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package java.util;

/**

 * An object that maps keys to values.  A map cannot contain duplicate keys;

 * each key can map to at most one value.

 *

 * <p>This interface takes the place of the <tt>Dictionary</tt> class, which

 * was a totally abstract class rather than an interface.

 *

 * <p>The <tt>Map</tt> interface provides three <i>collection views</i>, which

 * allow a map's contents to be viewed as a set of keys, collection of values,

 * or set of key-value mappings.  The <i>order</i> of a map is defined as

 * the order in which the iterators on the map's collection views return their

 * elements.  Some map implementations, like the <tt>TreeMap</tt> class, make

 * specific guarantees as to their order; others, like the <tt>HashMap</tt>

 * class, do not.

 *

 * <p>Note: great care must be exercised if mutable objects are used as map

 * keys.  The behavior of a map is not specified if the value of an object is

 * changed in a manner that affects <tt>equals</tt> comparisons while the

 * object is a key in the map.  A special case of this prohibition is that it

 * is not permissible for a map to contain itself as a key.  While it is

 * permissible for a map to contain itself as a value, extreme caution is

 * advised: the <tt>equals</tt> and <tt>hashCode</tt> methods are no longer

 * well defined on such a map.

 *

 * <p>All general-purpose map implementation classes should provide two

 * "standard" constructors: a void (no arguments) constructor which creates an

 * empty map, and a constructor with a single argument of type <tt>Map</tt>,

 * which creates a new map with the same key-value mappings as its argument.

 * In effect, the latter constructor allows the user to copy any map,

 * producing an equivalent map of the desired class.  There is no way to

 * enforce this recommendation (as interfaces cannot contain constructors) but

 * all of the general-purpose map implementations in the JDK comply.

 *

 * <p>The "destructive" methods contained in this interface, that is, the

 * methods that modify the map on which they operate, are specified to throw

 * <tt>UnsupportedOperationException</tt> if this map does not support the

 * operation.  If this is the case, these methods may, but are not required

 * to, throw an <tt>UnsupportedOperationException</tt> if the invocation would

 * have no effect on the map.  For example, invoking the {@link #putAll(Map)}

 * method on an unmodifiable map may, but is not required to, throw the

 * exception if the map whose mappings are to be "superimposed" is empty.

 *

 * <p>Some map implementations have restrictions on the keys and values they

 * may contain.  For example, some implementations prohibit null keys and

 * values, and some have restrictions on the types of their keys.  Attempting

 * to insert an ineligible key or value throws an unchecked exception,

 * typically <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.

 * Attempting to query the presence of an ineligible key or value may throw an

 * exception, or it may simply return false; some implementations will exhibit

 * the former behavior and some will exhibit the latter.  More generally,

 * attempting an operation on an ineligible key or value whose completion

 * would not result in the insertion of an ineligible element into the map may

 * throw an exception or it may succeed, at the option of the implementation.

 * Such exceptions are marked as "optional" in the specification for this

 * interface.

 *

 * <p>This interface is a member of the

 * <a href="{@docRoot}/../technotes/guides/collections/index.html">

 * Java Collections Framework</a>.

 *

 * <p>Many methods in Collections Framework interfaces are defined

 * in terms of the {@link Object#equals(Object) equals} method.  For

 * example, the specification for the {@link #containsKey(Object)

 * containsKey(Object key)} method says: "returns <tt>true</tt> if and

 * only if this map contains a mapping for a key <tt>k</tt> such that

 * <tt>(key==null ? k==null : key.equals(k))</tt>." This specification should

 * <i>not</i> be construed to imply that invoking <tt>Map.containsKey</tt>

 * with a non-null argument <tt>key</tt> will cause <tt>key.equals(k)</tt> to

 * be invoked for any key <tt>k</tt>.  Implementations are free to

 * implement optimizations whereby the <tt>equals</tt> invocation is avoided,

 * for example, by first comparing the hash codes of the two keys.  (The

 * {@link Object#hashCode()} specification guarantees that two objects with

 * unequal hash codes cannot be equal.)  More generally, implementations of

 * the various Collections Framework interfaces are free to take advantage of

 * the specified behavior of underlying {@link Object} methods wherever the

 * implementor deems it appropriate.

 *

 * @param <K> the type of keys maintained by this map

 * @param <V> the type of mapped values

 *

 * @author  Josh Bloch

 * @see HashMap

 * @see TreeMap

 * @see Hashtable

 * @see SortedMap

 * @see Collection

 * @see Set

 * @since 1.2

 */

public interface Map<K,V> {

    // Query Operations

    /**

     * Returns the number of key-value mappings in this map.  If the

     * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns

     * <tt>Integer.MAX_VALUE</tt>.

     *

     * @return the number of key-value mappings in this map

     */

    int size();

    /**

     * Returns <tt>true</tt> if this map contains no key-value mappings.

     *

     * @return <tt>true</tt> if this map contains no key-value mappings

     */

    boolean isEmpty();

    /**

     * Returns <tt>true</tt> if this map contains a mapping for the specified

     * key.  More formally, returns <tt>true</tt> if and only if

     * this map contains a mapping for a key <tt>k</tt> such that

     * <tt>(key==null ? k==null : key.equals(k))</tt>.  (There can be

     * at most one such mapping.)

     *

     * @param key key whose presence in this map is to be tested

     * @return <tt>true</tt> if this map contains a mapping for the specified

     *         key

     * @throws ClassCastException if the key is of an inappropriate type for

     * @throws NullPointerException if the specified key is null and this map

     */

    boolean containsKey(Object key);

    /**

     * Returns <tt>true</tt> if this map maps one or more keys to the

     * specified value.  More formally, returns <tt>true</tt> if and only if

     * this map contains at least one mapping to a value <tt>v</tt> such that

     * <tt>(value==null ? v==null : value.equals(v))</tt>.  This operation

     * will probably require time linear in the map size for most

     * implementations of the <tt>Map</tt> interface.

     *

     * @param value value whose presence in this map is to be tested

     * @return <tt>true</tt> if this map maps one or more keys to the

     *         specified value

     * @throws ClassCastException if the value is of an inappropriate type for

     * @throws NullPointerException if the specified value is null and this

     */

    boolean containsValue(Object value);

    /**

     * Returns the value to which the specified key is mapped,

     * or {@code null} if this map contains no mapping for the key.

     *

     * <p>More formally, if this map contains a mapping from a key

     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :

     * key.equals(k))}, then this method returns {@code v}; otherwise

     * it returns {@code null}.  (There can be at most one such mapping.)

     *

     * <p>If this map permits null values, then a return value of

     * {@code null} does not <i>necessarily</i> indicate that the map

     * contains no mapping for the key; it's also possible that the map

     * explicitly maps the key to {@code null}.  The {@link #containsKey

     * containsKey} operation may be used to distinguish these two cases.

     *

     * @param key the key whose associated value is to be returned

     * @return the value to which the specified key is mapped, or

     *         {@code null} if this map contains no mapping for the key

     * @throws ClassCastException if the key is of an inappropriate type for

     * @throws NullPointerException if the specified key is null and this map

     */

    V get(Object key);

    // Modification Operations

    /**

     * Associates the specified value with the specified key in this map

     * (optional operation).  If the map previously contained a mapping for

     * the key, the old value is replaced by the specified value.  (A map

     * <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only

     * if {@link #containsKey(Object) m.containsKey(k)} would return

     * <tt>true</tt>.)

     *

     * @param key key with which the specified value is to be associated

     * @param value value to be associated with the specified key

     * @return the previous value associated with <tt>key</tt>, or

     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.

     *         (A <tt>null</tt> return can also indicate that the map

     *         previously associated <tt>null</tt> with <tt>key</tt>,

     *         if the implementation supports <tt>null</tt> values.)

     * @throws UnsupportedOperationException if the <tt>put</tt> operation

     *         is not supported by this map

     * @throws ClassCastException if the class of the specified key or value

     *         prevents it from being stored in this map

     * @throws NullPointerException if the specified key or value is null

     *         and this map does not permit null keys or values

     * @throws IllegalArgumentException if some property of the specified key

     *         or value prevents it from being stored in this map

     */

    V put(K key, V value);

    /**

     * Removes the mapping for a key from this map if it is present

     * (optional operation).   More formally, if this map contains a mapping

     * from key <tt>k</tt> to value <tt>v</tt> such that

     * <code>(key==null ?  k==null : key.equals(k))</code>, that mapping

     * is removed.  (The map can contain at most one such mapping.)

     *

     * <p>Returns the value to which this map previously associated the key,

     * or <tt>null</tt> if the map contained no mapping for the key.

     *

     * <p>If this map permits null values, then a return value of

     * <tt>null</tt> does not <i>necessarily</i> indicate that the map

     * contained no mapping for the key; it's also possible that the map

     * explicitly mapped the key to <tt>null</tt>.

     *

     * <p>The map will not contain a mapping for the specified key once the

     * call returns.

     *

     * @param key key whose mapping is to be removed from the map

     * @return the previous value associated with <tt>key</tt>, or

     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.

     * @throws UnsupportedOperationException if the <tt>remove</tt> operation

     *         is not supported by this map

     * @throws ClassCastException if the key is of an inappropriate type for

     * @throws NullPointerException if the specified key is null and this

     */

    V remove(Object key);

    // Bulk Operations

    /**

     * Copies all of the mappings from the specified map to this map

     * (optional operation).  The effect of this call is equivalent to that

     * of calling {@link #put(Object,Object) put(k, v)} on this map once

     * for each mapping from key <tt>k</tt> to value <tt>v</tt> in the

     * specified map.  The behavior of this operation is undefined if the

     * specified map is modified while the operation is in progress.

     *

     * @param m mappings to be stored in this map

     * @throws UnsupportedOperationException if the <tt>putAll</tt> operation

     *         is not supported by this map

     * @throws ClassCastException if the class of a key or value in the

     *         specified map prevents it from being stored in this map

     * @throws NullPointerException if the specified map is null, or if

     *         this map does not permit null keys or values, and the

     *         specified map contains null keys or values

     * @throws IllegalArgumentException if some property of a key or value in

     *         the specified map prevents it from being stored in this map

     */

    void putAll(Map<? extends K, ? extends V> m);

    /**

     * Removes all of the mappings from this map (optional operation).

     * The map will be empty after this call returns.

     *

     * @throws UnsupportedOperationException if the <tt>clear</tt> operation

     *         is not supported by this map

     */

    void clear();

    // Views

    /**

     * Returns a {@link Set} view of the keys contained in this map.

     * The set is backed by the map, so changes to the map are

     * reflected in the set, and vice-versa.  If the map is modified

     * while an iteration over the set is in progress (except through

     * the iterator's own <tt>remove</tt> operation), the results of

     * the iteration are undefined.  The set supports element removal,

     * which removes the corresponding mapping from the map, via the

     * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,

     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>

     * operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>

     * operations.

     *

     * @return a set view of the keys contained in this map

     */

    Set<K> keySet();

    /**

     * Returns a {@link Collection} view of the values contained in this map.

     * The collection is backed by the map, so changes to the map are

     * reflected in the collection, and vice-versa.  If the map is

     * modified while an iteration over the collection is in progress

     * (except through the iterator's own <tt>remove</tt> operation),

     * the results of the iteration are undefined.  The collection

     * supports element removal, which removes the corresponding

     * mapping from the map, via the <tt>Iterator.remove</tt>,

     * <tt>Collection.remove</tt>, <tt>removeAll</tt>,

     * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not

     * support the <tt>add</tt> or <tt>addAll</tt> operations.

     *

     * @return a collection view of the values contained in this map

     */

    Collection<V> values();

    /**

     * Returns a {@link Set} view of the mappings contained in this map.

     * The set is backed by the map, so changes to the map are

     * reflected in the set, and vice-versa.  If the map is modified

     * while an iteration over the set is in progress (except through

     * the iterator's own <tt>remove</tt> operation, or through the

     * <tt>setValue</tt> operation on a map entry returned by the

     * iterator) the results of the iteration are undefined.  The set

     * supports element removal, which removes the corresponding

     * mapping from the map, via the <tt>Iterator.remove</tt>,

     * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and

     * <tt>clear</tt> operations.  It does not support the

     * <tt>add</tt> or <tt>addAll</tt> operations.

     *

     * @return a set view of the mappings contained in this map

     */

    Set<Map.Entry<K, V>> entrySet();

    /**

     * A map entry (key-value pair).  The <tt>Map.entrySet</tt> method returns

     * a collection-view of the map, whose elements are of this class.  The

     * <i>only</i> way to obtain a reference to a map entry is from the

     * iterator of this collection-view.  These <tt>Map.Entry</tt> objects are

     * valid <i>only</i> for the duration of the iteration; more formally,

     * the behavior of a map entry is undefined if the backing map has been

     * modified after the entry was returned by the iterator, except through

     * the <tt>setValue</tt> operation on the map entry.

     *

     * @see Map#entrySet()

     * @since 1.2

     */

    interface Entry<K,V> {

        /**

         * Returns the key corresponding to this entry.

         *

         * @return the key corresponding to this entry

         * @throws IllegalStateException implementations may, but are not

         *         required to, throw this exception if the entry has been

         *         removed from the backing map.

         */

        K getKey();

        /**

         * Returns the value corresponding to this entry.  If the mapping

         * has been removed from the backing map (by the iterator's

         * <tt>remove</tt> operation), the results of this call are undefined.

         *

         * @return the value corresponding to this entry

         * @throws IllegalStateException implementations may, but are not

         *         required to, throw this exception if the entry has been

         *         removed from the backing map.

         */

        V getValue();

        /**

         * Replaces the value corresponding to this entry with the specified

         * value (optional operation).  (Writes through to the map.)  The

         * behavior of this call is undefined if the mapping has already been

         * removed from the map (by the iterator's <tt>remove</tt> operation).

         *

         * @param value new value to be stored in this entry

         * @return old value corresponding to the entry

         * @throws UnsupportedOperationException if the <tt>put</tt> operation

         *         is not supported by the backing map

         * @throws ClassCastException if the class of the specified value

         *         prevents it from being stored in the backing map

         * @throws NullPointerException if the backing map does not permit

         *         null values, and the specified value is null

         * @throws IllegalArgumentException if some property of this value

         *         prevents it from being stored in the backing map

         * @throws IllegalStateException implementations may, but are not

         *         required to, throw this exception if the entry has been

         *         removed from the backing map.

         */

        V setValue(V value);

        /**

         * Compares the specified object with this entry for equality.

         * Returns <tt>true</tt> if the given object is also a map entry and

         * the two entries represent the same mapping.  More formally, two

         * entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping

         * if<pre>

         *     (e1.getKey()==null ?

         *      e2.getKey()==null : e1.getKey().equals(e2.getKey()))  &&

         *     (e1.getValue()==null ?

         *      e2.getValue()==null : e1.getValue().equals(e2.getValue()))

         * </pre>

         * This ensures that the <tt>equals</tt> method works properly across

         * different implementations of the <tt>Map.Entry</tt> interface.

         *

         * @param o object to be compared for equality with this map entry

         * @return <tt>true</tt> if the specified object is equal to this map

         *         entry

         */

        boolean equals(Object o);

        /**

         * Returns the hash code value for this map entry.  The hash code

         * of a map entry <tt>e</tt> is defined to be: <pre>

         *     (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^

         *     (e.getValue()==null ? 0 : e.getValue().hashCode())

         * </pre>

         * This ensures that <tt>e1.equals(e2)</tt> implies that

         * <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries

         * <tt>e1</tt> and <tt>e2</tt>, as required by the general

         * contract of <tt>Object.hashCode</tt>.

         *

         * @return the hash code value for this map entry

         * @see Object#hashCode()

         * @see Object#equals(Object)

         * @see #equals(Object)

         */

        int hashCode();

    }

    // Comparison and hashing

    /**

     * Compares the specified object with this map for equality.  Returns

     * <tt>true</tt> if the given object is also a map and the two maps

     * represent the same mappings.  More formally, two maps <tt>m1</tt> and

     * <tt>m2</tt> represent the same mappings if

     * <tt>m1.entrySet().equals(m2.entrySet())</tt>.  This ensures that the

     * <tt>equals</tt> method works properly across different implementations

     * of the <tt>Map</tt> interface.

     *

     * @param o object to be compared for equality with this map

     * @return <tt>true</tt> if the specified object is equal to this map

     */

    boolean equals(Object o);

    /**

     * Returns the hash code value for this map.  The hash code of a map is

     * defined to be the sum of the hash codes of each entry in the map's

     * <tt>entrySet()</tt> view.  This ensures that <tt>m1.equals(m2)</tt>

     * implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps

     * <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of

     * {@link Object#hashCode}.

     *

     * @return the hash code value for this map

     * @see Map.Entry#hashCode()

     * @see Object#equals(Object)

     * @see #equals(Object)

     */

    int hashCode();

}