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. * *

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. * *

Some map operations which perform recursive traversal of the map may fail * with an exception for self-referential instances where the map directly or * indirectly contains itself. This includes the {@code clone()}, * {@code equals()}, {@code hashCode()} and {@code toString()} methods. * Implementations may optionally handle the self-referential scenario, however * most current implementations do not do so. * *

This interface is a member of the * * Java Collections Framework. * * @param the type of keys maintained by this map * @param 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 { // Query Operations /** * Returns the number of key-value mappings in this map. If the * map contains more than Integer.MAX_VALUE elements, returns * Integer.MAX_VALUE. * * @return the number of key-value mappings in this map */ int size(); /** * Returns true if this map contains no key-value mappings. * * @return true if this map contains no key-value mappings */ boolean isEmpty(); /** * Returns true if this map contains a mapping for the specified * key. More formally, returns true if and only if * this map contains a mapping for a key k such that * (key==null ? k==null : key.equals(k)). (There can be * at most one such mapping.) * * @param key key whose presence in this map is to be tested * @return true if this map contains a mapping for the specified * 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) */ boolean containsKey(Object key); /** * Returns true if this map maps one or more keys to the * specified value. More formally, returns true if and only if * this map contains at least one mapping to a value v such that * (value==null ? v==null : value.equals(v)). This operation * will probably require time linear in the map size for most * implementations of the Map interface. * * @param value value whose presence in this map is to be tested * @return true if this map maps one or more keys to the * specified value * @throws ClassCastException if the value is of an inappropriate type for * this map * (optional) * @throws NullPointerException if the specified value is null and this * map does not permit null values * (optional) */ 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. * *

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 * (key==null ? k==null : key.equals(k)), that mapping * is removed. (The map can contain at most one such mapping.) * *

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 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 remove operation), * the results of the iteration are undefined. The collection * supports element removal, which removes the corresponding * mapping from the map, via the Iterator.remove, * Collection.remove, removeAll, * retainAll and clear operations. It does not * support the add or addAll operations. * * @return a collection view of the values contained in this map */ Collection 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 remove operation, or through the * setValue 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 Iterator.remove, * Set.remove, removeAll, retainAll and * clear operations. It does not support the * add or addAll operations. * * @return a set view of the mappings contained in this map */ Set> entrySet(); /** * A map entry (key-value pair). The Map.entrySet method returns * a collection-view of the map, whose elements are of this class. The * only way to obtain a reference to a map entry is from the * iterator of this collection-view. These Map.Entry objects are * valid only 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 setValue operation on the map entry. * * @see Map#entrySet() * @since 1.2 */ interface Entry { /** * 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 * remove 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 remove operation). * * @param value new value to be stored in this entry * @return old value corresponding to the entry * @throws UnsupportedOperationException if the put 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 true if the given object is also a map entry and * the two entries represent the same mapping. More formally, two * entries e1 and e2 represent the same mapping * if

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

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

The returned comparator is serializable and throws {@link * NullPointerException} when comparing an entry with a null key. * * @param the {@link Comparable} type of then map keys * @param the type of the map values * @return a comparator that compares {@link Map.Entry} in natural order on key. * @see Comparable * @since 1.8 */ public static , V> Comparator> comparingByKey() { return (Comparator> & Serializable) (c1, c2) -> c1.getKey().compareTo(c2.getKey()); } /** * Returns a comparator that compares {@link Map.Entry} in natural order on value. * *

The returned comparator is serializable and throws {@link * NullPointerException} when comparing an entry with null values. * * @param the type of the map keys * @param the {@link Comparable} type of the map values * @return a comparator that compares {@link Map.Entry} in natural order on value. * @see Comparable * @since 1.8 */ public static > Comparator> comparingByValue() { return (Comparator> & Serializable) (c1, c2) -> c1.getValue().compareTo(c2.getValue()); } /** * Returns a comparator that compares {@link Map.Entry} by key using the given * {@link Comparator}. * *

The returned comparator is serializable if the specified comparator * is also serializable. * * @param the type of the map keys * @param the type of the map values * @param cmp the key {@link Comparator} * @return a comparator that compares {@link Map.Entry} by the key. * @since 1.8 */ public static Comparator> comparingByKey(Comparator cmp) { Objects.requireNonNull(cmp); return (Comparator> & Serializable) (c1, c2) -> cmp.compare(c1.getKey(), c2.getKey()); } /** * Returns a comparator that compares {@link Map.Entry} by value using the given * {@link Comparator}. * *

The returned comparator is serializable if the specified comparator * is also serializable. * * @param the type of the map keys * @param the type of the map values * @param cmp the value {@link Comparator} * @return a comparator that compares {@link Map.Entry} by the value. * @since 1.8 */ public static Comparator> comparingByValue(Comparator cmp) { Objects.requireNonNull(cmp); return (Comparator> & Serializable) (c1, c2) -> cmp.compare(c1.getValue(), c2.getValue()); } } // 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(); // Defaultable methods /** * Returns the value to which the specified key is mapped, or * {@code defaultValue} if this map contains no mapping for the key. * * @implSpec * The default implementation makes no guarantees about synchronization * or atomicity properties of this method. Any implementation providing * atomicity guarantees must override this method and document its * concurrency properties. * * @param key the key whose associated value is to be returned * @param defaultValue the default mapping of the key * @return the value to which the specified key is mapped, or * {@code defaultValue} 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) * @since 1.8 */ default V getOrDefault(Object key, V defaultValue) { V v; return (((v = get(key)) != null) || containsKey(key)) ? v : defaultValue; } /** * Performs the given action for each entry in this map until all entries * have been processed or the action throws an exception. Unless * otherwise specified by the implementing class, actions are performed in * the order of entry set iteration (if an iteration order is specified.) * Exceptions thrown by the action are relayed to the caller. * * @implSpec * The default implementation is equivalent to, for this {@code map}: *

 {@code
     * for (Map.Entry entry : map.entrySet())
     *     action.accept(entry.getKey(), entry.getValue());
     * }

 {@code
     * for (Map.Entry entry : map.entrySet())
     *     entry.setValue(function.apply(entry.getKey(), entry.getValue()));
     * }

 {@code
     * V v = map.get(key);
     * if (v == null)
     *     v = map.put(key, value);
     *
     * return v;
     * }

 {@code
     * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
     *     map.remove(key);
     *     return true;
     * } else
     *     return false;
     * }

 {@code
     * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
     *     map.put(key, newValue);
     *     return true;
     * } else
     *     return false;
     * }

 {@code
     * if (map.containsKey(key)) {
     *     return map.put(key, value);
     * } else
     *     return null;
     * }

 {@code
     * map.computeIfAbsent(key, k -> new Value(f(k)));
     * }

 {@code
     * map.computeIfAbsent(key, k -> new HashSet()).add(v);
     * }

 {@code
     * if (map.get(key) == null) {
     *     V newValue = mappingFunction.apply(key);
     *     if (newValue != null)
     *         map.put(key, newValue);
     * }
     * }

 {@code
     * if (map.get(key) != null) {
     *     V oldValue = map.get(key);
     *     V newValue = remappingFunction.apply(key, oldValue);
     *     if (newValue != null)
     *         map.put(key, newValue);
     *     else
     *         map.remove(key);
     * }
     * }

 {@code
     * map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))}

 {@code
     * V oldValue = map.get(key);
     * V newValue = remappingFunction.apply(key, oldValue);
     * if (oldValue != null ) {
     *    if (newValue != null)
     *       map.put(key, newValue);
     *    else
     *       map.remove(key);
     * } else {
     *    if (newValue != null)
     *       map.put(key, newValue);
     *    else
     *       return null;
     * }
     * }

 {@code
     * map.merge(key, msg, String::concat)
     * }

 {@code
     * V oldValue = map.get(key);
     * V newValue = (oldValue == null) ? value :
     *              remappingFunction.apply(oldValue, value);
     * if (newValue == null)
     *     map.remove(key);
     * else
     *     map.put(key, newValue);
     * }