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   25   
   26   package java.util;
   27   
   28   /**
   29    * An object that maps keys to values.  A map cannot contain duplicate keys;
   30    * each key can map to at most one value.
   31    *
   32    * <p>This interface takes the place of the <tt>Dictionary</tt> class, which
   33    * was a totally abstract class rather than an interface.
   34    *
   35    * <p>The <tt>Map</tt> interface provides three <i>collection views</i>, which
   36    * allow a map's contents to be viewed as a set of keys, collection of values,
   37    * or set of key-value mappings.  The <i>order</i> of a map is defined as
   38    * the order in which the iterators on the map's collection views return their
   39    * elements.  Some map implementations, like the <tt>TreeMap</tt> class, make
   40    * specific guarantees as to their order; others, like the <tt>HashMap</tt>
   41    * class, do not.
   42    *
   43    * <p>Note: great care must be exercised if mutable objects are used as map
   44    * keys.  The behavior of a map is not specified if the value of an object is
   45    * changed in a manner that affects <tt>equals</tt> comparisons while the
   46    * object is a key in the map.  A special case of this prohibition is that it
   47    * is not permissible for a map to contain itself as a key.  While it is
   48    * permissible for a map to contain itself as a value, extreme caution is
   49    * advised: the <tt>equals</tt> and <tt>hashCode</tt> methods are no longer
   50    * well defined on such a map.
   51    *
   52    * <p>All general-purpose map implementation classes should provide two
   53    * "standard" constructors: a void (no arguments) constructor which creates an
   54    * empty map, and a constructor with a single argument of type <tt>Map</tt>,
   55    * which creates a new map with the same key-value mappings as its argument.
   56    * In effect, the latter constructor allows the user to copy any map,
   57    * producing an equivalent map of the desired class.  There is no way to
   58    * enforce this recommendation (as interfaces cannot contain constructors) but
   59    * all of the general-purpose map implementations in the JDK comply.
   60    *
   61    * <p>The "destructive" methods contained in this interface, that is, the
   62    * methods that modify the map on which they operate, are specified to throw
   63    * <tt>UnsupportedOperationException</tt> if this map does not support the
   64    * operation.  If this is the case, these methods may, but are not required
   65    * to, throw an <tt>UnsupportedOperationException</tt> if the invocation would
   66    * have no effect on the map.  For example, invoking the {@link #putAll(Map)}
   67    * method on an unmodifiable map may, but is not required to, throw the
   68    * exception if the map whose mappings are to be "superimposed" is empty.
   69    *
   70    * <p>Some map implementations have restrictions on the keys and values they
   71    * may contain.  For example, some implementations prohibit null keys and
   72    * values, and some have restrictions on the types of their keys.  Attempting
   73    * to insert an ineligible key or value throws an unchecked exception,
   74    * typically <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.
   75    * Attempting to query the presence of an ineligible key or value may throw an
   76    * exception, or it may simply return false; some implementations will exhibit
   77    * the former behavior and some will exhibit the latter.  More generally,
   78    * attempting an operation on an ineligible key or value whose completion
   79    * would not result in the insertion of an ineligible element into the map may
   80    * throw an exception or it may succeed, at the option of the implementation.
   81    * Such exceptions are marked as "optional" in the specification for this
   82    * interface.
   83    *
   84    * <p>This interface is a member of the
   85    * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   86    * Java Collections Framework</a>.
   87    *
   88    * <p>Many methods in Collections Framework interfaces are defined
   89    * in terms of the {@link Object#equals(Object) equals} method.  For
   90    * example, the specification for the {@link #containsKey(Object)
   91    * containsKey(Object key)} method says: "returns <tt>true</tt> if and
   92    * only if this map contains a mapping for a key <tt>k</tt> such that
   93    * <tt>(key==null ? k==null : key.equals(k))</tt>." This specification should
   94    * <i>not</i> be construed to imply that invoking <tt>Map.containsKey</tt>
   95    * with a non-null argument <tt>key</tt> will cause <tt>key.equals(k)</tt> to
   96    * be invoked for any key <tt>k</tt>.  Implementations are free to
   97    * implement optimizations whereby the <tt>equals</tt> invocation is avoided,
   98    * for example, by first comparing the hash codes of the two keys.  (The
   99    * {@link Object#hashCode()} specification guarantees that two objects with
  100    * unequal hash codes cannot be equal.)  More generally, implementations of
  101    * the various Collections Framework interfaces are free to take advantage of
  102    * the specified behavior of underlying {@link Object} methods wherever the
  103    * implementor deems it appropriate.
  104    *
  105    * @param <K> the type of keys maintained by this map
  106    * @param <V> the type of mapped values
  107    *
  108    * @author  Josh Bloch
  109    * @see HashMap
  110    * @see TreeMap
  111    * @see Hashtable
  112    * @see SortedMap
  113    * @see Collection
  114    * @see Set
  115    * @since 1.2
  116    */
  117   public interface Map<K,V> {
  118       // Query Operations
  119   
  120       /**
  121        * Returns the number of key-value mappings in this map.  If the
  122        * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
  123        * <tt>Integer.MAX_VALUE</tt>.
  124        *
  125        * @return the number of key-value mappings in this map
  126        */
  127       int size();
  128   
  129       /**
  130        * Returns <tt>true</tt> if this map contains no key-value mappings.
  131        *
  132        * @return <tt>true</tt> if this map contains no key-value mappings
  133        */
  134       boolean isEmpty();
  135   
  136       /**
  137        * Returns <tt>true</tt> if this map contains a mapping for the specified
  138        * key.  More formally, returns <tt>true</tt> if and only if
  139        * this map contains a mapping for a key <tt>k</tt> such that
  140        * <tt>(key==null ? k==null : key.equals(k))</tt>.  (There can be
  141        * at most one such mapping.)
  142        *
  143        * @param key key whose presence in this map is to be tested
  144        * @return <tt>true</tt> if this map contains a mapping for the specified
  145        *         key
  146        * @throws ClassCastException if the key is of an inappropriate type for
  147        *         this map
  148        * (<a href="Collection.html#optional-restrictions">optional</a>)
  149        * @throws NullPointerException if the specified key is null and this map
  150        *         does not permit null keys
  151        * (<a href="Collection.html#optional-restrictions">optional</a>)
  152        */
  153       boolean containsKey(Object key);
  154   
  155       /**
  156        * Returns <tt>true</tt> if this map maps one or more keys to the
  157        * specified value.  More formally, returns <tt>true</tt> if and only if
  158        * this map contains at least one mapping to a value <tt>v</tt> such that
  159        * <tt>(value==null ? v==null : value.equals(v))</tt>.  This operation
  160        * will probably require time linear in the map size for most
  161        * implementations of the <tt>Map</tt> interface.
  162        *
  163        * @param value value whose presence in this map is to be tested
  164        * @return <tt>true</tt> if this map maps one or more keys to the
  165        *         specified value
  166        * @throws ClassCastException if the value is of an inappropriate type for
  167        *         this map
  168        * (<a href="Collection.html#optional-restrictions">optional</a>)
  169        * @throws NullPointerException if the specified value is null and this
  170        *         map does not permit null values
  171        * (<a href="Collection.html#optional-restrictions">optional</a>)
  172        */
  173       boolean containsValue(Object value);
  174   
  175       /**
  176        * Returns the value to which the specified key is mapped,
  177        * or {@code null} if this map contains no mapping for the key.
  178        *
  179        * <p>More formally, if this map contains a mapping from a key
  180        * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
  181        * key.equals(k))}, then this method returns {@code v}; otherwise
  182        * it returns {@code null}.  (There can be at most one such mapping.)
  183        *
  184        * <p>If this map permits null values, then a return value of
  185        * {@code null} does not <i>necessarily</i> indicate that the map
  186        * contains no mapping for the key; it's also possible that the map
  187        * explicitly maps the key to {@code null}.  The {@link #containsKey
  188        * containsKey} operation may be used to distinguish these two cases.
  189        *
  190        * @param key the key whose associated value is to be returned
  191        * @return the value to which the specified key is mapped, or
  192        *         {@code null} if this map contains no mapping for the key
  193        * @throws ClassCastException if the key is of an inappropriate type for
  194        *         this map
  195        * (<a href="Collection.html#optional-restrictions">optional</a>)
  196        * @throws NullPointerException if the specified key is null and this map
  197        *         does not permit null keys
  198        * (<a href="Collection.html#optional-restrictions">optional</a>)
  199        */
  200       V get(Object key);
  201   
  202       // Modification Operations
  203   
  204       /**
  205        * Associates the specified value with the specified key in this map
  206        * (optional operation).  If the map previously contained a mapping for
  207        * the key, the old value is replaced by the specified value.  (A map
  208        * <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only
  209        * if {@link #containsKey(Object) m.containsKey(k)} would return
  210        * <tt>true</tt>.)
  211        *
  212        * @param key key with which the specified value is to be associated
  213        * @param value value to be associated with the specified key
  214        * @return the previous value associated with <tt>key</tt>, or
  215        *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
  216        *         (A <tt>null</tt> return can also indicate that the map
  217        *         previously associated <tt>null</tt> with <tt>key</tt>,
  218        *         if the implementation supports <tt>null</tt> values.)
  219        * @throws UnsupportedOperationException if the <tt>put</tt> operation
  220        *         is not supported by this map
  221        * @throws ClassCastException if the class of the specified key or value
  222        *         prevents it from being stored in this map
  223        * @throws NullPointerException if the specified key or value is null
  224        *         and this map does not permit null keys or values
  225        * @throws IllegalArgumentException if some property of the specified key
  226        *         or value prevents it from being stored in this map
  227        */
  228       V put(K key, V value);
  229   
  230       /**
  231        * Removes the mapping for a key from this map if it is present
  232        * (optional operation).   More formally, if this map contains a mapping
  233        * from key <tt>k</tt> to value <tt>v</tt> such that
  234        * <code>(key==null ?  k==null : key.equals(k))</code>, that mapping
  235        * is removed.  (The map can contain at most one such mapping.)
  236        *
  237        * <p>Returns the value to which this map previously associated the key,
  238        * or <tt>null</tt> if the map contained no mapping for the key.
  239        *
  240        * <p>If this map permits null values, then a return value of
  241        * <tt>null</tt> does not <i>necessarily</i> indicate that the map
  242        * contained no mapping for the key; it's also possible that the map
  243        * explicitly mapped the key to <tt>null</tt>.
  244        *
  245        * <p>The map will not contain a mapping for the specified key once the
  246        * call returns.
  247        *
  248        * @param key key whose mapping is to be removed from the map
  249        * @return the previous value associated with <tt>key</tt>, or
  250        *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
  251        * @throws UnsupportedOperationException if the <tt>remove</tt> operation
  252        *         is not supported by this map
  253        * @throws ClassCastException if the key is of an inappropriate type for
  254        *         this map
  255        * (<a href="Collection.html#optional-restrictions">optional</a>)
  256        * @throws NullPointerException if the specified key is null and this
  257        *         map does not permit null keys
  258        * (<a href="Collection.html#optional-restrictions">optional</a>)
  259        */
  260       V remove(Object key);
  261   
  262   
  263       // Bulk Operations
  264   
  265       /**
  266        * Copies all of the mappings from the specified map to this map
  267        * (optional operation).  The effect of this call is equivalent to that
  268        * of calling {@link #put(Object,Object) put(k, v)} on this map once
  269        * for each mapping from key <tt>k</tt> to value <tt>v</tt> in the
  270        * specified map.  The behavior of this operation is undefined if the
  271        * specified map is modified while the operation is in progress.
  272        *
  273        * @param m mappings to be stored in this map
  274        * @throws UnsupportedOperationException if the <tt>putAll</tt> operation
  275        *         is not supported by this map
  276        * @throws ClassCastException if the class of a key or value in the
  277        *         specified map prevents it from being stored in this map
  278        * @throws NullPointerException if the specified map is null, or if
  279        *         this map does not permit null keys or values, and the
  280        *         specified map contains null keys or values
  281        * @throws IllegalArgumentException if some property of a key or value in
  282        *         the specified map prevents it from being stored in this map
  283        */
  284       void putAll(Map<? extends K, ? extends V> m);
  285   
  286       /**
  287        * Removes all of the mappings from this map (optional operation).
  288        * The map will be empty after this call returns.
  289        *
  290        * @throws UnsupportedOperationException if the <tt>clear</tt> operation
  291        *         is not supported by this map
  292        */
  293       void clear();
  294   
  295   
  296       // Views
  297   
  298       /**
  299        * Returns a {@link Set} view of the keys contained in this map.
  300        * The set is backed by the map, so changes to the map are
  301        * reflected in the set, and vice-versa.  If the map is modified
  302        * while an iteration over the set is in progress (except through
  303        * the iterator's own <tt>remove</tt> operation), the results of
  304        * the iteration are undefined.  The set supports element removal,
  305        * which removes the corresponding mapping from the map, via the
  306        * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
  307        * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
  308        * operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>
  309        * operations.
  310        *
  311        * @return a set view of the keys contained in this map
  312        */
  313       Set<K> keySet();
  314   
  315       /**
  316        * Returns a {@link Collection} view of the values contained in this map.
  317        * The collection is backed by the map, so changes to the map are
  318        * reflected in the collection, and vice-versa.  If the map is
  319        * modified while an iteration over the collection is in progress
  320        * (except through the iterator's own <tt>remove</tt> operation),
  321        * the results of the iteration are undefined.  The collection
  322        * supports element removal, which removes the corresponding
  323        * mapping from the map, via the <tt>Iterator.remove</tt>,
  324        * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
  325        * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not
  326        * support the <tt>add</tt> or <tt>addAll</tt> operations.
  327        *
  328        * @return a collection view of the values contained in this map
  329        */
  330       Collection<V> values();
  331   
  332       /**
  333        * Returns a {@link Set} view of the mappings contained in this map.
  334        * The set is backed by the map, so changes to the map are
  335        * reflected in the set, and vice-versa.  If the map is modified
  336        * while an iteration over the set is in progress (except through
  337        * the iterator's own <tt>remove</tt> operation, or through the
  338        * <tt>setValue</tt> operation on a map entry returned by the
  339        * iterator) the results of the iteration are undefined.  The set
  340        * supports element removal, which removes the corresponding
  341        * mapping from the map, via the <tt>Iterator.remove</tt>,
  342        * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
  343        * <tt>clear</tt> operations.  It does not support the
  344        * <tt>add</tt> or <tt>addAll</tt> operations.
  345        *
  346        * @return a set view of the mappings contained in this map
  347        */
  348       Set<Map.Entry<K, V>> entrySet();
  349   
  350       /**
  351        * A map entry (key-value pair).  The <tt>Map.entrySet</tt> method returns
  352        * a collection-view of the map, whose elements are of this class.  The
  353        * <i>only</i> way to obtain a reference to a map entry is from the
  354        * iterator of this collection-view.  These <tt>Map.Entry</tt> objects are
  355        * valid <i>only</i> for the duration of the iteration; more formally,
  356        * the behavior of a map entry is undefined if the backing map has been
  357        * modified after the entry was returned by the iterator, except through
  358        * the <tt>setValue</tt> operation on the map entry.
  359        *
  360        * @see Map#entrySet()
  361        * @since 1.2
  362        */
  363       interface Entry<K,V> {
  364           /**
  365            * Returns the key corresponding to this entry.
  366            *
  367            * @return the key corresponding to this entry
  368            * @throws IllegalStateException implementations may, but are not
  369            *         required to, throw this exception if the entry has been
  370            *         removed from the backing map.
  371            */
  372           K getKey();
  373   
  374           /**
  375            * Returns the value corresponding to this entry.  If the mapping
  376            * has been removed from the backing map (by the iterator's
  377            * <tt>remove</tt> operation), the results of this call are undefined.
  378            *
  379            * @return the value corresponding to this entry
  380            * @throws IllegalStateException implementations may, but are not
  381            *         required to, throw this exception if the entry has been
  382            *         removed from the backing map.
  383            */
  384           V getValue();
  385   
  386           /**
  387            * Replaces the value corresponding to this entry with the specified
  388            * value (optional operation).  (Writes through to the map.)  The
  389            * behavior of this call is undefined if the mapping has already been
  390            * removed from the map (by the iterator's <tt>remove</tt> operation).
  391            *
  392            * @param value new value to be stored in this entry
  393            * @return old value corresponding to the entry
  394            * @throws UnsupportedOperationException if the <tt>put</tt> operation
  395            *         is not supported by the backing map
  396            * @throws ClassCastException if the class of the specified value
  397            *         prevents it from being stored in the backing map
  398            * @throws NullPointerException if the backing map does not permit
  399            *         null values, and the specified value is null
  400            * @throws IllegalArgumentException if some property of this value
  401            *         prevents it from being stored in the backing map
  402            * @throws IllegalStateException implementations may, but are not
  403            *         required to, throw this exception if the entry has been
  404            *         removed from the backing map.
  405            */
  406           V setValue(V value);
  407   
  408           /**
  409            * Compares the specified object with this entry for equality.
  410            * Returns <tt>true</tt> if the given object is also a map entry and
  411            * the two entries represent the same mapping.  More formally, two
  412            * entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping
  413            * if<pre>
  414            *     (e1.getKey()==null ?
  415            *      e2.getKey()==null : e1.getKey().equals(e2.getKey()))  &amp;&amp;
  416            *     (e1.getValue()==null ?
  417            *      e2.getValue()==null : e1.getValue().equals(e2.getValue()))
  418            * </pre>
  419            * This ensures that the <tt>equals</tt> method works properly across
  420            * different implementations of the <tt>Map.Entry</tt> interface.
  421            *
  422            * @param o object to be compared for equality with this map entry
  423            * @return <tt>true</tt> if the specified object is equal to this map
  424            *         entry
  425            */
  426           boolean equals(Object o);
  427   
  428           /**
  429            * Returns the hash code value for this map entry.  The hash code
  430            * of a map entry <tt>e</tt> is defined to be: <pre>
  431            *     (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
  432            *     (e.getValue()==null ? 0 : e.getValue().hashCode())
  433            * </pre>
  434            * This ensures that <tt>e1.equals(e2)</tt> implies that
  435            * <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries
  436            * <tt>e1</tt> and <tt>e2</tt>, as required by the general
  437            * contract of <tt>Object.hashCode</tt>.
  438            *
  439            * @return the hash code value for this map entry
  440            * @see Object#hashCode()
  441            * @see Object#equals(Object)
  442            * @see #equals(Object)
  443            */
  444           int hashCode();
  445       }
  446   
  447       // Comparison and hashing
  448   
  449       /**
  450        * Compares the specified object with this map for equality.  Returns
  451        * <tt>true</tt> if the given object is also a map and the two maps
  452        * represent the same mappings.  More formally, two maps <tt>m1</tt> and
  453        * <tt>m2</tt> represent the same mappings if
  454        * <tt>m1.entrySet().equals(m2.entrySet())</tt>.  This ensures that the
  455        * <tt>equals</tt> method works properly across different implementations
  456        * of the <tt>Map</tt> interface.
  457        *
  458        * @param o object to be compared for equality with this map
  459        * @return <tt>true</tt> if the specified object is equal to this map
  460        */
  461       boolean equals(Object o);
  462   
  463       /**
  464        * Returns the hash code value for this map.  The hash code of a map is
  465        * defined to be the sum of the hash codes of each entry in the map's
  466        * <tt>entrySet()</tt> view.  This ensures that <tt>m1.equals(m2)</tt>
  467        * implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps
  468        * <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of
  469        * {@link Object#hashCode}.
  470        *
  471        * @return the hash code value for this map
  472        * @see Map.Entry#hashCode()
  473        * @see Object#equals(Object)
  474        * @see #equals(Object)
  475        */
  476       int hashCode();
  477   
  478   }

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