Package com.google.common.collect

Interface Multimap<K,​V>

  • 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
    @GwtCompatible
public interface Multimap<K,​V>
    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:

    • asMap(), mentioned above
    • keys(), keySet(), values(), entries(), which are similar to the corresponding view collections of Map
    • and, notably, even the collection returned by get(key) is an active view of the values corresponding to key

    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). The recommended subinterfaces provide a much stronger guarantee.

    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

    As always, prefer the immutable implementations, ImmutableListMultimap and ImmutableSetMultimap. General-purpose mutable implementations are listed above under "All Known Implementing Classes". You can also create a custom multimap, backed by any Map and Collection types, using the Multimaps.newMultimap family of methods. Finally, another popular way to obtain a multimap is using Multimaps.index. See the Multimaps class for these and other static utilities related to multimaps.

    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 (imported from Google Collections Library)

    • Method Summary

      All Methods Instance Methods Abstract Methods 
      Modifier and Type Method Description
      java.util.Map<K,​java.util.Collection<V>> asMap()
      Returns a map view that associates each key with the corresponding values in the multimap.
      void clear()
      Removes all key-value pairs from the multimap.
      boolean containsEntry​(java.lang.Object key, java.lang.Object value)
      Returns true if the multimap contains the specified key-value pair.
      boolean containsKey​(java.lang.Object key)
      Returns true if the multimap contains any values for the specified key.
      boolean containsValue​(java.lang.Object value)
      Returns true if the multimap contains the specified value for any key.
      java.util.Collection<java.util.Map.Entry<K,​V>> entries()
      Returns a collection of all key-value pairs.
      boolean equals​(java.lang.Object obj)
      Compares the specified object with this multimap for equality.
      java.util.Collection<V> get​(K key)
      Returns a collection view containing the values associated with key in this multimap, if any.
      int hashCode()
      Returns the hash code for this multimap.
      boolean isEmpty()
      Returns true if the multimap contains no key-value pairs.
      Multiset<K> keys()
      Returns a collection, which may contain duplicates, of all keys.
      java.util.Set<K> keySet()
      Returns the set of all keys, each appearing once in the returned set.
      boolean put​(K key, V value)
      Stores a key-value pair in the multimap.
      boolean putAll​(Multimap<? extends K,​? extends V> multimap)
      Copies all of another multimap's key-value pairs into this multimap.
      boolean putAll​(K key, java.lang.Iterable<? extends V> values)
      Stores key-value pairs in this multimap with one key and multiple values.
      boolean remove​(java.lang.Object key, java.lang.Object value)
      Removes a single key-value pair from the multimap.
      java.util.Collection<V> removeAll​(java.lang.Object key)
      Removes all values associated with a given key.
      java.util.Collection<V> replaceValues​(K key, java.lang.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 the multimap.
      java.util.Collection<V> values()
      Returns a collection of all values in the multimap.

    • Method Detail

      • size

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

      • isEmpty

        boolean isEmpty()
        Returns true if the multimap contains no key-value pairs.

      • containsKey

        boolean containsKey​(@Nullable
                    java.lang.Object key)
        Returns true if the multimap contains any values for the specified key.
        Parameters:
        key - key to search for in multimap

      • containsValue

        boolean containsValue​(@Nullable
                      java.lang.Object value)
        Returns true if the multimap contains the specified value for any key.
        Parameters:
        value - value to search for in multimap

      • containsEntry

        boolean containsEntry​(@Nullable
                      java.lang.Object key,
                      @Nullable
                      java.lang.Object value)
        Returns true if the multimap contains the specified key-value pair.
        Parameters:
        key - key to search for in multimap
        value - value to search for in multimap

      • put

        boolean put​(@Nullable
            K key,
            @Nullable
            V value)
        Stores a key-value pair in the 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.

        Parameters:
        key - key to store in the multimap
        value - value to store in the multimap
        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

        boolean remove​(@Nullable
               java.lang.Object key,
               @Nullable
               java.lang.Object value)
        Removes a single key-value pair from the multimap.
        Parameters:
        key - key of entry to remove from the multimap
        value - value of entry to remove the multimap
        Returns:
        true if the multimap changed

      • putAll

        boolean putAll​(@Nullable
               K key,
               java.lang.Iterable<? extends V> values)
        Stores key-value pairs in this multimap with one key and multiple values.

        This is equivalent to 

           
 
   for (V value : values) {
     put(key, value);
   }

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

        Parameters:
        key - key to store in the multimap
        values - values to store in the multimap
        Returns:
        true if the multimap changed

      • putAll

        boolean putAll​(Multimap<? extends K,​? extends V> multimap)
        Copies all of another multimap's key-value pairs into this multimap. The order in which the mappings are added is determined by multimap.entries().
        Parameters:
        multimap - mappings to store in this multimap
        Returns:
        true if the multimap changed

      • replaceValues

        java.util.Collection<V> replaceValues​(@Nullable
                                      K key,
                                      java.lang.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).

        Parameters:
        key - key to store in the multimap
        values - values to store in the multimap
        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

        java.util.Collection<V> removeAll​(@Nullable
                                  java.lang.Object key)
        Removes all values associated with a given 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.

        Parameters:
        key - key of entries to remove from the multimap
        Returns:
        the collection of removed values, or an empty collection if no values were associated with the provided key. The 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.

      • get

        java.util.Collection<V> get​(@Nullable
                            K key)
        Returns a collection view containing the values associated with key in this multimap, if any. Note that even when (containsKey(key) is false, get(key) still returns an empty collection, not null.

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

        Parameters:
        key - key to search for in multimap
        Returns:
        a view collection containing the zero or more values that the key maps to

      • keySet

        java.util.Set<K> keySet()
        Returns the set of all keys, each appearing once in the returned set. Changes to the returned set will update the underlying multimap, and vice versa.

        Note that the key set contains a key if and only if this multimap maps that key to at least one value.

        Returns:
        the collection of distinct keys

      • keys

        Multiset<K> keys()
        Returns a collection, which may contain duplicates, of all keys. The number of times of key appears in the returned multiset equals the number of mappings the key has in the multimap. Changes to the returned multiset will update the underlying multimap, and vice versa.
        Returns:
        a multiset with keys corresponding to the distinct keys of the multimap and frequencies corresponding to the number of values that each key maps to

      • values

        java.util.Collection<V> values()
        Returns a collection of all values in the multimap. Changes to the returned collection will update the underlying multimap, and vice versa.
        Returns:
        collection of values, which may include the same value multiple times if it occurs in multiple mappings

      • entries

        java.util.Collection<java.util.Map.Entry<K,​V>> entries()
        Returns a collection of all key-value pairs. Changes to the returned collection will update the underlying multimap, and vice versa. The entries collection does not support the add or addAll operations.
        Returns:
        collection of map entries consisting of key-value pairs

      • asMap

        java.util.Map<K,​java.util.Collection<V>> asMap()
        Returns a map view that associates each key with the corresponding values in the multimap. Changes to the returned map, such as element removal, will update the underlying multimap. The map does not support setValue() on its entries, put, or putAll.

        When passed a key that is present in the map, asMap().get(Object) has the same behavior as get(K), returning a live collection. When passed a key that is not present, however, asMap().get(Object) returns null instead of an empty collection.

        Returns:
        a map view from a key to its collection of values

      • equals

        boolean equals​(@Nullable
               java.lang.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 java.lang.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().

        Overrides:
        hashCode in class java.lang.Object