1.hashCode()==c2.hashCode()</tt>.
 494      *
 495      * @return the hash code value for this collection
 496      *
 497      * @see Object#hashCode()
 498      * @see Object#equals(Object)
 499      */
 500     int hashCode();
 501 
 502     /**
 503      * Creates a {@link Spliterator} over the elements in this collection.
 504      *
 505      * <p>The returned {@code Spliterator} must report the characteristic
 506      * {@link Spliterator#SIZED}; implementations should document any additional
 507      * characteristic values reported by the returned spliterator.  If
 508      * this collection contains no elements then the returned spliterator is
 509      * only required to report {@link Spliterator#SIZED} and is not required to
 510      * report additional characteristic values (if any).
 511      *
 512      * <p>The default implementation should be overridden by subclasses that
 513      * can return a more efficient spliterator.  In order to
 514      * preserve expected laziness behavior for the {@link #stream()} and
 515      * {@link #parallelStream()}} methods, spliterators should either have the
 516      * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
 517      * <em><a href="Spliterator.html#binding">late-binding</a></em>.
 518      * If none of these is practical, the overriding class should describe the
 519      * spliterator's documented policy of binding and structural interference,
 520      * and should override the {@link #stream()} and {@link #parallelStream()}
 521      * methods to create streams using a {@code Supplier} of the spliterator,
 522      * as in:
 523      * <pre>{@code
 524      *     Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
 525      * }</pre>
 526      * <p>These requirements ensure that streams produced by the
 527      * {@link #stream()} and {@link #parallelStream()} methods will reflect the
 528      * contents of the collection as of initiation of the terminal stream
 529      * operation.
 530      *
 531      * @implSpec
 532      * The default implementation creates a
 533      * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
 534      * from the collections's {@code Iterator}.  The spliterator inherits the
 535      * <em>fail-fast</em> properties of the collection's iterator.
 536      *
 537      * @implNote
 538      * The returned {@code Spliterator} additionally reports
 539      * {@link Spliterator#SUBSIZED}.
 540      *
 541      * <p>If a spliterator covers no elements then the reporting of additional
 542      * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED},
 543      * does not aid clients to control, specialize or simplify computation.
 544      * However, this does enable shared use of an immutable and empty
 545      * spliterator instance (see {@link Spliterators#emptySpliterator()}) for
 546      * empty collections, and enables clients to determine if such a spliterator
 547      * covers no elements.
 548      *
 549      * @return a {@code Spliterator} over the elements in this collection
 550      * @since 1.8
 551      */
 552     default Spliterator<E> spliterator() {
 553         return Spliterators.spliterator(this, 0);
 554     }
 555 
 556     /**
 557      * Returns a sequential {@code Stream} with this collection as its source.
 558      *
 559      * <p>This method should be overridden when the {@link #spliterator()}
 560      * method cannot return a spliterator that is {@code IMMUTABLE},
 561      * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
 562      * for details.)
 563      *
 564      * @implSpec
 565      * The default implementation creates a sequential {@code Stream} from the
 566      * collection's {@code Spliterator}.
 567      *
 568      * @return a sequential {@code Stream} over the elements in this collection
 569      * @since 1.8
 570      */
 571     default Stream<E> stream() {
 572         return StreamSupport.stream(spliterator(), false);
 573     }
 574 
 575     /**
 576      * Returns a possibly parallel {@code Stream} with this collection as its
 577      * source.  It is allowable for this method to return a sequential stream.
 578      *
 579      * <p>This method should be overridden when the {@link #spliterator()}
 580      * method cannot return a spliterator that is {@code IMMUTABLE},
 581      * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
 582      * for details.)
 583      *
 584      * @implSpec
 585      * The default implementation creates a parallel {@code Stream} from the
 586      * collection's {@code Spliterator}.
 587      *
 588      * @return a possibly parallel {@code Stream} over the elements in this
 589      * collection
 590      * @since 1.8
 591      */
 592     default Stream<E> parallelStream() {
 593         return StreamSupport.stream(spliterator(), true);
 594     }
 595 }