@GwtCompatible public interface Multimap<K,V>
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:
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).
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!
...
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
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.
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.
Multimaps are commonly used in places where a Map<K, Collection<V>>
would otherwise
have appeared. The differences include:
put
.
get
never returns null
, only an empty collection.
size()
.
Collections.min(multimap.values())
finds the smallest value across all keys.
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.
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
.
Modifier and Type | Method and 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.
|
default void |
forEach(BiConsumer<? super K,? super V> action)
Performs the given action for all key-value pairs contained in this multimap.
|
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(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 |
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 |
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() ). |
int size()
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.
boolean isEmpty()
true
if this multimap contains no key-value pairs. Equivalent to size()
== 0
, but can in some cases be more efficient.boolean containsKey(@CompatibleWith(value="K") @Nullable Object key)
true
if this multimap contains at least one key-value pair with the key key
.boolean containsValue(@CompatibleWith(value="V") @Nullable Object value)
true
if this multimap contains at least one key-value pair with the value
value
.boolean containsEntry(@CompatibleWith(value="K") @Nullable Object key, @CompatibleWith(value="V") @Nullable Object value)
true
if this multimap contains at least one key-value pair with the key key
and the value value
.@CanIgnoreReturnValue boolean put(K key, V value)
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.
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@CanIgnoreReturnValue boolean remove(@CompatibleWith(value="K") @Nullable Object key, @CompatibleWith(value="V") @Nullable Object value)
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.true
if the multimap changed@CanIgnoreReturnValue boolean putAll(K key, Iterable<? extends V> values)
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.
true
if the multimap changed@CanIgnoreReturnValue boolean putAll(Multimap<? extends K,? extends V> multimap)
multimap
in this multimap, in the order returned by
multimap.entries()
.true
if the multimap changed@CanIgnoreReturnValue Collection<V> replaceValues(K key, Iterable<? extends V> values)
If values
is empty, this is equivalent to removeAll(key)
.
@CanIgnoreReturnValue Collection<V> removeAll(@CompatibleWith(value="K") @Nullable Object 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.
Collection<V> get(K key)
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.
Set<K> keySet()
Changes to the returned set will update the underlying multimap, and vice versa. However, adding to the returned set is not possible.
Multiset<K> keys()
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.
Collection<V> values()
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.
Collection<Map.Entry<K,V>> entries()
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.
default void forEach(BiConsumer<? super K,? super V> action)
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())
.
Map<K,Collection<V>> asMap()
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
.
boolean equals(@Nullable Object obj)
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.
equals
in class Object
obj
- the reference object with which to compare.true
if this object is the same as the obj
argument; false
otherwise.Object.hashCode()
,
HashMap
int hashCode()
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.
hashCode
in class Object
Object.equals(java.lang.Object)
,
System.identityHashCode(java.lang.Object)
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