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@GwtCompatible public interface Multiset<E>
A collection that supports order-independent equality, like 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.
| Nested Class Summary | |
|---|---|
static interface |
Multiset.Entry<E>
An unmodifiable element-count pair for a multiset. |
| Method Summary | |
|---|---|
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)
Returns true 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 into Multiset.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()
|
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)
|
boolean |
retainAll(Collection<?> c)
|
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. |
String |
toString()
|
| Methods inherited from interface java.util.Collection |
|---|
addAll, clear, isEmpty, size, toArray, toArray |
| Method Detail |
|---|
int count(@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
int add(@Nullable
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 element
NullPointerException - 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.
int remove(@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
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 multiset
IllegalArgumentException - if count is negative
NullPointerException - 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.
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 multiset
true if the condition for modification was met. This
implies that the multiset was indeed modified, unless
oldCount == newCount.
IllegalArgumentException - if oldCount or newCount is
negative
NullPointerException - 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.
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 Objectint 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 ObjectString toString()
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].
toString in class ObjectIterator<E> iterator()
Elements that occur multiple times in the multiset will appear multiple times in this iterator, though not necessarily sequentially.
iterator in interface Collection<E>iterator in interface Iterable<E>
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 for
true 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 multiset
true if this multiset contains at least one occurrence of
each element contained in elements
NullPointerException - if elements is nullboolean 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.
add in interface Collection<E>element - the element to add one occurrence of; may be null only if
explicitly allowed by the implementation
true always, since this call is required to modify the
multiset, unlike other Collection types
NullPointerException - if element is null and this
implementation does not permit null elements
IllegalArgumentException - if Integer.MAX_VALUE occurrences
of element are already contained in this multiset
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.
remove in interface Collection<E>element - the element to remove one occurrence of
true if an occurrence was found and removedboolean 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>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>Multisets.retainOccurrences(Multiset, Multiset)
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