Guava: Google Core Libraries for Java 33.1.0-jre · com.google.common.collect.Multimap

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 Note

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 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

ImmutableListMultimap
ImmutableSetMultimap
Configure your own mutable multimap with MultimapBuilder
LinkedListMultimap (for one unusual kind of mutable Multimap)

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.

Method Summary

Modifier and Type	Method and Description
public 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.
public void	clear() Removes all key-value pairs from the multimap, leaving it empty.
public boolean	containsEntry(Object key, Object value) Returns `true` if this multimap contains at least one key-value pair with the key `key` and the value `value`.
public boolean	containsKey(Object key) Returns `true` if this multimap contains at least one key-value pair with the key `key`.
public boolean	containsValue(Object value) Returns `true` if this multimap contains at least one key-value pair with the value `value`.
public Collection<Map.Entry<K, V>>	entries() Returns a view collection of all key-value pairs contained in this multimap, as `Entry` instances.
public boolean	equals(Object obj) Compares the specified object with this multimap for equality.
public default void	forEach(BiConsumer<? super K, ? super V> action) Performs the given action for all key-value pairs contained in this multimap.
public Collection<V>	get(K key) Returns a view collection of the values associated with `key` in this multimap, if any.
public int	hashCode() Returns the hash code for this multimap.
public boolean	isEmpty() Returns `true` if this multimap contains no key-value pairs.
public Multiset<K>	keys() Returns a view collection containing the key from each key-value pair in this multimap, without collapsing duplicates.
public Set<K>	keySet() Returns a view collection of all distinct keys contained in this multimap.
public boolean	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 put(K key, V value) Stores a key-value pair in this multimap.
public boolean	Returns: `true` if the multimap changed 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`.
public boolean	Returns: `true` if the multimap changed putAll(Multimap<? extends K, ? extends V> multimap) Stores all key-value pairs of `multimap` in this multimap, in the order returned by `multimap.entries()`.
public boolean	Returns: `true` if the multimap changed remove(Object key, Object value) Removes a single key-value pair with the key `key` and the value `value` from this multimap, if such exists.
public Collection<V>	Returns: the values that were removed (possibly empty). The returned collection may be modifiable, but updating it will have no effect on the multimap. removeAll(Object key) Removes all values associated with the key `key`.
public Collection<V>	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. replaceValues(K key, Iterable<? extends V> values) Stores a collection of values with the same key, replacing any existing values for that key.
public int	size() Returns the number of key-value pairs in this multimap.
public 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 Detail

asMap back to summary

asMap	back to summary
public 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`.

public 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.

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

containsEntry	back to summary
public boolean containsEntry(Object key, Object value) Returns `true` if this multimap contains at least one key-value pair with the key `key` and the value `value`.

containsKey	back to summary
public boolean containsKey(Object key) Returns `true` if this multimap contains at least one key-value pair with the key `key`.

containsValue	back to summary
public boolean containsValue(Object value) Returns `true` if this multimap contains at least one key-value pair with the value `value`.

entries back to summary

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

public Collection<Map.Entry<K, V>> entries()

Returns a view collection of all key-value pairs contained in this multimap, as 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.

equals back to summary

equals	back to summary
public boolean equals(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. Annotations @Override

public boolean equals(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.

Annotations

@Override

forEach back to summary

forEach	back to summary
public default void forEach(BiConsumer<? super K, ? super V> action) Performs the given action for all key-value pairs contained in this multimap. If an ordering is specified by the `Multimap` implementation, actions will be performed in the order of iteration of `entries()`. Exceptions thrown by the action are relayed to the caller. To loop over all keys and their associated value collections, write `Multimaps.asMap(multimap).forEach((key, valueCollection) -> action())`. Since 21.0

public default void forEach(BiConsumer<? super K, ? super V> action)

Performs the given action for all key-value pairs contained in this multimap. If an ordering is specified by the Multimap implementation, actions will be performed in the order of iteration of entries(). Exceptions thrown by the action are relayed to the caller.

To loop over all keys and their associated value collections, write Multimaps.asMap(multimap).forEach((key, valueCollection) -> action()).

Since: 21.0

get back to summary

get	back to summary
public 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.

public 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.

hashCode back to summary

hashCode	back to summary
public 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 `Multimap#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. Annotations @Override

public 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 Multimap#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.

Annotations

@Override

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

keys back to summary

keys	back to summary
public 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.

public 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.

keySet	back to summary
public 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.

keySet

back to summary

public 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.

put back to summary

put	back to summary
public 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:boolean `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 Annotations @CanIgnoreReturnValue

public 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:boolean

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

Annotations

@CanIgnoreReturnValue

putAll back to summary

putAll	back to summary
public 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:boolean `true` if the multimap changed Annotations @CanIgnoreReturnValue

public 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:boolean

true if the multimap changed

Annotations

@CanIgnoreReturnValue

putAll back to summary

putAll	back to summary
public 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:boolean `true` if the multimap changed Annotations @CanIgnoreReturnValue

public 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:boolean

true if the multimap changed

Annotations

@CanIgnoreReturnValue

remove back to summary

remove	back to summary
public boolean remove(Object key, 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:boolean `true` if the multimap changed Annotations @CanIgnoreReturnValue

public boolean remove(Object key, 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:boolean

true if the multimap changed

Annotations

@CanIgnoreReturnValue

removeAll back to summary

removeAll	back to summary
public Collection<V> removeAll(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:Collection<V> the values that were removed (possibly empty). The returned collection may be modifiable, but updating it will have no effect on the multimap. Annotations @CanIgnoreReturnValue

public Collection<V> removeAll(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:Collection<V>

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

Annotations

@CanIgnoreReturnValue

replaceValues back to summary

replaceValues	back to summary
public 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:Collection<V> 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. Annotations @CanIgnoreReturnValue

public 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:Collection<V>

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.

Annotations

@CanIgnoreReturnValue

size	back to summary
public 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.

values back to summary

values	back to summary
public 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.

public 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.

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

Example

Views

Subinterfaces

Comparison to a map of collections

Implementations

Other Notes

Method Summary

Method Detail