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25  
26  package java.util;
27  
28  /**
29   * A collection that contains no duplicate elements.  More formally, sets
30   * contain no pair of elements <code>e1</code> and <code>e2</code> such that
31   * <code>e1.equals(e2)</code>, and at most one null element.  As implied by
32   * its name, this interface models the mathematical <i>set</i> abstraction.
33   *
34   * <p>The <tt>Set</tt> interface places additional stipulations, beyond those
35   * inherited from the <tt>Collection</tt> interface, on the contracts of all
36   * constructors and on the contracts of the <tt>add</tt>, <tt>equals</tt> and
37   * <tt>hashCode</tt> methods.  Declarations for other inherited methods are
38   * also included here for convenience.  (The specifications accompanying these
39   * declarations have been tailored to the <tt>Set</tt> interface, but they do
40   * not contain any additional stipulations.)
41   *
42   * <p>The additional stipulation on constructors is, not surprisingly,
43   * that all constructors must create a set that contains no duplicate elements
44   * (as defined above).
45   *
46   * <p>Note: Great care must be exercised if mutable objects are used as set
47   * elements.  The behavior of a set is not specified if the value of an object
48   * is changed in a manner that affects <tt>equals</tt> comparisons while the
49   * object is an element in the set.  A special case of this prohibition is
50   * that it is not permissible for a set to contain itself as an element.
51   *
52   * <p>Some set implementations have restrictions on the elements that
53   * they may contain.  For example, some implementations prohibit null elements,
54   * and some have restrictions on the types of their elements.  Attempting to
55   * add an ineligible element throws an unchecked exception, typically
56   * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.  Attempting
57   * to query the presence of an ineligible element may throw an exception,
58   * or it may simply return false; some implementations will exhibit the former
59   * behavior and some will exhibit the latter.  More generally, attempting an
60   * operation on an ineligible element whose completion would not result in
61   * the insertion of an ineligible element into the set may throw an
62   * exception or it may succeed, at the option of the implementation.
63   * Such exceptions are marked as "optional" in the specification for this
64   * interface.
65   *
66   * <p>This interface is a member of the
67   * <a href="{@docRoot}/../technotes/guides/collections/index.html">
68   * Java Collections Framework</a>.
69   *
70   * @param <E> the type of elements maintained by this set
71   *
72   * @author  Josh Bloch
73   * @author  Neal Gafter
74   * @see Collection
75   * @see List
76   * @see SortedSet
77   * @see HashSet
78   * @see TreeSet
79   * @see AbstractSet
80   * @see Collections#singleton(java.lang.Object)
81   * @see Collections#EMPTY_SET
82   * @since 1.2
83   */
84  
85  public interface Set<E> extends Collection<E> {
86      // Query Operations
87  
88      /**
89       * Returns the number of elements in this set (its cardinality).  If this
90       * set contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
91       * <tt>Integer.MAX_VALUE</tt>.
92       *
93       * @return the number of elements in this set (its cardinality)
94       */
95      int size();
96  
97      /**
98       * Returns <tt>true</tt> if this set contains no elements.
99       *
100      * @return <tt>true</tt> if this set contains no elements
101      */
102     boolean isEmpty();
103 
104     /**
105      * Returns <tt>true</tt> if this set contains the specified element.
106      * More formally, returns <tt>true</tt> if and only if this set
107      * contains an element <tt>e</tt> such that
108      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
109      *
110      * @param o element whose presence in this set is to be tested
111      * @return <tt>true</tt> if this set contains the specified element
112      * @throws ClassCastException if the type of the specified element
113      *         is incompatible with this set
114      * (<a href="Collection.html#optional-restrictions">optional</a>)
115      * @throws NullPointerException if the specified element is null and this
116      *         set does not permit null elements
117      * (<a href="Collection.html#optional-restrictions">optional</a>)
118      */
119     boolean contains(Object o);
120 
121     /**
122      * Returns an iterator over the elements in this set.  The elements are
123      * returned in no particular order (unless this set is an instance of some
124      * class that provides a guarantee).
125      *
126      * @return an iterator over the elements in this set
127      */
128     Iterator<E> iterator();
129 
130     /**
131      * Returns an array containing all of the elements in this set.
132      * If this set makes any guarantees as to what order its elements
133      * are returned by its iterator, this method must return the
134      * elements in the same order.
135      *
136      * <p>The returned array will be "safe" in that no references to it
137      * are maintained by this set.  (In other words, this method must
138      * allocate a new array even if this set is backed by an array).
139      * The caller is thus free to modify the returned array.
140      *
141      * <p>This method acts as bridge between array-based and collection-based
142      * APIs.
143      *
144      * @return an array containing all the elements in this set
145      */
146     Object[] toArray();
147 
148     /**
149      * Returns an array containing all of the elements in this set; the
150      * runtime type of the returned array is that of the specified array.
151      * If the set fits in the specified array, it is returned therein.
152      * Otherwise, a new array is allocated with the runtime type of the
153      * specified array and the size of this set.
154      *
155      * <p>If this set fits in the specified array with room to spare
156      * (i.e., the array has more elements than this set), the element in
157      * the array immediately following the end of the set is set to
158      * <tt>null</tt>.  (This is useful in determining the length of this
159      * set <i>only</i> if the caller knows that this set does not contain
160      * any null elements.)
161      *
162      * <p>If this set makes any guarantees as to what order its elements
163      * are returned by its iterator, this method must return the elements
164      * in the same order.
165      *
166      * <p>Like the {@link #toArray()} method, this method acts as bridge between
167      * array-based and collection-based APIs.  Further, this method allows
168      * precise control over the runtime type of the output array, and may,
169      * under certain circumstances, be used to save allocation costs.
170      *
171      * <p>Suppose <tt>x</tt> is a set known to contain only strings.
172      * The following code can be used to dump the set into a newly allocated
173      * array of <tt>String</tt>:
174      *
175      * <pre>
176      *     String[] y = x.toArray(new String[0]);</pre>
177      *
178      * Note that <tt>toArray(new Object[0])</tt> is identical in function to
179      * <tt>toArray()</tt>.
180      *
181      * @param a the array into which the elements of this set are to be
182      *        stored, if it is big enough; otherwise, a new array of the same
183      *        runtime type is allocated for this purpose.
184      * @return an array containing all the elements in this set
185      * @throws ArrayStoreException if the runtime type of the specified array
186      *         is not a supertype of the runtime type of every element in this
187      *         set
188      * @throws NullPointerException if the specified array is null
189      */
190     <T> T[] toArray(T[] a);
191 
192 
193     // Modification Operations
194 
195     /**
196      * Adds the specified element to this set if it is not already present
197      * (optional operation).  More formally, adds the specified element
198      * <tt>e</tt> to this set if the set contains no element <tt>e2</tt>
199      * such that
200      * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>.
201      * If this set already contains the element, the call leaves the set
202      * unchanged and returns <tt>false</tt>.  In combination with the
203      * restriction on constructors, this ensures that sets never contain
204      * duplicate elements.
205      *
206      * <p>The stipulation above does not imply that sets must accept all
207      * elements; sets may refuse to add any particular element, including
208      * <tt>null</tt>, and throw an exception, as described in the
209      * specification for {@link Collection#add Collection.add}.
210      * Individual set implementations should clearly document any
211      * restrictions on the elements that they may contain.
212      *
213      * @param e element to be added to this set
214      * @return <tt>true</tt> if this set did not already contain the specified
215      *         element
216      * @throws UnsupportedOperationException if the <tt>add</tt> operation
217      *         is not supported by this set
218      * @throws ClassCastException if the class of the specified element
219      *         prevents it from being added to this set
220      * @throws NullPointerException if the specified element is null and this
221      *         set does not permit null elements
222      * @throws IllegalArgumentException if some property of the specified element
223      *         prevents it from being added to this set
224      */
225     boolean add(E e);
226 
227 
228     /**
229      * Removes the specified element from this set if it is present
230      * (optional operation).  More formally, removes an element <tt>e</tt>
231      * such that
232      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>, if
233      * this set contains such an element.  Returns <tt>true</tt> if this set
234      * contained the element (or equivalently, if this set changed as a
235      * result of the call).  (This set will not contain the element once the
236      * call returns.)
237      *
238      * @param o object to be removed from this set, if present
239      * @return <tt>true</tt> if this set contained the specified element
240      * @throws ClassCastException if the type of the specified element
241      *         is incompatible with this set
242      * (<a href="Collection.html#optional-restrictions">optional</a>)
243      * @throws NullPointerException if the specified element is null and this
244      *         set does not permit null elements
245      * (<a href="Collection.html#optional-restrictions">optional</a>)
246      * @throws UnsupportedOperationException if the <tt>remove</tt> operation
247      *         is not supported by this set
248      */
249     boolean remove(Object o);
250 
251 
252     // Bulk Operations
253 
254     /**
255      * Returns <tt>true</tt> if this set contains all of the elements of the
256      * specified collection.  If the specified collection is also a set, this
257      * method returns <tt>true</tt> if it is a <i>subset</i> of this set.
258      *
259      * @param  c collection to be checked for containment in this set
260      * @return <tt>true</tt> if this set contains all of the elements of the
261      *         specified collection
262      * @throws ClassCastException if the types of one or more elements
263      *         in the specified collection are incompatible with this
264      *         set
265      * (<a href="Collection.html#optional-restrictions">optional</a>)
266      * @throws NullPointerException if the specified collection contains one
267      *         or more null elements and this set does not permit null
268      *         elements
269      * (<a href="Collection.html#optional-restrictions">optional</a>),
270      *         or if the specified collection is null
271      * @see    #contains(Object)
272      */
273     boolean containsAll(Collection<?> c);
274 
275     /**
276      * Adds all of the elements in the specified collection to this set if
277      * they're not already present (optional operation).  If the specified
278      * collection is also a set, the <tt>addAll</tt> operation effectively
279      * modifies this set so that its value is the <i>union</i> of the two
280      * sets.  The behavior of this operation is undefined if the specified
281      * collection is modified while the operation is in progress.
282      *
283      * @param  c collection containing elements to be added to this set
284      * @return <tt>true</tt> if this set changed as a result of the call
285      *
286      * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
287      *         is not supported by this set
288      * @throws ClassCastException if the class of an element of the
289      *         specified collection prevents it from being added to this set
290      * @throws NullPointerException if the specified collection contains one
291      *         or more null elements and this set does not permit null
292      *         elements, or if the specified collection is null
293      * @throws IllegalArgumentException if some property of an element of the
294      *         specified collection prevents it from being added to this set
295      * @see #add(Object)
296      */
297     boolean addAll(Collection<? extends E> c);
298 
299     /**
300      * Retains only the elements in this set that are contained in the
301      * specified collection (optional operation).  In other words, removes
302      * from this set all of its elements that are not contained in the
303      * specified collection.  If the specified collection is also a set, this
304      * operation effectively modifies this set so that its value is the
305      * <i>intersection</i> of the two sets.
306      *
307      * @param  c collection containing elements to be retained in this set
308      * @return <tt>true</tt> if this set changed as a result of the call
309      * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
310      *         is not supported by this set
311      * @throws ClassCastException if the class of an element of this set
312      *         is incompatible with the specified collection
313      * (<a href="Collection.html#optional-restrictions">optional</a>)
314      * @throws NullPointerException if this set contains a null element and the
315      *         specified collection does not permit null elements
316      *         (<a href="Collection.html#optional-restrictions">optional</a>),
317      *         or if the specified collection is null
318      * @see #remove(Object)
319      */
320     boolean retainAll(Collection<?> c);
321 
322     /**
323      * Removes from this set all of its elements that are contained in the
324      * specified collection (optional operation).  If the specified
325      * collection is also a set, this operation effectively modifies this
326      * set so that its value is the <i>asymmetric set difference</i> of
327      * the two sets.
328      *
329      * @param  c collection containing elements to be removed from this set
330      * @return <tt>true</tt> if this set changed as a result of the call
331      * @throws UnsupportedOperationException if the <tt>removeAll</tt> operation
332      *         is not supported by this set
333      * @throws ClassCastException if the class of an element of this set
334      *         is incompatible with the specified collection
335      * (<a href="Collection.html#optional-restrictions">optional</a>)
336      * @throws NullPointerException if this set contains a null element and the
337      *         specified collection does not permit null elements
338      *         (<a href="Collection.html#optional-restrictions">optional</a>),
339      *         or if the specified collection is null
340      * @see #remove(Object)
341      * @see #contains(Object)
342      */
343     boolean removeAll(Collection<?> c);
344 
345     /**
346      * Removes all of the elements from this set (optional operation).
347      * The set will be empty after this call returns.
348      *
349      * @throws UnsupportedOperationException if the <tt>clear</tt> method
350      *         is not supported by this set
351      */
352     void clear();
353 
354 
355     // Comparison and hashing
356 
357     /**
358      * Compares the specified object with this set for equality.  Returns
359      * <tt>true</tt> if the specified object is also a set, the two sets
360      * have the same size, and every member of the specified set is
361      * contained in this set (or equivalently, every member of this set is
362      * contained in the specified set).  This definition ensures that the
363      * equals method works properly across different implementations of the
364      * set interface.
365      *
366      * @param o object to be compared for equality with this set
367      * @return <tt>true</tt> if the specified object is equal to this set
368      */
369     boolean equals(Object o);
370 
371     /**
372      * Returns the hash code value for this set.  The hash code of a set is
373      * defined to be the sum of the hash codes of the elements in the set,
374      * where the hash code of a <tt>null</tt> element is defined to be zero.
375      * This ensures that <tt>s1.equals(s2)</tt> implies that
376      * <tt>s1.hashCode()==s2.hashCode()</tt> for any two sets <tt>s1</tt>
377      * and <tt>s2</tt>, as required by the general contract of
378      * {@link Object#hashCode}.
379      *
380      * @return the hash code value for this set
381      * @see Object#equals(Object)
382      * @see Set#equals(Object)
383      */
384     int hashCode();
385 
386     /**
387      * Creates a {@code Spliterator} over the elements in this set.
388      *
389      * <p>The {@code Spliterator} reports {@link Spliterator#DISTINCT}.
390      * Implementations should document the reporting of additional
391      * characteristic values.
392      *
393      * @implSpec
394      * The default implementation creates a
395      * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
396      * from the set's {@code Iterator}.  The spliterator inherits the
397      * <em>fail-fast</em> properties of the set's iterator.
398      * <p>
399      * The created {@code Spliterator} additionally reports
400      * {@link Spliterator#SIZED}.
401      *
402      * @implNote
403      * The created {@code Spliterator} additionally reports
404      * {@link Spliterator#SUBSIZED}.
405      *
406      * @return a {@code Spliterator} over the elements in this set
407      * @since 1.8
408      */
409     @Override
410     default Spliterator<E> spliterator() {
411         return Spliterators.spliterator(this, Spliterator.DISTINCT);
412     }
413 }