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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>Some collection operations which perform recursive traversal of the
108  * collection may fail with an exception for self-referential instances where
109  * the collection directly or indirectly contains itself. This includes the
110  * {@code clone()}, {@code equals()}, {@code hashCode()} and {@code toString()}
111  * methods. Implementations may optionally handle the self-referential scenario,
112  * however most current implementations do not do so.
113  *
114  * <p>This interface is a member of the
115  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
116  * Java Collections Framework</a>.
117  *
118  * @implSpec
119  * The default method implementations (inherited or otherwise) do not apply any
120  * synchronization protocol.  If a {@code Collection} implementation has a
121  * specific synchronization protocol, then it must override default
122  * implementations to apply that protocol.
123  *
124  * @param <E> the type of elements in this collection
125  *
126  * @author  Josh Bloch
127  * @author  Neal Gafter
128  * @see     Set
129  * @see     List
130  * @see     Map
131  * @see     SortedSet
132  * @see     SortedMap
133  * @see     HashSet
134  * @see     TreeSet
135  * @see     ArrayList
136  * @see     LinkedList
137  * @see     Vector
138  * @see     Collections
139  * @see     Arrays
140  * @see     AbstractCollection
141  * @since 1.2
142  */
143 
144 public interface Collection<E> extends Iterable<E> {
145     // Query Operations
146 
147     /**
148      * Returns the number of elements in this collection.  If this collection
149      * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
150      * <tt>Integer.MAX_VALUE</tt>.
151      *
152      * @return the number of elements in this collection
153      */
154     int size();
155 
156     /**
157      * Returns <tt>true</tt> if this collection contains no elements.
158      *
159      * @return <tt>true</tt> if this collection contains no elements
160      */
161     boolean isEmpty();
162 
163     /**
164      * Returns <tt>true</tt> if this collection contains the specified element.
165      * More formally, returns <tt>true</tt> if and only if this collection
166      * contains at least one element <tt>e</tt> such that
167      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
168      *
169      * @param o element whose presence in this collection is to be tested
170      * @return <tt>true</tt> if this collection contains the specified
171      *         element
172      * @throws ClassCastException if the type of the specified element
173      *         is incompatible with this collection
174      *         (<a href="#optional-restrictions">optional</a>)
175      * @throws NullPointerException if the specified element is null and this
176      *         collection does not permit null elements
177      *         (<a href="#optional-restrictions">optional</a>)
178      */
179     boolean contains(Object o);
180 
181     /**
182      * Returns an iterator over the elements in this collection.  There are no
183      * guarantees concerning the order in which the elements are returned
184      * (unless this collection is an instance of some class that provides a
185      * guarantee).
186      *
187      * @return an <tt>Iterator</tt> over the elements in this collection
188      */
189     Iterator<E> iterator();
190 
191     /**
192      * Returns an array containing all of the elements in this collection.
193      * If this collection makes any guarantees as to what order its elements
194      * are returned by its iterator, this method must return the elements in
195      * the same order.
196      *
197      * <p>The returned array will be "safe" in that no references to it are
198      * maintained by this collection.  (In other words, this method must
199      * allocate a new array even if this collection is backed by an array).
200      * The caller is thus free to modify the returned array.
201      *
202      * <p>This method acts as bridge between array-based and collection-based
203      * APIs.
204      *
205      * @return an array containing all of the elements in this collection
206      */
207     Object[] toArray();
208 
209     /**
210      * Returns an array containing all of the elements in this collection;
211      * the runtime type of the returned array is that of the specified array.
212      * If the collection fits in the specified array, it is returned therein.
213      * Otherwise, a new array is allocated with the runtime type of the
214      * specified array and the size of this collection.
215      *
216      * <p>If this collection fits in the specified array with room to spare
217      * (i.e., the array has more elements than this collection), the element
218      * in the array immediately following the end of the collection is set to
219      * <tt>null</tt>.  (This is useful in determining the length of this
220      * collection <i>only</i> if the caller knows that this collection does
221      * not contain any <tt>null</tt> elements.)
222      *
223      * <p>If this collection makes any guarantees as to what order its elements
224      * are returned by its iterator, this method must return the elements in
225      * the same order.
226      *
227      * <p>Like the {@link #toArray()} method, this method acts as bridge between
228      * array-based and collection-based APIs.  Further, this method allows
229      * precise control over the runtime type of the output array, and may,
230      * under certain circumstances, be used to save allocation costs.
231      *
232      * <p>Suppose <tt>x</tt> is a collection known to contain only strings.
233      * The following code can be used to dump the collection into a newly
234      * allocated array of <tt>String</tt>:
235      *
236      * <pre>
237      *     String[] y = x.toArray(new String[0]);</pre>
238      *
239      * Note that <tt>toArray(new Object[0])</tt> is identical in function to
240      * <tt>toArray()</tt>.
241      *
242      * @param <T> the runtime type of the array to contain the collection
243      * @param a the array into which the elements of this collection are to be
244      *        stored, if it is big enough; otherwise, a new array of the same
245      *        runtime type is allocated for this purpose.
246      * @return an array containing all of the elements in this collection
247      * @throws ArrayStoreException if the runtime type of the specified array
248      *         is not a supertype of the runtime type of every element in
249      *         this collection
250      * @throws NullPointerException if the specified array is null
251      */
252     <T> T[] toArray(T[] a);
253 
254     // Modification Operations
255 
256     /**
257      * Ensures that this collection contains the specified element (optional
258      * operation).  Returns <tt>true</tt> if this collection changed as a
259      * result of the call.  (Returns <tt>false</tt> if this collection does
260      * not permit duplicates and already contains the specified element.)<p>
261      *
262      * Collections that support this operation may place limitations on what
263      * elements may be added to this collection.  In particular, some
264      * collections will refuse to add <tt>null</tt> elements, and others will
265      * impose restrictions on the type of elements that may be added.
266      * Collection classes should clearly specify in their documentation any
267      * restrictions on what elements may be added.<p>
268      *
269      * If a collection refuses to add a particular element for any reason
270      * other than that it already contains the element, it <i>must</i> throw
271      * an exception (rather than returning <tt>false</tt>).  This preserves
272      * the invariant that a collection always contains the specified element
273      * after this call returns.
274      *
275      * @param e element whose presence in this collection is to be ensured
276      * @return <tt>true</tt> if this collection changed as a result of the
277      *         call
278      * @throws UnsupportedOperationException if the <tt>add</tt> operation
279      *         is not supported by this collection
280      * @throws ClassCastException if the class of the specified element
281      *         prevents it from being added to this collection
282      * @throws NullPointerException if the specified element is null and this
283      *         collection does not permit null elements
284      * @throws IllegalArgumentException if some property of the element
285      *         prevents it from being added to this collection
286      * @throws IllegalStateException if the element cannot be added at this
287      *         time due to insertion restrictions
288      */
289     boolean add(E e);
290 
291     /**
292      * Removes a single instance of the specified element from this
293      * collection, if it is present (optional operation).  More formally,
294      * removes an element <tt>e</tt> such that
295      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>, if
296      * this collection contains one or more such elements.  Returns
297      * <tt>true</tt> if this collection contained the specified element (or
298      * equivalently, if this collection changed as a result of the call).
299      *
300      * @param o element to be removed from this collection, if present
301      * @return <tt>true</tt> if an element was removed as a result of this call
302      * @throws ClassCastException if the type of the specified element
303      *         is incompatible with this collection
304      *         (<a href="#optional-restrictions">optional</a>)
305      * @throws NullPointerException if the specified element is null and this
306      *         collection does not permit null elements
307      *         (<a href="#optional-restrictions">optional</a>)
308      * @throws UnsupportedOperationException if the <tt>remove</tt> operation
309      *         is not supported by this collection
310      */
311     boolean remove(Object o);
312 
313 
314     // Bulk Operations
315 
316     /**
317      * Returns <tt>true</tt> if this collection contains all of the elements
318      * in the specified collection.
319      *
320      * @param  c collection to be checked for containment in this collection
321      * @return <tt>true</tt> if this collection contains all of the elements
322      *         in the specified collection
323      * @throws ClassCastException if the types of one or more elements
324      *         in the specified collection are incompatible with this
325      *         collection
326      *         (<a href="#optional-restrictions">optional</a>)
327      * @throws NullPointerException if the specified collection contains one
328      *         or more null elements and this collection does not permit null
329      *         elements
330      *         (<a href="#optional-restrictions">optional</a>),
331      *         or if the specified collection is null.
332      * @see    #contains(Object)
333      */
334     boolean containsAll(Collection<?> c);
335 
336     /**
337      * Adds all of the elements in the specified collection to this collection
338      * (optional operation).  The behavior of this operation is undefined if
339      * the specified collection is modified while the operation is in progress.
340      * (This implies that the behavior of this call is undefined if the
341      * specified collection is this collection, and this collection is
342      * nonempty.)
343      *
344      * @param c collection containing elements to be added to this collection
345      * @return <tt>true</tt> if this collection changed as a result of the call
346      * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
347      *         is not supported by this collection
348      * @throws ClassCastException if the class of an element of the specified
349      *         collection prevents it from being added to this collection
350      * @throws NullPointerException if the specified collection contains a
351      *         null element and this collection does not permit null elements,
352      *         or if the specified collection is null
353      * @throws IllegalArgumentException if some property of an element of the
354      *         specified collection prevents it from being added to this
355      *         collection
356      * @throws IllegalStateException if not all the elements can be added at
357      *         this time due to insertion restrictions
358      * @see #add(Object)
359      */
360     boolean addAll(Collection<? extends E> c);
361 
362     /**
363      * Removes all of this collection's elements that are also contained in the
364      * specified collection (optional operation).  After this call returns,
365      * this collection will contain no elements in common with the specified
366      * collection.
367      *
368      * @param c collection containing elements to be removed from this collection
369      * @return <tt>true</tt> if this collection changed as a result of the
370      *         call
371      * @throws UnsupportedOperationException if the <tt>removeAll</tt> method
372      *         is not supported by this collection
373      * @throws ClassCastException if the types of one or more elements
374      *         in this collection are incompatible with the specified
375      *         collection
376      *         (<a href="#optional-restrictions">optional</a>)
377      * @throws NullPointerException if this collection contains one or more
378      *         null elements and the specified collection does not support
379      *         null elements
380      *         (<a href="#optional-restrictions">optional</a>),
381      *         or if the specified collection is null
382      * @see #remove(Object)
383      * @see #contains(Object)
384      */
385     boolean removeAll(Collection<?> c);
386 
387     /**
388      * Removes all of the elements of this collection that satisfy the given
389      * predicate.  Errors or runtime exceptions thrown during iteration or by
390      * the predicate are relayed to the caller.
391      *
392      * @implSpec
393      * The default implementation traverses all elements of the collection using
394      * its {@link #iterator}.  Each matching element is removed using
395      * {@link Iterator#remove()}.  If the collection's iterator does not
396      * support removal then an {@code UnsupportedOperationException} will be
397      * thrown on the first matching element.
398      *
399      * @param filter a predicate which returns {@code true} for elements to be
400      *        removed
401      * @return {@code true} if any elements were removed
402      * @throws NullPointerException if the specified filter is null
403      * @throws UnsupportedOperationException if elements cannot be removed
404      *         from this collection.  Implementations may throw this exception if a
405      *         matching element cannot be removed or if, in general, removal is not
406      *         supported.
407      * @since 1.8
408      */
409     default boolean removeIf(Predicate<? super E> filter) {
410         Objects.requireNonNull(filter);
411         boolean removed = false;
412         final Iterator<E> each = iterator();
413         while (each.hasNext()) {
414             if (filter.test(each.next())) {
415                 each.remove();
416                 removed = true;
417             }
418         }
419         return removed;
420     }
421 
422     /**
423      * Retains only the elements in this collection that are contained in the
424      * specified collection (optional operation).  In other words, removes from
425      * this collection all of its elements that are not contained in the
426      * specified collection.
427      *
428      * @param c collection containing elements to be retained in this collection
429      * @return <tt>true</tt> if this collection changed as a result of the call
430      * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
431      *         is not supported by this collection
432      * @throws ClassCastException if the types of one or more elements
433      *         in this collection are incompatible with the specified
434      *         collection
435      *         (<a href="#optional-restrictions">optional</a>)
436      * @throws NullPointerException if this collection contains one or more
437      *         null elements and the specified collection does not permit null
438      *         elements
439      *         (<a href="#optional-restrictions">optional</a>),
440      *         or if the specified collection is null
441      * @see #remove(Object)
442      * @see #contains(Object)
443      */
444     boolean retainAll(Collection<?> c);
445 
446     /**
447      * Removes all of the elements from this collection (optional operation).
448      * The collection will be empty after this method returns.
449      *
450      * @throws UnsupportedOperationException if the <tt>clear</tt> operation
451      *         is not supported by this collection
452      */
453     void clear();
454 
455 
456     // Comparison and hashing
457 
458     /**
459      * Compares the specified object with this collection for equality. <p>
460      *
461      * While the <tt>Collection</tt> interface adds no stipulations to the
462      * general contract for the <tt>Object.equals</tt>, programmers who
463      * implement the <tt>Collection</tt> interface "directly" (in other words,
464      * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
465      * or a <tt>List</tt>) must exercise care if they choose to override the
466      * <tt>Object.equals</tt>.  It is not necessary to do so, and the simplest
467      * course of action is to rely on <tt>Object</tt>'s implementation, but
468      * the implementor may wish to implement a "value comparison" in place of
469      * the default "reference comparison."  (The <tt>List</tt> and
470      * <tt>Set</tt> interfaces mandate such value comparisons.)<p>
471      *
472      * The general contract for the <tt>Object.equals</tt> method states that
473      * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
474      * only if <tt>b.equals(a)</tt>).  The contracts for <tt>List.equals</tt>
475      * and <tt>Set.equals</tt> state that lists are only equal to other lists,
476      * and sets to other sets.  Thus, a custom <tt>equals</tt> method for a
477      * collection class that implements neither the <tt>List</tt> nor
478      * <tt>Set</tt> interface must return <tt>false</tt> when this collection
479      * is compared to any list or set.  (By the same logic, it is not possible
480      * to write a class that correctly implements both the <tt>Set</tt> and
481      * <tt>List</tt> interfaces.)
482      *
483      * @param o object to be compared for equality with this collection
484      * @return <tt>true</tt> if the specified object is equal to this
485      * collection
486      *
487      * @see Object#equals(Object)
488      * @see Set#equals(Object)
489      * @see List#equals(Object)
490      */
491     boolean equals(Object o);
492 
493     /**
494      * Returns the hash code value for this collection.  While the
495      * <tt>Collection</tt> interface adds no stipulations to the general
496      * contract for the <tt>Object.hashCode</tt> method, programmers should
497      * take note that any class that overrides the <tt>Object.equals</tt>
498      * method must also override the <tt>Object.hashCode</tt> method in order
499      * to satisfy the general contract for the <tt>Object.hashCode</tt> method.
500      * In particular, <tt>c1.equals(c2)</tt> implies that
501      * <tt>c1.hashCode()==c2.hashCode()</tt>.
502      *
503      * @return the hash code value for this collection
504      *
505      * @see Object#hashCode()
506      * @see Object#equals(Object)
507      */
508     int hashCode();
509 
510     /**
511      * Creates a {@link Spliterator} over the elements in this collection.
512      *
513      * Implementations should document characteristic values reported by the
514      * spliterator.  Such characteristic values are not required to be reported
515      * if the spliterator reports {@link Spliterator#SIZED} and this collection
516      * contains no elements.
517      *
518      * <p>The default implementation should be overridden by subclasses that
519      * can return a more efficient spliterator.  In order to
520      * preserve expected laziness behavior for the {@link #stream()} and
521      * {@link #parallelStream()}} methods, spliterators should either have the
522      * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
523      * <em><a href="Spliterator.html#binding">late-binding</a></em>.
524      * If none of these is practical, the overriding class should describe the
525      * spliterator's documented policy of binding and structural interference,
526      * and should override the {@link #stream()} and {@link #parallelStream()}
527      * methods to create streams using a {@code Supplier} of the spliterator,
528      * as in:
529      * <pre>{@code
530      *     Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
531      * }</pre>
532      * <p>These requirements ensure that streams produced by the
533      * {@link #stream()} and {@link #parallelStream()} methods will reflect the
534      * contents of the collection as of initiation of the terminal stream
535      * operation.
536      *
537      * @implSpec
538      * The default implementation creates a
539      * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
540      * from the collections's {@code Iterator}.  The spliterator inherits the
541      * <em>fail-fast</em> properties of the collection's iterator.
542      * <p>
543      * The created {@code Spliterator} reports {@link Spliterator#SIZED}.
544      *
545      * @implNote
546      * The created {@code Spliterator} additionally reports
547      * {@link Spliterator#SUBSIZED}.
548      *
549      * <p>If a spliterator covers no elements then the reporting of additional
550      * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED},
551      * does not aid clients to control, specialize or simplify computation.
552      * However, this does enable shared use of an immutable and empty
553      * spliterator instance (see {@link Spliterators#emptySpliterator()}) for
554      * empty collections, and enables clients to determine if such a spliterator
555      * covers no elements.
556      *
557      * @return a {@code Spliterator} over the elements in this collection
558      * @since 1.8
559      */
560     @Override
561     default Spliterator<E> spliterator() {
562         return Spliterators.spliterator(this, 0);
563     }
564 
565     /**
566      * Returns a sequential {@code Stream} with this collection as its source.
567      *
568      * <p>This method should be overridden when the {@link #spliterator()}
569      * method cannot return a spliterator that is {@code IMMUTABLE},
570      * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
571      * for details.)
572      *
573      * @implSpec
574      * The default implementation creates a sequential {@code Stream} from the
575      * collection's {@code Spliterator}.
576      *
577      * @return a sequential {@code Stream} over the elements in this collection
578      * @since 1.8
579      */
580     default Stream<E> stream() {
581         return StreamSupport.stream(spliterator(), false);
582     }
583 
584     /**
585      * Returns a possibly parallel {@code Stream} with this collection as its
586      * source.  It is allowable for this method to return a sequential stream.
587      *
588      * <p>This method should be overridden when the {@link #spliterator()}
589      * method cannot return a spliterator that is {@code IMMUTABLE},
590      * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
591      * for details.)
592      *
593      * @implSpec
594      * The default implementation creates a parallel {@code Stream} from the
595      * collection's {@code Spliterator}.
596      *
597      * @return a possibly parallel {@code Stream} over the elements in this
598      * collection
599      * @since 1.8
600      */
601     default Stream<E> parallelStream() {
602         return StreamSupport.stream(spliterator(), true);
603     }
604 }