# Interface MultiValuedMap<K,V>

Type Parameters:
  `K` - the type of the keys in this map
  `V` - the type of the values in this map

All Known Subinterfaces:
  `ListValuedMap<K,V>`, `SetValuedMap<K,V>`

All Known Implementing Classes:
  `AbstractListValuedMap`, `AbstractMultiValuedMap`, `AbstractMultiValuedMapDecorator`, `AbstractSetValuedMap`, `ArrayListValuedHashMap`, `ArrayListValuedLinkedHashMap`, `HashSetValuedHashMap`, `LinkedHashSetValuedLinkedHashMap`, `TransformedMultiValuedMap`, `UnmodifiableMultiValuedMap`

public interface MultiValuedMap<K,V>
Defines a map that holds a collection of values against each key.

A `MultiValuedMap` is a Map with slightly different semantics:

- Putting a value into the map will add the value to a `Collection` at that key.
- Getting a value will return a `Collection`, holding all the values put to that key.

For example:

     MultiValuedMap<Integer, String> map = new ArrayListValuedHashMap<>();
     map.put(1, "A");
     map.put(1, "B");
     map.put(1, "C");
     Collection<String> coll = map.get(1);

`coll` will be a collection containing "A", "B", "C".

Since:
  4.1

- ## Method Summary

  Modifier and Type
  Method
  Description
  `Map<K,Collection<V>>`
  `asMap()`
  Returns a view of this multivalued map as a `Map` from each distinct key to the non-empty collection of that key's associated values.
  `void`
  `clear()`
  Removes all of the mappings from this map (optional operation).
  `boolean`
  `containsKey(Object key)`
  Returns `true` if this map contains a mapping for the specified key.
  `boolean`
  `containsMapping(Object key, Object value)`
  Checks whether the map contains a mapping for the specified key and value.
  `boolean`
  `containsValue(Object value)`
  Checks whether the map contains at least one mapping for the specified value.
  `Collection<Map.Entry<K,V>>`
  `entries()`
  Returns a `Collection` view of the mappings contained in this multivalued map.
  `Collection<V>`
  `get(K key)`
  Gets a view collection of the values associated with the specified key.
  `boolean`
  `isEmpty()`
  Returns `true` if this map contains no key-value mappings.
  `MultiSet<K>`
  `keys()`
  Returns a `MultiSet` view of the keys contained in this multivalued map.
  `Set<K>`
  `keySet()`
  Returns a `Set` view of the keys contained in this multivalued map.
  `MapIterator<K,V>`
  `mapIterator()`
  Obtains a `MapIterator` over this multivalued map.
  `boolean`
  `put(K key, V value)`
  Adds a key-value mapping to this multivalued map.
  `boolean`
  `putAll(Map<? extends K,? extends V> map)`
  Copies all mappings from the specified map to this multivalued map (optional operation).
  `boolean`
  `putAll(K key, Iterable<? extends V> values)`
  Adds a mapping to the specified key for all values contained in the given Iterable.
  `boolean`
  `putAll(MultiValuedMap<? extends K,? extends V> map)`
  Copies all mappings from the specified map to this multivalued map (optional operation).
  `Collection<V>`
  `remove(Object key)`
  Removes all values associated with the specified key.
  `boolean`
  `removeMapping(Object key, Object item)`
  Removes a key-value mapping from the map.
  `int`
  `size()`
  Gets the total size of the map.
  `Collection<V>`
  `values()`
  Gets a `Collection` view of all values contained in this multivalued map.

- ## Method Details

  - ### asMap

    Map<K,Collection<V>> asMap()
    Returns a view of this multivalued map as a `Map` from each distinct key to the non-empty collection of that key's associated values.

    Note that `this.asMap().get(k)` is equivalent to `this.get(k)` only when `k` is a key contained in the multivalued map; otherwise it returns `null` as opposed to an empty collection.

    Changes to the returned map or the collections that serve as its values will update the underlying multivalued map, and vice versa. The map does not support `put` or `putAll`, nor do its entries support `setValue`.

    Returns:
      a map view of the mappings in this multivalued map

  - ### clear

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

    The map will be empty after this call returns.

    Throws:
      `UnsupportedOperationException` - if the map is unmodifiable

  - ### containsKey

    boolean containsKey(Object key)
    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.)
    Parameters:
      `key` - key whose presence in this map is to be tested
    Returns:
      true if this map contains a mapping for the specified key
    Throws:
      `NullPointerException` - if the specified key is null and this map does not permit null keys (optional)

  - ### containsMapping

    boolean containsMapping(Object key, Object value)
    Checks whether the map contains a mapping for the specified key and value.
    Parameters:
      `key` - the key to search for
      `value` - the value to search for
    Returns:
      true if the map contains the value

  - ### containsValue

    boolean containsValue(Object value)
    Checks whether the map contains at least one mapping for the specified value.
    Parameters:
      `value` - the value to search for
    Returns:
      true if the map contains the value
    Throws:
      `NullPointerException` - if the value is null and null values are not supported by the used collection types (optional)

  - ### entries

    Collection<Map.Entry<K,V>> entries()
    Returns a `Collection` view of the mappings contained in this multivalued map.

    The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa.

    Returns:
      a set view of the mappings contained in this map

  - ### get

    Collection<V> get(K key)
    Gets a view collection of the values associated with the specified key.

    This method will return an empty collection if `containsKey(Object)` returns `false`. Changes to the returned collection will update the underlying `MultiValuedMap` and vice-versa.

    Parameters:
      `key` - the key to retrieve
    Returns:
      the `Collection` of values, implementations should return an empty collection for no mapping
    Throws:
      `NullPointerException` - if the key is null and null keys are invalid (optional)

  - ### isEmpty

    boolean isEmpty()
    Returns `true` if this map contains no key-value mappings.
    Returns:
      `true` if this map contains no key-value mappings

  - ### keys

    MultiSet<K> keys()
    Returns a `MultiSet` view of the keys contained in this multivalued map.

    The `MultiSet.getCount(Object)` method of the returned multiset will give the same result a calling `get(Object).size()` for the same key.

    This multiset is backed by the map, so any changes in the map are reflected in the multiset.

    Returns:
      a multiset view of the keys contained in this map

  - ### keySet

    Set<K> keySet()
    Returns a `Set` view of the keys contained in this multivalued 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 result of the iteration is 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.

    Returns:
      a set view of the keys contained in this map

  - ### mapIterator

    MapIterator<K,V> mapIterator()
    Obtains a `MapIterator` over this multivalued map.

    A map iterator is an efficient way of iterating over maps. There is no need to access the entries collection or use `Map.Entry` objects.

    Returns:
      a map iterator

  - ### put

    boolean put(K key, V value)
    Adds a key-value mapping to this multivalued map.

    Unlike a normal `Map` the previous value is not replaced. Instead, the new value is added to the collection stored against the key. Depending on the collection type used, duplicate key-value mappings may be allowed.

    The method will return `true` if the size of the multivalued map has been increased because of this operation.

    Parameters:
      `key` - the key to store against
      `value` - the value to add to the collection at the key
    Returns:
      true if the map changed as a result of this put operation, or false if the map already contained the key-value mapping and the collection type does not allow duplicate values, for example when using a Set
    Throws:
      `UnsupportedOperationException` - if the put operation is not supported by this multivalued map, for example if it is unmodifiable
      `NullPointerException` - if the key or value is null and null is invalid (optional)
      `IllegalArgumentException` - if some aspect of the specified key or value prevents it from being stored in this multivalued map

  - ### putAll

    boolean putAll(K key, Iterable<? extends V> values)
    Adds a mapping to the specified key for all values contained in the given Iterable.
    Parameters:
      `key` - the key to store against
      `values` - the values to add to the collection at the key, may not be null
    Returns:
      true if the map changed as a result of this operation
    Throws:
      `NullPointerException` - if the specified iterable is null, or if this map does not permit null keys or values, and the specified key or values contain null (optional)

  - ### putAll

    boolean putAll(Map<? extends K,? extends V> map)
    Copies all mappings from the specified map to this multivalued map (optional operation).

    The effect of this call is equivalent to that of calling `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.

    Parameters:
      `map` - mappings to be stored in this map, may not be null
    Returns:
      true if the map changed as a result of this operation
    Throws:
      `UnsupportedOperationException` - if the `putAll` operation is not supported by this map
      `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 (optional)
      `IllegalArgumentException` - if some property of a key or value in the specified map prevents it from being stored in this map

  - ### putAll

    boolean putAll(MultiValuedMap<? extends K,? extends V> map)
    Copies all mappings from the specified map to this multivalued map (optional operation).

    The effect of this call is equivalent to that of calling `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.

    Parameters:
      `map` - mappings to be stored in this map, may not be null
    Returns:
      true if the map changed as a result of this operation
    Throws:
      `UnsupportedOperationException` - if the `putAll` operation is not supported by this map
      `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 (optional)
      `IllegalArgumentException` - if some property of a key or value in the specified map prevents it from being stored in this map

  - ### remove

    Collection<V> remove(Object key)
    Removes all values associated with the specified key.

    The returned collection may be modifiable, but updates will not be propagated to this multivalued map. In case no mapping was stored for the specified key, an empty, unmodifiable collection will be returned.

    Parameters:
      `key` - the key to remove values from
    Returns:
      the values that were removed
    Throws:
      `UnsupportedOperationException` - if the map is unmodifiable
      `NullPointerException` - if the key is null and null keys are invalid (optional)

  - ### removeMapping

    boolean removeMapping(Object key, Object item)
    Removes a key-value mapping from the map.

    The item is removed from the collection mapped to the specified key. Other values attached to that key are unaffected.

    If the last value for a key is removed, implementations typically return an empty collection from a subsequent `get(Object)`.

    Parameters:
      `key` - the key to remove from
      `item` - the item to remove
    Returns:
      true if the mapping was removed, false otherwise
    Throws:
      `UnsupportedOperationException` - if the map is unmodifiable
      `NullPointerException` - if the key or value is null and null is invalid (optional)

  - ### size

    int size()
    Gets the total size of the map.

    Implementations would return the total size of the map which is the count of the values from all keys.

    Returns:
      the total size of the map

  - ### values

    Collection<V> values()
    Gets a `Collection` view of all values contained in this multivalued map.

    Implementations typically return a collection containing the combination of values from all keys.

    Returns:
      a collection view of the values contained in this multivalued map