Interface Multiset<E extends @Nullable Object>
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- All Superinterfaces:
Collection<E>
,Iterable<E>
- All Known Subinterfaces:
SortedMultiset<E>
,com.google.common.collect.SortedMultisetBridge<E>
- All Known Implementing Classes:
ConcurrentHashMultiset
,EnumMultiset
,ForwardingMultiset
,ForwardingSortedMultiset
,ForwardingSortedMultiset.StandardDescendingMultiset
,HashMultiset
,ImmutableMultiset
,ImmutableSortedMultiset
,LinkedHashMultiset
,TreeMultiset
@GwtCompatible public interface Multiset<E extends @Nullable Object> extends Collection<E>
A collection that supports order-independent equality, likeSet
, 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 thanInteger.MAX_VALUE
occurrences of any one element.Multiset
refines the specifications of several methods fromCollection
. 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 exampleadd(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 operationsadd(Object)
orremove(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", andentrySet()
is similar but containsMultiset.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 twostatic
creation methods:create()
, returning an empty multiset, andcreate(Iterable<? extends E>)
, returning a multiset containing the given initial elements. This is simply a refinement ofCollection
'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, thesetCount
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.Warning: as with normal
Set
s, it is almost always a bad idea to modify an element (in a way that affects itsObject.equals(java.lang.Object)
behavior) while it is contained in a multiset. Undefined behavior and bugs will result.Common implementations include
ImmutableMultiset
,HashMultiset
, andConcurrentHashMultiset
.If your values may be zero, negative, or outside the range of an int, you may wish to use
AtomicLongMap
instead. Note, however, that unlikeMultiset
,AtomicLongMap
does not automatically remove zeros.See the Guava User Guide article on
Multiset
.- Since:
- 2.0
- Author:
- Kevin Bourrillion
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Nested Class Summary
Nested Classes Modifier and Type Interface Description static interface
Multiset.Entry<E extends @Nullable Object>
An unmodifiable element-count pair for a multiset.
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Method Summary
All Methods Instance Methods Abstract Methods Modifier and Type Method 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(Object element)
Determines whether this multiset contains the specified element.boolean
containsAll(Collection<?> elements)
Returnstrue
if this multiset contains at least one occurrence of each element in the specified collection.int
count(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 intoMultiset.Entry
instances, each providing an element of the multiset and the count of that element.boolean
equals(Object object)
Compares the specified object with this multiset for equality.int
hashCode()
Returns the hash code for this multiset.Iterator<E>
iterator()
Returns an iterator over the elements in this collection.boolean
remove(Object element)
Removes a single occurrence of the specified element from this multiset, if present.int
remove(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 insetCount(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.String
toString()
Returns a string representation of the object.-
Methods inherited from interface java.util.Collection
addAll, clear, isEmpty, parallelStream, removeIf, spliterator, stream, toArray, toArray, toArray
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Method Detail
-
size
int size()
Returns the total number of all occurrences of all elements in this multiset.Note: this method does not return the number of distinct elements in the multiset, which is given by
entrySet().size()
.- Specified by:
size
in interfaceCollection<E extends @Nullable Object>
- Returns:
- the number of elements in this collection
-
count
int count(@CompatibleWith("E") @CheckForNull Object element)
Returns the number of occurrences of an element in this multiset (the count of the element). Note that for anObject.equals(java.lang.Object)
-based multiset, this gives the same result asCollections.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.- Parameters:
element
- the element to count occurrences of- Returns:
- the number of occurrences of the element in this multiset; possibly zero but never negative
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add
@CanIgnoreReturnValue int add(E element, int occurrences)
Adds a number of occurrences of an element to this multiset. Note that ifoccurrences == 1
, this method has the identical effect toadd(Object)
. This method is functionally equivalent (except in the case of overflow) to the calladdAll(Collections.nCopies(element, occurrences))
, which would presumably perform much more poorly.- Parameters:
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.- Returns:
- the count of the element before the operation; possibly zero
- Throws:
IllegalArgumentException
- ifoccurrences
is negative, or if this operation would result in more thanInteger.MAX_VALUE
occurrences of the elementNullPointerException
- ifelement
is null and this implementation does not permit null elements. Note that ifoccurrences
is zero, the implementation may opt to return normally.
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add
@CanIgnoreReturnValue boolean add(E element)
Adds a single occurrence of the specified element to this multiset.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.- Specified by:
add
in interfaceCollection<E extends @Nullable Object>
- Parameters:
element
- the element to add one occurrence of; may be null only if explicitly allowed by the implementation- Returns:
true
always, since this call is required to modify the multiset, unlike otherCollection
types- Throws:
NullPointerException
- ifelement
is null and this implementation does not permit null elementsIllegalArgumentException
- ifInteger.MAX_VALUE
occurrences ofelement
are already contained in this multiset
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remove
@CanIgnoreReturnValue int remove(@CompatibleWith("E") @CheckForNull Object element, int occurrences)
Removes a number of occurrences of the specified element from this multiset. If the multiset contains fewer than this number of occurrences to begin with, all occurrences will be removed. Note that ifoccurrences == 1
, this is functionally equivalent to the callremove(element)
.- Parameters:
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.- Returns:
- the count of the element before the operation; possibly zero
- Throws:
IllegalArgumentException
- ifoccurrences
is negative
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remove
@CanIgnoreReturnValue boolean remove(@CheckForNull Object element)
Removes a single occurrence of the specified element from this multiset, if present.This method refines
Collection.remove(java.lang.Object)
to further specify that it may not throw an exception in response toelement
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.- Specified by:
remove
in interfaceCollection<E extends @Nullable Object>
- Parameters:
element
- the element to remove one occurrence of- Returns:
true
if an occurrence was found and removed
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setCount
@CanIgnoreReturnValue int setCount(E element, int count)
Adds or removes the necessary occurrences of an element such that the element attains the desired count.- Parameters:
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 multiset- Returns:
- the count of the element before the operation; possibly zero
- Throws:
IllegalArgumentException
- ifcount
is negativeNullPointerException
- ifelement
is null and this implementation does not permit null elements. Note that ifcount
is zero, the implementor may optionally return zero instead.
-
setCount
@CanIgnoreReturnValue boolean setCount(E element, int oldCount, int newCount)
Conditionally sets the count of an element to a new value, as described insetCount(Object, int)
, provided that the element has the expected current count. If the current count is notoldCount
, no change is made.- Parameters:
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 multiset- Returns:
true
if the condition for modification was met. This implies that the multiset was indeed modified, unlessoldCount == newCount
.- Throws:
IllegalArgumentException
- ifoldCount
ornewCount
is negativeNullPointerException
- ifelement
is null and the implementation does not permit null elements. Note that ifoldCount
andnewCount
are both zero, the implementor may optionally returntrue
instead.
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elementSet
Set<E> elementSet()
Returns the set of distinct elements contained in this multiset. The element set is backed by the same data as the multiset, so any change to either is immediately reflected in the other. The order of the elements in the element set is unspecified.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()
.- Returns:
- a view of the set of distinct elements in this multiset
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entrySet
Set<Multiset.Entry<E>> entrySet()
Returns a view of the contents of this multiset, grouped intoMultiset.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 theelementSet()
). 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 theEntry
instances themselves don't even have methods for modification. See the specific implementation class for more details on how its entry set handles modifications.- Returns:
- a set of entries representing the data of this multiset
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equals
boolean equals(@CheckForNull Object object)
Compares the specified object with this multiset for equality. Returnstrue
if the given object is also a multiset and contains equal elements with equal counts, regardless of order.- Specified by:
equals
in interfaceCollection<E extends @Nullable Object>
- Overrides:
equals
in classObject
- Parameters:
object
- the reference object with which to compare.- Returns:
true
if this object is the same as the obj argument;false
otherwise.- See Also:
Object.hashCode()
,HashMap
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hashCode
int hashCode()
Returns the hash code for this multiset. This is defined as the sum of((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.
- Specified by:
hashCode
in interfaceCollection<E extends @Nullable Object>
- Overrides:
hashCode
in classObject
- Returns:
- a hash code value for this object.
- See Also:
Object.equals(java.lang.Object)
,System.identityHashCode(java.lang.Object)
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toString
String toString()
Returns a string representation of the object. In general, thetoString
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 classObject
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 theentrySet()
, yielding a result such as[a x 3, c, d x 2, e]
.
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iterator
Iterator<E> iterator()
Returns an iterator over the elements in this collection. There are no guarantees concerning the order in which the elements are returned (unless this collection is an instance of some class that provides a guarantee).Elements that occur multiple times in the multiset will appear multiple times in this iterator, though not necessarily sequentially.
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contains
boolean contains(@CheckForNull Object element)
Determines whether this multiset contains the specified element.This method refines
Collection.contains(java.lang.Object)
to further specify that it may not throw an exception in response toelement
being null or of the wrong type.- Specified by:
contains
in interfaceCollection<E extends @Nullable Object>
- Parameters:
element
- the element to check for- Returns:
true
if this multiset contains at least one occurrence of the element
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containsAll
boolean containsAll(Collection<?> elements)
Returnstrue
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 ofelements
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 ifelements
contains several occurrences of an element and this multiset contains only one. This is no different than any other collection type likeList
, but it may be unexpected to the user of a multiset.- Specified by:
containsAll
in interfaceCollection<E extends @Nullable Object>
- Parameters:
elements
- the collection of elements to be checked for containment in this multiset- Returns:
true
if this multiset contains at least one occurrence of each element contained inelements
- Throws:
NullPointerException
- ifelements
is null- See Also:
Collection.contains(Object)
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removeAll
@CanIgnoreReturnValue boolean removeAll(Collection<?> c)
Removes all of this collection's elements that are also contained in the specified collection (optional operation). After this call returns, this collection will contain no elements in common with the specified collection.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 inc
, seeMultisets.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 ofelements
being null or of the wrong type.- Specified by:
removeAll
in interfaceCollection<E extends @Nullable Object>
- Parameters:
c
- collection containing elements to be removed from this collection- Returns:
true
if this collection changed as a result of the call- See Also:
Collection.remove(Object)
,Collection.contains(Object)
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retainAll
@CanIgnoreReturnValue boolean retainAll(Collection<?> c)
Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.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 inc
, seeMultisets.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 ofelements
being null or of the wrong type.- Specified by:
retainAll
in interfaceCollection<E extends @Nullable Object>
- Parameters:
c
- collection containing elements to be retained in this collection- Returns:
true
if this collection changed as a result of the call- See Also:
Multisets.retainOccurrences(Multiset, Multiset)
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