1 /*
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   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   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).
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  17  * You should have received a copy of the GNU General Public License version
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  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
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  22  * or visit www.oracle.com if you need additional information or have any
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  24  */
  25 
  26 package java.util;
  27 
  28 /**
  29  * The root interface in the <i>collection hierarchy</i>.  A collection
  30  * represents a group of objects, known as its <i>elements</i>.  Some
  31  * collections allow duplicate elements and others do not.  Some are ordered
  32  * and others unordered.  The JDK does not provide any <i>direct</i>
  33  * implementations of this interface: it provides implementations of more
  34  * specific subinterfaces like <tt>Set</tt> and <tt>List</tt>.  This interface
  35  * is typically used to pass collections around and manipulate them where
  36  * maximum generality is desired.
  37  *
  38  * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain
  39  * duplicate elements) should implement this interface directly.
  40  *
  41  * <p>All general-purpose <tt>Collection</tt> implementation classes (which
  42  * typically implement <tt>Collection</tt> indirectly through one of its
  43  * subinterfaces) should provide two "standard" constructors: a void (no
  44  * arguments) constructor, which creates an empty collection, and a
  45  * constructor with a single argument of type <tt>Collection</tt>, which
  46  * creates a new collection with the same elements as its argument.  In
  47  * effect, the latter constructor allows the user to copy any collection,
  48  * producing an equivalent collection of the desired implementation type.
  49  * There is no way to enforce this convention (as interfaces cannot contain
  50  * constructors) but all of the general-purpose <tt>Collection</tt>
  51  * implementations in the Java platform libraries comply.
  52  *
  53  * <p>The "destructive" methods contained in this interface, that is, the
  54  * methods that modify the collection on which they operate, are specified to
  55  * throw <tt>UnsupportedOperationException</tt> if this collection does not
  56  * support the operation.  If this is the case, these methods may, but are not
  57  * required to, throw an <tt>UnsupportedOperationException</tt> if the
  58  * invocation would have no effect on the collection.  For example, invoking
  59  * the {@link #addAll(Collection)} method on an unmodifiable collection may,
  60  * but is not required to, throw the exception if the collection to be added
  61  * is empty.
  62  *
  63  * <p><a name="optional-restrictions"/>
  64  * Some collection implementations have restrictions on the elements that
  65  * they may contain.  For example, some implementations prohibit null elements,
  66  * and some have restrictions on the types of their elements.  Attempting to
  67  * add an ineligible element throws an unchecked exception, typically
  68  * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.  Attempting
  69  * to query the presence of an ineligible element may throw an exception,
  70  * or it may simply return false; some implementations will exhibit the former
  71  * behavior and some will exhibit the latter.  More generally, attempting an
  72  * operation on an ineligible element whose completion would not result in
  73  * the insertion of an ineligible element into the collection may throw an
  74  * exception or it may succeed, at the option of the implementation.
  75  * Such exceptions are marked as "optional" in the specification for this
  76  * interface.
  77  *
  78  * <p>It is up to each collection to determine its own synchronization
  79  * policy.  In the absence of a stronger guarantee by the
  80  * implementation, undefined behavior may result from the invocation
  81  * of any method on a collection that is being mutated by another
  82  * thread; this includes direct invocations, passing the collection to
  83  * a method that might perform invocations, and using an existing
  84  * iterator to examine the collection.
  85  *
  86  * <p>Many methods in Collections Framework interfaces are defined in
  87  * terms of the {@link Object#equals(Object) equals} method.  For example,
  88  * the specification for the {@link #contains(Object) contains(Object o)}
  89  * method says: "returns <tt>true</tt> if and only if this collection
  90  * contains at least one element <tt>e</tt> such that
  91  * <tt>(o==null ? e==null : o.equals(e))</tt>."  This specification should
  92  * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt>
  93  * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be
  94  * invoked for any element <tt>e</tt>.  Implementations are free to implement
  95  * optimizations whereby the <tt>equals</tt> invocation is avoided, for
  96  * example, by first comparing the hash codes of the two elements.  (The
  97  * {@link Object#hashCode()} specification guarantees that two objects with
  98  * unequal hash codes cannot be equal.)  More generally, implementations of
  99  * the various Collections Framework interfaces are free to take advantage of
 100  * the specified behavior of underlying {@link Object} methods wherever the
 101  * implementor deems it appropriate.
 102  *
 103  * <p>This interface is a member of the
 104  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 105  * Java Collections Framework</a>.
 106  *
 107  * @param <E> the type of elements in this collection
 108  *
 109  * @author  Josh Bloch
 110  * @author  Neal Gafter
 111  * @see     Set
 112  * @see     List
 113  * @see     Map
 114  * @see     SortedSet
 115  * @see     SortedMap
 116  * @see     HashSet
 117  * @see     TreeSet
 118  * @see     ArrayList
 119  * @see     LinkedList
 120  * @see     Vector
 121  * @see     Collections
 122  * @see     Arrays
 123  * @see     AbstractCollection
 124  * @since 1.2
 125  */
 126 
 127 public interface Collection<E> extends Iterable<E> {
 128     // Query Operations
 129 
 130     /**
 131      * Returns the number of elements in this collection.  If this collection
 132      * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
 133      * <tt>Integer.MAX_VALUE</tt>.
 134      *
 135      * @return the number of elements in this collection
 136      */
 137     int size();
 138 
 139     /**
 140      * Returns <tt>true</tt> if this collection contains no elements.
 141      *
 142      * @return <tt>true</tt> if this collection contains no elements
 143      */
 144     boolean isEmpty();
 145 
 146     /**
 147      * Returns <tt>true</tt> if this collection contains the specified element.
 148      * More formally, returns <tt>true</tt> if and only if this collection
 149      * contains at least one element <tt>e</tt> such that
 150      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
 151      *
 152      * @param o element whose presence in this collection is to be tested
 153      * @return <tt>true</tt> if this collection contains the specified
 154      *         element
 155      * @throws ClassCastException if the type of the specified element
 156      *         is incompatible with this collection
 157      *         (<a href="#optional-restrictions">optional</a>)
 158      * @throws NullPointerException if the specified element is null and this
 159      *         collection does not permit null elements
 160      *         (<a href="#optional-restrictions">optional</a>)
 161      */
 162     boolean contains(Object o);
 163 
 164     /**
 165      * Returns an iterator over the elements in this collection.  There are no
 166      * guarantees concerning the order in which the elements are returned
 167      * (unless this collection is an instance of some class that provides a
 168      * guarantee).
 169      *
 170      * @return an <tt>Iterator</tt> over the elements in this collection
 171      */
 172     Iterator<E> iterator();
 173 
 174     /**
 175      * Returns an array containing all of the elements in this collection.
 176      * If this collection makes any guarantees as to what order its elements
 177      * are returned by its iterator, this method must return the elements in
 178      * the same order.
 179      *
 180      * <p>The returned array will be "safe" in that no references to it are
 181      * maintained by this collection.  (In other words, this method must
 182      * allocate a new array even if this collection is backed by an array).
 183      * The caller is thus free to modify the returned array.
 184      *
 185      * <p>This method acts as bridge between array-based and collection-based
 186      * APIs.
 187      *
 188      * @return an array containing all of the elements in this collection
 189      */
 190     Object[] toArray();
 191 
 192     /**
 193      * Returns an array containing all of the elements in this collection;
 194      * the runtime type of the returned array is that of the specified array.
 195      * If the collection fits in the specified array, it is returned therein.
 196      * Otherwise, a new array is allocated with the runtime type of the
 197      * specified array and the size of this collection.
 198      *
 199      * <p>If this collection fits in the specified array with room to spare
 200      * (i.e., the array has more elements than this collection), the element
 201      * in the array immediately following the end of the collection is set to
 202      * <tt>null</tt>.  (This is useful in determining the length of this
 203      * collection <i>only</i> if the caller knows that this collection does
 204      * not contain any <tt>null</tt> elements.)
 205      *
 206      * <p>If this collection makes any guarantees as to what order its elements
 207      * are returned by its iterator, this method must return the elements in
 208      * the same order.
 209      *
 210      * <p>Like the {@link #toArray()} method, this method acts as bridge between
 211      * array-based and collection-based APIs.  Further, this method allows
 212      * precise control over the runtime type of the output array, and may,
 213      * under certain circumstances, be used to save allocation costs.
 214      *
 215      * <p>Suppose <tt>x</tt> is a collection known to contain only strings.
 216      * The following code can be used to dump the collection into a newly
 217      * allocated array of <tt>String</tt>:
 218      *
 219      * <pre>
 220      *     String[] y = x.toArray(new String[0]);</pre>
 221      *
 222      * Note that <tt>toArray(new Object[0])</tt> is identical in function to
 223      * <tt>toArray()</tt>.
 224      *
 225      * @param a the array into which the elements of this collection are to be
 226      *        stored, if it is big enough; otherwise, a new array of the same
 227      *        runtime type is allocated for this purpose.
 228      * @return an array containing all of the elements in this collection
 229      * @throws ArrayStoreException if the runtime type of the specified array
 230      *         is not a supertype of the runtime type of every element in
 231      *         this collection
 232      * @throws NullPointerException if the specified array is null
 233      */
 234     <T> T[] toArray(T[] a);
 235 
 236     // Modification Operations
 237 
 238     /**
 239      * Ensures that this collection contains the specified element (optional
 240      * operation).  Returns <tt>true</tt> if this collection changed as a
 241      * result of the call.  (Returns <tt>false</tt> if this collection does
 242      * not permit duplicates and already contains the specified element.)<p>
 243      *
 244      * Collections that support this operation may place limitations on what
 245      * elements may be added to this collection.  In particular, some
 246      * collections will refuse to add <tt>null</tt> elements, and others will
 247      * impose restrictions on the type of elements that may be added.
 248      * Collection classes should clearly specify in their documentation any
 249      * restrictions on what elements may be added.<p>
 250      *
 251      * If a collection refuses to add a particular element for any reason
 252      * other than that it already contains the element, it <i>must</i> throw
 253      * an exception (rather than returning <tt>false</tt>).  This preserves
 254      * the invariant that a collection always contains the specified element
 255      * after this call returns.
 256      *
 257      * @param e element whose presence in this collection is to be ensured
 258      * @return <tt>true</tt> if this collection changed as a result of the
 259      *         call
 260      * @throws UnsupportedOperationException if the <tt>add</tt> operation
 261      *         is not supported by this collection
 262      * @throws ClassCastException if the class of the specified element
 263      *         prevents it from being added to this collection
 264      * @throws NullPointerException if the specified element is null and this
 265      *         collection does not permit null elements
 266      * @throws IllegalArgumentException if some property of the element
 267      *         prevents it from being added to this collection
 268      * @throws IllegalStateException if the element cannot be added at this
 269      *         time due to insertion restrictions
 270      */
 271     boolean add(E e);
 272 
 273     /**
 274      * Removes a single instance of the specified element from this
 275      * collection, if it is present (optional operation).  More formally,
 276      * removes an element <tt>e</tt> such that
 277      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>, if
 278      * this collection contains one or more such elements.  Returns
 279      * <tt>true</tt> if this collection contained the specified element (or
 280      * equivalently, if this collection changed as a result of the call).
 281      *
 282      * @param o element to be removed from this collection, if present
 283      * @return <tt>true</tt> if an element was removed as a result of this call
 284      * @throws ClassCastException if the type of the specified element
 285      *         is incompatible with this collection
 286      *         (<a href="#optional-restrictions">optional</a>)
 287      * @throws NullPointerException if the specified element is null and this
 288      *         collection does not permit null elements
 289      *         (<a href="#optional-restrictions">optional</a>)
 290      * @throws UnsupportedOperationException if the <tt>remove</tt> operation
 291      *         is not supported by this collection
 292      */
 293     boolean remove(Object o);
 294 
 295 
 296     // Bulk Operations
 297 
 298     /**
 299      * Returns <tt>true</tt> if this collection contains all of the elements
 300      * in the specified collection.
 301      *
 302      * @param  c collection to be checked for containment in this collection
 303      * @return <tt>true</tt> if this collection contains all of the elements
 304      *         in the specified collection
 305      * @throws ClassCastException if the types of one or more elements
 306      *         in the specified collection are incompatible with this
 307      *         collection
 308      *         (<a href="#optional-restrictions">optional</a>)
 309      * @throws NullPointerException if the specified collection contains one
 310      *         or more null elements and this collection does not permit null
 311      *         elements
 312      *         (<a href="#optional-restrictions">optional</a>),
 313      *         or if the specified collection is null.
 314      * @see    #contains(Object)
 315      */
 316     boolean containsAll(Collection<?> c);
 317 
 318     /**
 319      * Adds all of the elements in the specified collection to this collection
 320      * (optional operation).  The behavior of this operation is undefined if
 321      * the specified collection is modified while the operation is in progress.
 322      * (This implies that the behavior of this call is undefined if the
 323      * specified collection is this collection, and this collection is
 324      * nonempty.)
 325      *
 326      * @param c collection containing elements to be added to this collection
 327      * @return <tt>true</tt> if this collection changed as a result of the call
 328      * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
 329      *         is not supported by this collection
 330      * @throws ClassCastException if the class of an element of the specified
 331      *         collection prevents it from being added to this collection
 332      * @throws NullPointerException if the specified collection contains a
 333      *         null element and this collection does not permit null elements,
 334      *         or if the specified collection is null
 335      * @throws IllegalArgumentException if some property of an element of the
 336      *         specified collection prevents it from being added to this
 337      *         collection
 338      * @throws IllegalStateException if not all the elements can be added at
 339      *         this time due to insertion restrictions
 340      * @see #add(Object)
 341      */
 342     boolean addAll(Collection<? extends E> c);
 343 
 344     /**
 345      * Removes all of this collection's elements that are also contained in the
 346      * specified collection (optional operation).  After this call returns,
 347      * this collection will contain no elements in common with the specified
 348      * collection.
 349      *
 350      * @param c collection containing elements to be removed from this collection
 351      * @return <tt>true</tt> if this collection changed as a result of the
 352      *         call
 353      * @throws UnsupportedOperationException if the <tt>removeAll</tt> method
 354      *         is not supported by this collection
 355      * @throws ClassCastException if the types of one or more elements
 356      *         in this collection are incompatible with the specified
 357      *         collection
 358      *         (<a href="#optional-restrictions">optional</a>)
 359      * @throws NullPointerException if this collection contains one or more
 360      *         null elements and the specified collection does not support
 361      *         null elements
 362      *         (<a href="#optional-restrictions">optional</a>),
 363      *         or if the specified collection is null
 364      * @see #remove(Object)
 365      * @see #contains(Object)
 366      */
 367     boolean removeAll(Collection<?> c);
 368 
 369     /**
 370      * Retains only the elements in this collection that are contained in the
 371      * specified collection (optional operation).  In other words, removes from
 372      * this collection all of its elements that are not contained in the
 373      * specified collection.
 374      *
 375      * @param c collection containing elements to be retained in this collection
 376      * @return <tt>true</tt> if this collection changed as a result of the call
 377      * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
 378      *         is not supported by this collection
 379      * @throws ClassCastException if the types of one or more elements
 380      *         in this collection are incompatible with the specified
 381      *         collection
 382      *         (<a href="#optional-restrictions">optional</a>)
 383      * @throws NullPointerException if this collection contains one or more
 384      *         null elements and the specified collection does not permit null
 385      *         elements
 386      *         (<a href="#optional-restrictions">optional</a>),
 387      *         or if the specified collection is null
 388      * @see #remove(Object)
 389      * @see #contains(Object)
 390      */
 391     boolean retainAll(Collection<?> c);
 392 
 393     /**
 394      * Removes all of the elements from this collection (optional operation).
 395      * The collection will be empty after this method returns.
 396      *
 397      * @throws UnsupportedOperationException if the <tt>clear</tt> operation
 398      *         is not supported by this collection
 399      */
 400     void clear();
 401 
 402 
 403     // Comparison and hashing
 404 
 405     /**
 406      * Compares the specified object with this collection for equality. <p>
 407      *
 408      * While the <tt>Collection</tt> interface adds no stipulations to the
 409      * general contract for the <tt>Object.equals</tt>, programmers who
 410      * implement the <tt>Collection</tt> interface "directly" (in other words,
 411      * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
 412      * or a <tt>List</tt>) must exercise care if they choose to override the
 413      * <tt>Object.equals</tt>.  It is not necessary to do so, and the simplest
 414      * course of action is to rely on <tt>Object</tt>'s implementation, but
 415      * the implementor may wish to implement a "value comparison" in place of
 416      * the default "reference comparison."  (The <tt>List</tt> and
 417      * <tt>Set</tt> interfaces mandate such value comparisons.)<p>
 418      *
 419      * The general contract for the <tt>Object.equals</tt> method states that
 420      * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
 421      * only if <tt>b.equals(a)</tt>).  The contracts for <tt>List.equals</tt>
 422      * and <tt>Set.equals</tt> state that lists are only equal to other lists,
 423      * and sets to other sets.  Thus, a custom <tt>equals</tt> method for a
 424      * collection class that implements neither the <tt>List</tt> nor
 425      * <tt>Set</tt> interface must return <tt>false</tt> when this collection
 426      * is compared to any list or set.  (By the same logic, it is not possible
 427      * to write a class that correctly implements both the <tt>Set</tt> and
 428      * <tt>List</tt> interfaces.)
 429      *
 430      * @param o object to be compared for equality with this collection
 431      * @return <tt>true</tt> if the specified object is equal to this
 432      * collection
 433      *
 434      * @see Object#equals(Object)
 435      * @see Set#equals(Object)
 436      * @see List#equals(Object)
 437      */
 438     boolean equals(Object o);
 439 
 440     /**
 441      * Returns the hash code value for this collection.  While the
 442      * <tt>Collection</tt> interface adds no stipulations to the general
 443      * contract for the <tt>Object.hashCode</tt> method, programmers should
 444      * take note that any class that overrides the <tt>Object.equals</tt>
 445      * method must also override the <tt>Object.hashCode</tt> method in order
 446      * to satisfy the general contract for the <tt>Object.hashCode</tt> method.
 447      * In particular, <tt>c1.equals(c2)</tt> implies that
 448      * <tt>c1.hashCode()==c2.hashCode()</tt>.
 449      *
 450      * @return the hash code value for this collection
 451      *
 452      * @see Object#hashCode()
 453      * @see Object#equals(Object)
 454      */
 455     int hashCode();
 456 }