1 /*
   2  * Copyright 1997-2006 Sun Microsystems, Inc.  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.  Sun designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
  22  * CA 95054 USA or visit www.sun.com if you need additional information or
  23  * have any questions.
  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>Some collection implementations have restrictions on the elements that
  64  * they may contain.  For example, some implementations prohibit null elements,
  65  * and some have restrictions on the types of their elements.  Attempting to
  66  * add an ineligible element throws an unchecked exception, typically
  67  * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.  Attempting
  68  * to query the presence of an ineligible element may throw an exception,
  69  * or it may simply return false; some implementations will exhibit the former
  70  * behavior and some will exhibit the latter.  More generally, attempting an
  71  * operation on an ineligible element whose completion would not result in
  72  * the insertion of an ineligible element into the collection may throw an
  73  * exception or it may succeed, at the option of the implementation.
  74  * Such exceptions are marked as "optional" in the specification for this
  75  * interface.
  76  *
  77  * <p>It is up to each collection to determine its own synchronization
  78  * policy.  In the absence of a stronger guarantee by the
  79  * implementation, undefined behavior may result from the invocation
  80  * of any method on a collection that is being mutated by another
  81  * thread; this includes direct invocations, passing the collection to
  82  * a method that might perform invocations, and using an existing
  83  * iterator to examine the collection.
  84  *
  85  * <p>Many methods in Collections Framework interfaces are defined in
  86  * terms of the {@link Object#equals(Object) equals} method.  For example,
  87  * the specification for the {@link #contains(Object) contains(Object o)}
  88  * method says: "returns <tt>true</tt> if and only if this collection
  89  * contains at least one element <tt>e</tt> such that
  90  * <tt>(o==null ? e==null : o.equals(e))</tt>."  This specification should
  91  * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt>
  92  * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be
  93  * invoked for any element <tt>e</tt>.  Implementations are free to implement
  94  * optimizations whereby the <tt>equals</tt> invocation is avoided, for
  95  * example, by first comparing the hash codes of the two elements.  (The
  96  * {@link Object#hashCode()} specification guarantees that two objects with
  97  * unequal hash codes cannot be equal.)  More generally, implementations of
  98  * the various Collections Framework interfaces are free to take advantage of
  99  * the specified behavior of underlying {@link Object} methods wherever the
 100  * implementor deems it appropriate.
 101  *
 102  * <p>This interface is a member of the
 103  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 104  * Java Collections Framework</a>.
 105  *
 106  * @author  Josh Bloch
 107  * @author  Neal Gafter
 108  * @see     Set
 109  * @see     List
 110  * @see     Map
 111  * @see     SortedSet
 112  * @see     SortedMap
 113  * @see     HashSet
 114  * @see     TreeSet
 115  * @see     ArrayList
 116  * @see     LinkedList
 117  * @see     Vector
 118  * @see     Collections
 119  * @see     Arrays
 120  * @see     AbstractCollection
 121  * @since 1.2
 122  */
 123 
 124 public interface Collection<E> extends Iterable<E> {
 125     // Query Operations
 126 
 127     /**
 128      * Returns the number of elements in this collection.  If this collection
 129      * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
 130      * <tt>Integer.MAX_VALUE</tt>.
 131      *
 132      * @return the number of elements in this collection
 133      */
 134     int size();
 135 
 136     /**
 137      * Returns <tt>true</tt> if this collection contains no elements.
 138      *
 139      * @return <tt>true</tt> if this collection contains no elements
 140      */
 141     boolean isEmpty();
 142 
 143     /**
 144      * Returns <tt>true</tt> if this collection contains the specified element.
 145      * More formally, returns <tt>true</tt> if and only if this collection
 146      * contains at least one element <tt>e</tt> such that
 147      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
 148      *
 149      * @param o element whose presence in this collection is to be tested
 150      * @return <tt>true</tt> if this collection contains the specified
 151      *         element
 152      * @throws ClassCastException if the type of the specified element
 153      *         is incompatible with this collection (optional)
 154      * @throws NullPointerException if the specified element is null and this
 155      *         collection does not permit null elements (optional)
 156      */
 157     boolean contains(Object o);
 158 
 159     /**
 160      * Returns an iterator over the elements in this collection.  There are no
 161      * guarantees concerning the order in which the elements are returned
 162      * (unless this collection is an instance of some class that provides a
 163      * guarantee).
 164      *
 165      * @return an <tt>Iterator</tt> over the elements in this collection
 166      */
 167     Iterator<E> iterator();
 168 
 169     /**
 170      * Returns an array containing all of the elements in this collection.
 171      * If this collection makes any guarantees as to what order its elements
 172      * are returned by its iterator, this method must return the elements in
 173      * the same order.
 174      *
 175      * <p>The returned array will be "safe" in that no references to it are
 176      * maintained by this collection.  (In other words, this method must
 177      * allocate a new array even if this collection is backed by an array).
 178      * The caller is thus free to modify the returned array.
 179      *
 180      * <p>This method acts as bridge between array-based and collection-based
 181      * APIs.
 182      *
 183      * @return an array containing all of the elements in this collection
 184      */
 185     Object[] toArray();
 186 
 187     /**
 188      * Returns an array containing all of the elements in this collection;
 189      * the runtime type of the returned array is that of the specified array.
 190      * If the collection fits in the specified array, it is returned therein.
 191      * Otherwise, a new array is allocated with the runtime type of the
 192      * specified array and the size of this collection.
 193      *
 194      * <p>If this collection fits in the specified array with room to spare
 195      * (i.e., the array has more elements than this collection), the element
 196      * in the array immediately following the end of the collection is set to
 197      * <tt>null</tt>.  (This is useful in determining the length of this
 198      * collection <i>only</i> if the caller knows that this collection does
 199      * not contain any <tt>null</tt> elements.)
 200      *
 201      * <p>If this collection makes any guarantees as to what order its elements
 202      * are returned by its iterator, this method must return the elements in
 203      * the same order.
 204      *
 205      * <p>Like the {@link #toArray()} method, this method acts as bridge between
 206      * array-based and collection-based APIs.  Further, this method allows
 207      * precise control over the runtime type of the output array, and may,
 208      * under certain circumstances, be used to save allocation costs.
 209      *
 210      * <p>Suppose <tt>x</tt> is a collection known to contain only strings.
 211      * The following code can be used to dump the collection into a newly
 212      * allocated array of <tt>String</tt>:
 213      *
 214      * <pre>
 215      *     String[] y = x.toArray(new String[0]);</pre>
 216      *
 217      * Note that <tt>toArray(new Object[0])</tt> is identical in function to
 218      * <tt>toArray()</tt>.
 219      *
 220      * @param a the array into which the elements of this collection are to be
 221      *        stored, if it is big enough; otherwise, a new array of the same
 222      *        runtime type is allocated for this purpose.
 223      * @return an array containing all of the elements in this collection
 224      * @throws ArrayStoreException if the runtime type of the specified array
 225      *         is not a supertype of the runtime type of every element in
 226      *         this collection
 227      * @throws NullPointerException if the specified array is null
 228      */
 229     <T> T[] toArray(T[] a);
 230 
 231     // Modification Operations
 232 
 233     /**
 234      * Ensures that this collection contains the specified element (optional
 235      * operation).  Returns <tt>true</tt> if this collection changed as a
 236      * result of the call.  (Returns <tt>false</tt> if this collection does
 237      * not permit duplicates and already contains the specified element.)<p>
 238      *
 239      * Collections that support this operation may place limitations on what
 240      * elements may be added to this collection.  In particular, some
 241      * collections will refuse to add <tt>null</tt> elements, and others will
 242      * impose restrictions on the type of elements that may be added.
 243      * Collection classes should clearly specify in their documentation any
 244      * restrictions on what elements may be added.<p>
 245      *
 246      * If a collection refuses to add a particular element for any reason
 247      * other than that it already contains the element, it <i>must</i> throw
 248      * an exception (rather than returning <tt>false</tt>).  This preserves
 249      * the invariant that a collection always contains the specified element
 250      * after this call returns.
 251      *
 252      * @param e element whose presence in this collection is to be ensured
 253      * @return <tt>true</tt> if this collection changed as a result of the
 254      *         call
 255      * @throws UnsupportedOperationException if the <tt>add</tt> operation
 256      *         is not supported by this collection
 257      * @throws ClassCastException if the class of the specified element
 258      *         prevents it from being added to this collection
 259      * @throws NullPointerException if the specified element is null and this
 260      *         collection does not permit null elements
 261      * @throws IllegalArgumentException if some property of the element
 262      *         prevents it from being added to this collection
 263      * @throws IllegalStateException if the element cannot be added at this
 264      *         time due to insertion restrictions
 265      */
 266     boolean add(E e);
 267 
 268     /**
 269      * Removes a single instance of the specified element from this
 270      * collection, if it is present (optional operation).  More formally,
 271      * removes an element <tt>e</tt> such that
 272      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>, if
 273      * this collection contains one or more such elements.  Returns
 274      * <tt>true</tt> if this collection contained the specified element (or
 275      * equivalently, if this collection changed as a result of the call).
 276      *
 277      * @param o element to be removed from this collection, if present
 278      * @return <tt>true</tt> if an element was removed as a result of this call
 279      * @throws ClassCastException if the type of the specified element
 280      *         is incompatible with this collection (optional)
 281      * @throws NullPointerException if the specified element is null and this
 282      *         collection does not permit null elements (optional)
 283      * @throws UnsupportedOperationException if the <tt>remove</tt> operation
 284      *         is not supported by this collection
 285      */
 286     boolean remove(Object o);
 287 
 288 
 289     // Bulk Operations
 290 
 291     /**
 292      * Returns <tt>true</tt> if this collection contains all of the elements
 293      * in the specified collection.
 294      *
 295      * @param  c collection to be checked for containment in this collection
 296      * @return <tt>true</tt> if this collection contains all of the elements
 297      *         in the specified collection
 298      * @throws ClassCastException if the types of one or more elements
 299      *         in the specified collection are incompatible with this
 300      *         collection (optional)
 301      * @throws NullPointerException if the specified collection contains one
 302      *         or more null elements and this collection does not permit null
 303      *         elements (optional), or if the specified collection is null
 304      * @see    #contains(Object)
 305      */
 306     boolean containsAll(Collection<?> c);
 307 
 308     /**
 309      * Adds all of the elements in the specified collection to this collection
 310      * (optional operation).  The behavior of this operation is undefined if
 311      * the specified collection is modified while the operation is in progress.
 312      * (This implies that the behavior of this call is undefined if the
 313      * specified collection is this collection, and this collection is
 314      * nonempty.)
 315      *
 316      * @param c collection containing elements to be added to this collection
 317      * @return <tt>true</tt> if this collection changed as a result of the call
 318      * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
 319      *         is not supported by this collection
 320      * @throws ClassCastException if the class of an element of the specified
 321      *         collection prevents it from being added to this collection
 322      * @throws NullPointerException if the specified collection contains a
 323      *         null element and this collection does not permit null elements,
 324      *         or if the specified collection is null
 325      * @throws IllegalArgumentException if some property of an element of the
 326      *         specified collection prevents it from being added to this
 327      *         collection
 328      * @throws IllegalStateException if not all the elements can be added at
 329      *         this time due to insertion restrictions
 330      * @see #add(Object)
 331      */
 332     boolean addAll(Collection<? extends E> c);
 333 
 334     /**
 335      * Removes all of this collection's elements that are also contained in the
 336      * specified collection (optional operation).  After this call returns,
 337      * this collection will contain no elements in common with the specified
 338      * collection.
 339      *
 340      * @param c collection containing elements to be removed from this collection
 341      * @return <tt>true</tt> if this collection changed as a result of the
 342      *         call
 343      * @throws UnsupportedOperationException if the <tt>removeAll</tt> method
 344      *         is not supported by this collection
 345      * @throws ClassCastException if the types of one or more elements
 346      *         in this collection are incompatible with the specified
 347      *         collection (optional)
 348      * @throws NullPointerException if this collection contains one or more
 349      *         null elements and the specified collection does not support
 350      *         null elements (optional), or if the specified collection is null
 351      * @see #remove(Object)
 352      * @see #contains(Object)
 353      */
 354     boolean removeAll(Collection<?> c);
 355 
 356     /**
 357      * Retains only the elements in this collection that are contained in the
 358      * specified collection (optional operation).  In other words, removes from
 359      * this collection all of its elements that are not contained in the
 360      * specified collection.
 361      *
 362      * @param c collection containing elements to be retained in this collection
 363      * @return <tt>true</tt> if this collection changed as a result of the call
 364      * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
 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 (optional)
 369      * @throws NullPointerException if this collection contains one or more
 370      *         null elements and the specified collection does not permit null
 371      *         elements (optional), or if the specified collection is null
 372      * @see #remove(Object)
 373      * @see #contains(Object)
 374      */
 375     boolean retainAll(Collection<?> c);
 376 
 377     /**
 378      * Removes all of the elements from this collection (optional operation).
 379      * The collection will be empty after this method returns.
 380      *
 381      * @throws UnsupportedOperationException if the <tt>clear</tt> operation
 382      *         is not supported by this collection
 383      */
 384     void clear();
 385 
 386 
 387     // Comparison and hashing
 388 
 389     /**
 390      * Compares the specified object with this collection for equality. <p>
 391      *
 392      * While the <tt>Collection</tt> interface adds no stipulations to the
 393      * general contract for the <tt>Object.equals</tt>, programmers who
 394      * implement the <tt>Collection</tt> interface "directly" (in other words,
 395      * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
 396      * or a <tt>List</tt>) must exercise care if they choose to override the
 397      * <tt>Object.equals</tt>.  It is not necessary to do so, and the simplest
 398      * course of action is to rely on <tt>Object</tt>'s implementation, but
 399      * the implementor may wish to implement a "value comparison" in place of
 400      * the default "reference comparison."  (The <tt>List</tt> and
 401      * <tt>Set</tt> interfaces mandate such value comparisons.)<p>
 402      *
 403      * The general contract for the <tt>Object.equals</tt> method states that
 404      * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
 405      * only if <tt>b.equals(a)</tt>).  The contracts for <tt>List.equals</tt>
 406      * and <tt>Set.equals</tt> state that lists are only equal to other lists,
 407      * and sets to other sets.  Thus, a custom <tt>equals</tt> method for a
 408      * collection class that implements neither the <tt>List</tt> nor
 409      * <tt>Set</tt> interface must return <tt>false</tt> when this collection
 410      * is compared to any list or set.  (By the same logic, it is not possible
 411      * to write a class that correctly implements both the <tt>Set</tt> and
 412      * <tt>List</tt> interfaces.)
 413      *
 414      * @param o object to be compared for equality with this collection
 415      * @return <tt>true</tt> if the specified object is equal to this
 416      * collection
 417      *
 418      * @see Object#equals(Object)
 419      * @see Set#equals(Object)
 420      * @see List#equals(Object)
 421      */
 422     boolean equals(Object o);
 423 
 424     /**
 425      * Returns the hash code value for this collection.  While the
 426      * <tt>Collection</tt> interface adds no stipulations to the general
 427      * contract for the <tt>Object.hashCode</tt> method, programmers should
 428      * take note that any class that overrides the <tt>Object.equals</tt>
 429      * method must also override the <tt>Object.hashCode</tt> method in order
 430      * to satisfy the general contract for the <tt>Object.hashCode</tt>method.
 431      * In particular, <tt>c1.equals(c2)</tt> implies that
 432      * <tt>c1.hashCode()==c2.hashCode()</tt>.
 433      *
 434      * @return the hash code value for this collection
 435      *
 436      * @see Object#hashCode()
 437      * @see Object#equals(Object)
 438      */
 439     int hashCode();
 440 }