@GwtCompatible public interface Multiset<E> extends Collection<E>
Set
, but may have duplicate
elements. A multiset is also sometimes called a bag.
Elements of a multiset that are equal to one another are referred to as occurrences of
the same single element. The total number of occurrences of an element in a multiset is called
the count of that element (the terms "frequency" and "multiplicity" are equivalent, but
not used in this API). Since the count of an element is represented as an int
, a multiset
may never contain more than Integer.MAX_VALUE
occurrences of any one element.
Multiset
refines the specifications of several methods from Collection
. It
also defines an additional query operation, count(java.lang.Object)
, which returns the count of an
element. There are five new bulk-modification operations, for example add(Object, int)
,
to add or remove multiple occurrences of an element at once, or to set the count of an element to
a specific value. These modification operations are optional, but implementations which support
the standard collection operations add(Object)
or remove(Object)
are encouraged
to implement the related methods as well. Finally, two collection views are provided: elementSet()
contains the distinct elements of the multiset "with duplicates collapsed", and
entrySet()
is similar but contains Multiset.Entry
instances, each providing
both a distinct element and the count of that element.
In addition to these required methods, implementations of Multiset
are expected to
provide two static
creation methods: create()
, returning an empty multiset, and
create(Iterable<? extends E>)
, returning a multiset containing the given initial
elements. This is simply a refinement of Collection
's constructor recommendations,
reflecting the new developments of Java 5.
As with other collection types, the modification operations are optional, and should throw
UnsupportedOperationException
when they are not implemented. Most implementations should
support either all add operations or none of them, all removal operations or none of them, and if
and only if all of these are supported, the setCount
methods as well.
A multiset uses Object.equals(java.lang.Object)
to determine whether two instances should be considered
"the same," unless specified otherwise by the implementation.
Common implementations include ImmutableMultiset
, HashMultiset
, and ConcurrentHashMultiset
.
If your values may be zero, negative, or outside the range of an int, you may wish to use
AtomicLongMap
instead. Note, however, that unlike
Multiset
, AtomicLongMap
does not automatically remove zeros.
See the Guava User Guide article on Multiset
.
Modifier and Type | Interface and Description |
---|---|
static interface |
Multiset.Entry<E>
An unmodifiable element-count pair for a multiset.
|
Modifier and Type | Method and Description |
---|---|
boolean |
add(E element)
Adds a single occurrence of the specified element to this multiset.
|
int |
add(E element,
int occurrences)
Adds a number of occurrences of an element to this multiset.
|
boolean |
contains(@Nullable Object element)
Determines whether this multiset contains the specified element.
|
boolean |
containsAll(Collection<?> elements)
Returns
true if this multiset contains at least one occurrence of each element in the
specified collection. |
int |
count(@Nullable Object element)
Returns the number of occurrences of an element in this multiset (the count of the
element).
|
Set<E> |
elementSet()
Returns the set of distinct elements contained in this multiset.
|
Set<Multiset.Entry<E>> |
entrySet()
Returns a view of the contents of this multiset, grouped into
Multiset.Entry instances,
each providing an element of the multiset and the count of that element. |
boolean |
equals(@Nullable Object object)
Compares the specified object with this multiset for equality.
|
default void |
forEach(Consumer<? super E> action)
Performs the given action for each element of the
Iterable
until all elements have been processed or the action throws an
exception. |
default void |
forEachEntry(ObjIntConsumer<? super E> action)
Runs the specified action for each distinct element in this multiset, and the number of
occurrences of that element.
|
int |
hashCode()
Returns the hash code for this multiset.
|
Iterator<E> |
iterator()
Returns an iterator over the elements in this collection.
|
boolean |
remove(@Nullable Object element)
Removes a single occurrence of the specified element from this multiset, if present.
|
int |
remove(@Nullable Object element,
int occurrences)
Removes a number of occurrences of the specified element from this multiset.
|
boolean |
removeAll(Collection<?> c)
Removes all of this collection's elements that are also contained in the
specified collection (optional operation).
|
boolean |
retainAll(Collection<?> c)
Retains only the elements in this collection that are contained in the
specified collection (optional operation).
|
int |
setCount(E element,
int count)
Adds or removes the necessary occurrences of an element such that the element attains the
desired count.
|
boolean |
setCount(E element,
int oldCount,
int newCount)
Conditionally sets the count of an element to a new value, as described in
setCount(Object, int) , provided that the element has the expected current count. |
int |
size()
Returns the total number of all occurrences of all elements in this multiset.
|
default Spliterator<E> |
spliterator()
Creates a
Spliterator over the elements in this collection. |
String |
toString()
Returns a string representation of the object.
|
addAll, clear, isEmpty, parallelStream, removeIf, stream, toArray, toArray
int size()
Note: this method does not return the number of distinct elements in the
multiset, which is given by entrySet().size()
.
size
in interface Collection<E>
int count(@CompatibleWith(value="E") @Nullable Object element)
Object.equals(java.lang.Object)
-based multiset, this gives the same result as
Collections.frequency(java.util.Collection<?>, java.lang.Object)
(which would presumably perform more poorly).
Note: the utility method Iterables.frequency(java.lang.Iterable<?>, java.lang.Object)
generalizes this operation; it
correctly delegates to this method when dealing with a multiset, but it can also accept any
other iterable type.
element
- the element to count occurrences of@CanIgnoreReturnValue int add(E element, int occurrences)
occurrences ==
1
, this method has the identical effect to add(Object)
. This method is functionally
equivalent (except in the case of overflow) to the call addAll(Collections.nCopies(element, occurrences))
, which would presumably perform much more
poorly.element
- the element to add occurrences of; may be null only if explicitly allowed by the
implementationoccurrences
- the number of occurrences of the element to add. May be zero, in which case
no change will be made.IllegalArgumentException
- if occurrences
is negative, or if this operation would
result in more than Integer.MAX_VALUE
occurrences of the elementNullPointerException
- if element
is null and this implementation does not permit
null elements. Note that if occurrences
is zero, the implementation may opt to
return normally.@CanIgnoreReturnValue boolean add(E element)
This method refines Collection.add(E)
, which only ensures the presence of the
element, to further specify that a successful call must always increment the count of the
element, and the overall size of the collection, by one.
To both add the element and obtain the previous count of that element, use add
(element, 1)
instead.
add
in interface Collection<E>
element
- the element to add one occurrence of; may be null only if explicitly allowed by
the implementationtrue
always, since this call is required to modify the multiset, unlike other
Collection
typesNullPointerException
- if element
is null and this implementation does not permit
null elementsIllegalArgumentException
- if Integer.MAX_VALUE
occurrences of element
are already contained in this multiset@CanIgnoreReturnValue int remove(@CompatibleWith(value="E") @Nullable Object element, int occurrences)
occurrences == 1
, this is functionally equivalent to the call remove(element)
.element
- the element to conditionally remove occurrences ofoccurrences
- the number of occurrences of the element to remove. May be zero, in which
case no change will be made.IllegalArgumentException
- if occurrences
is negative@CanIgnoreReturnValue boolean remove(@Nullable Object element)
This method refines Collection.remove(java.lang.Object)
to further specify that it may not
throw an exception in response to element
being null or of the wrong type.
To both remove the element and obtain the previous count of that element, use remove
(element, 1)
instead.
remove
in interface Collection<E>
element
- the element to remove one occurrence oftrue
if an occurrence was found and removed@CanIgnoreReturnValue int setCount(E element, int count)
element
- the element to add or remove occurrences of; may be null only if explicitly
allowed by the implementationcount
- the desired count of the element in this multisetIllegalArgumentException
- if count
is negativeNullPointerException
- if element
is null and this implementation does not permit
null elements. Note that if count
is zero, the implementor may optionally return
zero instead.@CanIgnoreReturnValue boolean setCount(E element, int oldCount, int newCount)
setCount(Object, int)
, provided that the element has the expected current count. If the
current count is not oldCount
, no change is made.element
- the element to conditionally set the count of; may be null only if explicitly
allowed by the implementationoldCount
- the expected present count of the element in this multisetnewCount
- the desired count of the element in this multisettrue
if the condition for modification was met. This implies that the multiset
was indeed modified, unless oldCount == newCount
.IllegalArgumentException
- if oldCount
or newCount
is negativeNullPointerException
- if element
is null and the implementation does not permit
null elements. Note that if oldCount
and newCount
are both zero, the
implementor may optionally return true
instead.Set<E> elementSet()
If the element set supports any removal operations, these necessarily cause all occurrences of the removed element(s) to be removed from the multiset. Implementations are not expected to support the add operations, although this is possible.
A common use for the element set is to find the number of distinct elements in the multiset:
elementSet().size()
.
Set<Multiset.Entry<E>> entrySet()
Multiset.Entry
instances,
each providing an element of the multiset and the count of that element. This set contains
exactly one entry for each distinct element in the multiset (thus it always has the same size
as the elementSet()
). The order of the elements in the element set is unspecified.
The entry set is backed by the same data as the multiset, so any change to either is
immediately reflected in the other. However, multiset changes may or may not be reflected in
any Entry
instances already retrieved from the entry set (this is
implementation-dependent). Furthermore, implementations are not required to support
modifications to the entry set at all, and the Entry
instances themselves don't even
have methods for modification. See the specific implementation class for more details on how
its entry set handles modifications.
@Beta default void forEachEntry(ObjIntConsumer<? super E> action)
Multiset
implementations, this may be more
efficient than iterating over the entrySet()
either explicitly or with entrySet().forEach(action)
.boolean equals(@Nullable Object object)
true
if the
given object is also a multiset and contains equal elements with equal counts, regardless of
order.equals
in interface Collection<E>
equals
in class Object
object
- 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()
((element == null) ? 0 : element.hashCode()) ^ count(element)
over all distinct elements in the multiset. It follows that a multiset and its entry set always have the same hash code.
hashCode
in interface Collection<E>
hashCode
in class Object
Object.equals(java.lang.Object)
,
System.identityHashCode(java.lang.Object)
String toString()
toString
method returns a string that
"textually represents" this object. The result should
be a concise but informative representation that is easy for a
person to read.
It is recommended that all subclasses override this method.
The toString
method for class Object
returns a string consisting of the name of the class of which the
object is an instance, the at-sign character `@
', and
the unsigned hexadecimal representation of the hash code of the
object. In other words, this method returns a string equal to the
value of:
getClass().getName() + '@' + Integer.toHexString(hashCode())
It is recommended, though not mandatory, that this method return the result of invoking
toString()
on the entrySet()
, yielding a result such as [a x 3, c, d x 2,
e]
.
Iterator<E> iterator()
Elements that occur multiple times in the multiset will appear multiple times in this iterator, though not necessarily sequentially.
boolean contains(@Nullable Object element)
This method refines Collection.contains(java.lang.Object)
to further specify that it may not
throw an exception in response to element
being null or of the wrong type.
contains
in interface Collection<E>
element
- the element to check fortrue
if this multiset contains at least one occurrence of the elementboolean containsAll(Collection<?> elements)
true
if this multiset contains at least one occurrence of each element in the
specified collection.
This method refines Collection.containsAll(java.util.Collection<?>)
to further specify that it may not
throw an exception in response to any of elements
being null or of the wrong type.
Note: this method does not take into account the occurrence count of an element in
the two collections; it may still return true
even if elements
contains several
occurrences of an element and this multiset contains only one. This is no different than any
other collection type like List
, but it may be unexpected to the user of a multiset.
containsAll
in interface Collection<E>
elements
- the collection of elements to be checked for containment in this multisettrue
if this multiset contains at least one occurrence of each element
contained in elements
NullPointerException
- if elements
is nullCollection.contains(Object)
@CanIgnoreReturnValue boolean removeAll(Collection<?> c)
Note: This method ignores how often any element might appear in c
, and only
cares whether or not an element appears at all. If you wish to remove one occurrence in this
multiset for every occurrence in c
, see Multisets.removeOccurrences(Multiset,
Multiset)
.
This method refines Collection.removeAll(java.util.Collection<?>)
to further specify that it may not
throw an exception in response to any of elements
being null or of the wrong type.
removeAll
in interface Collection<E>
c
- collection containing elements to be removed from this collectionCollection.remove(Object)
,
Collection.contains(Object)
@CanIgnoreReturnValue boolean retainAll(Collection<?> c)
Note: This method ignores how often any element might appear in c
, and only
cares whether or not an element appears at all. If you wish to remove one occurrence in this
multiset for every occurrence in c
, see Multisets.retainOccurrences(Multiset,
Multiset)
.
This method refines Collection.retainAll(java.util.Collection<?>)
to further specify that it may not
throw an exception in response to any of elements
being null or of the wrong type.
retainAll
in interface Collection<E>
c
- collection containing elements to be retained in this collectionMultisets.retainOccurrences(Multiset, Multiset)
default void forEach(Consumer<? super E> action)
Iterable
until all elements have been processed or the action throws an
exception. Unless otherwise specified by the implementing class,
actions are performed in the order of iteration (if an iteration order
is specified). Exceptions thrown by the action are relayed to the
caller.
Elements that occur multiple times in the multiset will be passed to the Consumer
correspondingly many times, though not necessarily sequentially.
default Spliterator<E> spliterator()
java.util.Collection
Spliterator
over the elements in this collection.
Implementations should document characteristic values reported by the
spliterator. Such characteristic values are not required to be reported
if the spliterator reports Spliterator.SIZED
and this collection
contains no elements.
The default implementation should be overridden by subclasses that
can return a more efficient spliterator. In order to
preserve expected laziness behavior for the Collection.stream()
and
Collection.parallelStream()
} methods, spliterators should either have the
characteristic of IMMUTABLE
or CONCURRENT
, or be
late-binding.
If none of these is practical, the overriding class should describe the
spliterator's documented policy of binding and structural interference,
and should override the Collection.stream()
and Collection.parallelStream()
methods to create streams using a Supplier
of the spliterator,
as in:
Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
These requirements ensure that streams produced by the
Collection.stream()
and Collection.parallelStream()
methods will reflect the
contents of the collection as of initiation of the terminal stream
operation.
spliterator
in interface Collection<E>
spliterator
in interface Iterable<E>
Spliterator
over the elements in this collectionCopyright © 2010–2019. All rights reserved.