This interface takes the place of the Dictionary class, which + * was a totally abstract class rather than an interface. + * + *
The Map interface provides three collection views, which + * allow a map's contents to be viewed as a set of keys, collection of values, + * or set of key-value mappings. The order 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 TreeMap class, make + * specific guarantees as to their order; others, like the HashMap + * class, do not. + * + *
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 equals 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 equals and hashCode methods are no longer + * well defined on such a map. + * + *
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 Map, + * 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. + * + *
The "destructive" methods contained in this interface, that is, the + * methods that modify the map on which they operate, are specified to throw + * UnsupportedOperationException if this map does not support the + * operation. If this is the case, these methods may, but are not required + * to, throw an UnsupportedOperationException 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. + * + *
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 NullPointerException or ClassCastException. + * 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. + * + *
This interface is a member of the + * + * Java Collections Framework. + * + *
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 true if and
+ * only if this map contains a mapping for a key k such that
+ * (key==null ? k==null : key.equals(k))." This specification should
+ * not be construed to imply that invoking Map.containsKey
+ * with a non-null argument key will cause key.equals(k) to
+ * be invoked for any key k. Implementations are free to
+ * implement optimizations whereby the equals 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 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.)
+ *
+ * If this map permits null values, then a return value of
+ * {@code null} does not necessarily 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
+ * this map
+ * (optional)
+ * @throws NullPointerException if the specified key is null and this map
+ * does not permit null keys
+ * (optional)
+ */
+ 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
+ * m is said to contain a mapping for a key k if and only
+ * if {@link #containsKey(Object) m.containsKey(k)} would return
+ * true.)
+ *
+ * @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 key, or
+ * null if there was no mapping for key.
+ * (A null return can also indicate that the map
+ * previously associated null with key,
+ * if the implementation supports null values.)
+ * @throws UnsupportedOperationException if the put 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 k to value v such that
+ * Returns the value to which this map previously associated the key,
+ * or null if the map contained no mapping for the key.
+ *
+ * If this map permits null values, then a return value of
+ * null does not necessarily indicate that the map
+ * contained no mapping for the key; it's also possible that the map
+ * explicitly mapped the key to null.
+ *
+ * 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 key, or
+ * null if there was no mapping for key.
+ * @throws UnsupportedOperationException if the remove operation
+ * is not supported by this map
+ * @throws ClassCastException if the key is of an inappropriate type for
+ * this map
+ * (optional)
+ * @throws NullPointerException if the specified key is null and this
+ * map does not permit null keys
+ * (optional)
+ */
+ 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 k to value v 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 putAll 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 m);
+
+ /**
+ * Removes all of the mappings from this map (optional operation).
+ * The map will be empty after this call returns.
+ *
+ * @throws UnsupportedOperationException if the clear 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 remove operation), the results of
+ * the iteration are undefined. The set supports element removal,
+ * which removes the corresponding mapping from the map, via the
+ * Iterator.remove, Set.remove,
+ * removeAll, retainAll, and clear
+ * operations. It does not support the add or addAll
+ * operations.
+ *
+ * @return a set view of the keys contained in this map
+ */
+ Set(key==null ? k==null : key.equals(k)), that mapping
+ * is removed. (The map can contain at most one such mapping.)
+ *
+ *
+ * (e1.getKey()==null ?
+ * e2.getKey()==null : e1.getKey().equals(e2.getKey())) &&
+ * (e1.getValue()==null ?
+ * e2.getValue()==null : e1.getValue().equals(e2.getValue()))
+ *
+ * This ensures that the equals method works properly across
+ * different implementations of the Map.Entry interface.
+ *
+ * @param o object to be compared for equality with this map entry
+ * @return true 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 e is defined to be:
+ * (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
+ * (e.getValue()==null ? 0 : e.getValue().hashCode())
+ *
+ * This ensures that e1.equals(e2) implies that
+ * e1.hashCode()==e2.hashCode() for any two Entries
+ * e1 and e2, as required by the general
+ * contract of Object.hashCode.
+ *
+ * @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
+ * true if the given object is also a map and the two maps
+ * represent the same mappings. More formally, two maps m1 and
+ * m2 represent the same mappings if
+ * m1.entrySet().equals(m2.entrySet()). This ensures that the
+ * equals method works properly across different implementations
+ * of the Map interface.
+ *
+ * @param o object to be compared for equality with this map
+ * @return true 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
+ * entrySet() view. This ensures that m1.equals(m2)
+ * implies that m1.hashCode()==m2.hashCode() for any two maps
+ * m1 and m2, 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();
+
+}