Interface Multimap<K extends @Nullable Object, V extends @Nullable Object>

All Known Subinterfaces:
ListMultimap<K,V>, SetMultimap<K,V>, SortedSetMultimap<K,V>
All Known Implementing Classes:
ArrayListMultimap, ForwardingListMultimap, ForwardingMultimap, ForwardingSetMultimap, ForwardingSortedSetMultimap, HashMultimap, ImmutableListMultimap, ImmutableMultimap, ImmutableSetMultimap, LinkedHashMultimap, LinkedListMultimap, TreeMultimap
@DoNotMock("Use ImmutableMultimap, HashMultimap, or another implementation") @GwtCompatible public interface Multimap<K extends @Nullable Object, V extends @Nullable Object>
A collection that maps keys to values, similar to Map, but in which each key may be associated with multiple values. You can visualize the contents of a multimap either as a map from keys to nonempty collections of values:
  • a → 1, 2
  • b → 3
... or as a single "flattened" collection of key-value pairs:
  • a → 1
  • a → 2
  • b → 3

Important: although the first interpretation resembles how most multimaps are implemented, the design of the Multimap API is based on the second form. So, using the multimap shown above as an example, the size() is 3, not 2, and the values() collection is [1, 2, 3], not [[1, 2], [3]]. For those times when the first style is more useful, use the multimap's asMap() view (or create a Map<K, Collection<V>> in the first place).

Example

The following code: 

ListMultimap<String, String> multimap = ArrayListMultimap.create();
for (President pres : US_PRESIDENTS_IN_ORDER) {
  multimap.put(pres.firstName(), pres.lastName());
}
for (String firstName : multimap.keySet()) {
  List<String> lastNames = multimap.get(firstName);
  out.println(firstName + ": " + lastNames);
}
... produces output such as: 
Zachary: [Taylor]
John: [Adams, Adams, Tyler, Kennedy]  // Remember, Quincy!
George: [Washington, Bush, Bush]
Grover: [Cleveland, Cleveland]        // Two, non-consecutive terms, rep'ing NJ!
...

Views

Much of the power of the multimap API comes from the view collections it provides. These always reflect the latest state of the multimap itself. When they support modification, the changes are write-through (they automatically update the backing multimap). These view collections are:

The collections returned by the replaceValues and removeAll methods, which contain values that have just been removed from the multimap, are naturally not views.

Subinterfaces

Instead of using the Multimap interface directly, prefer the subinterfaces ListMultimap and SetMultimap. These take their names from the fact that the collections they return from get behave like (and, of course, implement) List and Set, respectively.

For example, the "presidents" code snippet above used a ListMultimap; if it had used a SetMultimap instead, two presidents would have vanished, and last names might or might not appear in chronological order.

Warning: instances of type Multimap may not implement Object.equals(java.lang.Object) in the way you expect. Multimaps containing the same key-value pairs, even in the same order, may or may not be equal and may or may not have the same hashCode. The recommended subinterfaces provide much stronger guarantees.

Comparison to a map of collections

Multimaps are commonly used in places where a Map<K, Collection<V>> would otherwise have appeared. The differences include:

  • There is no need to populate an empty collection before adding an entry with put.
  • get never returns null, only an empty collection.
  • A key is contained in the multimap if and only if it maps to at least one value. Any operation that causes a key to have zero associated values has the effect of removing that key from the multimap.
  • The total entry count is available as size().
  • Many complex operations become easier; for example, Collections.min(multimap.values()) finds the smallest value across all keys.

Implementations

Guava contains a number of other multimap implementations, such as ArrayListMultimap. In new code, we recommend using MultimapBuilder instead: It provides better control of how keys and values are stored.

Other Notes

As with Map, the behavior of a Multimap is not specified if key objects already present in the multimap change in a manner that affects equals comparisons. Use caution if mutable objects are used as keys in a Multimap.

All methods that modify the multimap are optional. The view collections returned by the multimap may or may not be modifiable. Any modification method that is not supported will throw UnsupportedOperationException.

See the Guava User Guide article on Multimap.

Since:
2.0
Author:
Jared Levy

  • Method Summary

    Modifier and Type
    Method
    Description
    Map<K, Collection<V>>
    asMap()
    Returns a view of this multimap as a Map from each distinct key to the nonempty collection of that key's associated values.
    void
    clear()
    Removes all key-value pairs from the multimap, leaving it empty.
    boolean
    containsEntry(@Nullable Object key, @Nullable Object value)
    Returns true if this multimap contains at least one key-value pair with the key key and the value value.
    boolean
    containsKey(@Nullable Object key)
    Returns true if this multimap contains at least one key-value pair with the key key.
    boolean
    containsValue(@Nullable Object value)
    Returns true if this multimap contains at least one key-value pair with the value value.
    Collection<Map.Entry<K,V>>
    entries()
    Returns a view collection of all key-value pairs contained in this multimap, as Map.Entry instances.
    boolean
    equals(@Nullable Object obj)
    Compares the specified object with this multimap for equality.
    Collection<V>
    get(K key)
    Returns a view collection of the values associated with key in this multimap, if any.
    int
    hashCode()
    Returns the hash code for this multimap.
    boolean
    isEmpty()
    Returns true if this multimap contains no key-value pairs.
    Multiset<K>
    keys()
    Returns a view collection containing the key from each key-value pair in this multimap, without collapsing duplicates.
    Set<K>
    keySet()
    Returns a view collection of all distinct keys contained in this multimap.
    boolean
    put(K key, V value)
    Stores a key-value pair in this multimap.
    boolean
    putAll(Multimap<? extends K, ? extends V> multimap)
    Stores all key-value pairs of multimap in this multimap, in the order returned by multimap.entries().
    boolean
    putAll(K key, Iterable<? extends V> values)
    Stores a key-value pair in this multimap for each of values, all using the same key, key.
    boolean
    remove(@Nullable Object key, @Nullable Object value)
    Removes a single key-value pair with the key key and the value value from this multimap, if such exists.
    Collection<V>
    removeAll(@Nullable Object key)
    Removes all values associated with the key key.
    Collection<V>
    replaceValues(K key, Iterable<? extends V> values)
    Stores a collection of values with the same key, replacing any existing values for that key.
    int
    size()
    Returns the number of key-value pairs in this multimap.
    Collection<V>
    values()
    Returns a view collection containing the value from each key-value pair contained in this multimap, without collapsing duplicates (so values().size() == size()).

  • Method Details

    • size

      int size()
      Returns the number of key-value pairs in this multimap.

      Note: this method does not return the number of distinct keys in the multimap, which is given by keySet().size() or asMap().size(). See the opening section of the Multimap class documentation for clarification.

    • isEmpty

      boolean isEmpty()
      Returns true if this multimap contains no key-value pairs. Equivalent to size() == 0, but can in some cases be more efficient.

    • containsKey

      boolean containsKey(@CompatibleWith("K") @Nullable Object key)
      Returns true if this multimap contains at least one key-value pair with the key key.

    • containsValue

      boolean containsValue(@CompatibleWith("V") @Nullable Object value)
      Returns true if this multimap contains at least one key-value pair with the value value.

    • containsEntry

      boolean containsEntry(@CompatibleWith("K") @Nullable Object key, @CompatibleWith("V") @Nullable Object value)
      Returns true if this multimap contains at least one key-value pair with the key key and the value value.

    • put

      @CanIgnoreReturnValue boolean put(K key, V value)
      Stores a key-value pair in this multimap.

      Some multimap implementations allow duplicate key-value pairs, in which case put always adds a new key-value pair and increases the multimap size by 1. Other implementations prohibit duplicates, and storing a key-value pair that's already in the multimap has no effect.

      Returns:
      true if the method increased the size of the multimap, or false if the multimap already contained the key-value pair and doesn't allow duplicates

    • remove

      @CanIgnoreReturnValue boolean remove(@CompatibleWith("K") @Nullable Object key, @CompatibleWith("V") @Nullable Object value)
      Removes a single key-value pair with the key key and the value value from this multimap, if such exists. If multiple key-value pairs in the multimap fit this description, which one is removed is unspecified.
      Returns:
      true if the multimap changed

    • putAll

      @CanIgnoreReturnValue boolean putAll(K key, Iterable<? extends V> values)
      Stores a key-value pair in this multimap for each of values, all using the same key, key. Equivalent to (but expected to be more efficient than): 
      for (V value : values) {
  put(key, value);
}

      In particular, this is a no-op if values is empty.

      Returns:
      true if the multimap changed

    • putAll

      @CanIgnoreReturnValue boolean putAll(Multimap<? extends K, ? extends V> multimap)
      Stores all key-value pairs of multimap in this multimap, in the order returned by multimap.entries().
      Returns:
      true if the multimap changed

    • replaceValues

      @CanIgnoreReturnValue Collection<V> replaceValues(K key, Iterable<? extends V> values)
      Stores a collection of values with the same key, replacing any existing values for that key.

      If values is empty, this is equivalent to removeAll(key).

      Returns:
      the collection of replaced values, or an empty collection if no values were previously associated with the key. The collection may be modifiable, but updating it will have no effect on the multimap.

    • removeAll

      @CanIgnoreReturnValue Collection<V> removeAll(@CompatibleWith("K") @Nullable Object key)
      Removes all values associated with the key key.

      Once this method returns, key will not be mapped to any values, so it will not appear in keySet(), asMap(), or any other views.

      Returns:
      the values that were removed (possibly empty). The returned collection may be modifiable, but updating it will have no effect on the multimap.

    • clear

      void clear()
      Removes all key-value pairs from the multimap, leaving it empty.

    • get

      Collection<V> get(K key)
      Returns a view collection of the values associated with key in this multimap, if any. Note that when containsKey(key) is false, this returns an empty collection, not null.

      Changes to the returned collection will update the underlying multimap, and vice versa.

    • keySet

      Set<K> keySet()
      Returns a view collection of all distinct keys contained in this multimap. Note that the key set contains a key if and only if this multimap maps that key to at least one value.

      Changes to the returned set will update the underlying multimap, and vice versa. However, adding to the returned set is not possible.

    • keys

      Multiset<K> keys()
      Returns a view collection containing the key from each key-value pair in this multimap, without collapsing duplicates. This collection has the same size as this multimap, and keys().count(k) == get(k).size() for all k.

      Changes to the returned multiset will update the underlying multimap, and vice versa. However, adding to the returned collection is not possible.

    • values

      Collection<V> values()
      Returns a view collection containing the value from each key-value pair contained in this multimap, without collapsing duplicates (so values().size() == size()).

      Changes to the returned collection will update the underlying multimap, and vice versa. However, adding to the returned collection is not possible.

    • entries

      Collection<Map.Entry<K,V>> entries()
      Returns a view collection of all key-value pairs contained in this multimap, as Map.Entry instances.

      Changes to the returned collection or the entries it contains will update the underlying multimap, and vice versa. However, adding to the returned collection is not possible.

    • asMap

      Map<K, Collection<V>> asMap()
      Returns a view of this multimap as a Map from each distinct key to the nonempty 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 multimap; 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 multimap, and vice versa. The map does not support put or putAll, nor do its entries support setValue.

    • equals

      boolean equals(@Nullable Object obj)
      Compares the specified object with this multimap for equality. Two multimaps are equal when their map views, as returned by asMap(), are also equal.

      In general, two multimaps with identical key-value mappings may or may not be equal, depending on the implementation. For example, two SetMultimap instances with the same key-value mappings are equal, but equality of two ListMultimap instances depends on the ordering of the values for each key.

      A non-empty SetMultimap cannot be equal to a non-empty ListMultimap, since their asMap() views contain unequal collections as values. However, any two empty multimaps are equal, because they both have empty asMap() views.

      Overrides:
      equals in class Object

    • hashCode

      int hashCode()
      Returns the hash code for this multimap.

      The hash code of a multimap is defined as the hash code of the map view, as returned by asMap().

      In general, two multimaps with identical key-value mappings may or may not have the same hash codes, depending on the implementation. For example, two SetMultimap instances with the same key-value mappings will have the same hashCode, but the hashCode of ListMultimap instances depends on the ordering of the values for each key.

      Overrides:
      hashCode in class Object