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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 (see "Note on
element equivalence", below) 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:
newInstance()
, returning an empty multiset, and newInstance(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.
(TODO: after writing the below section, I became very unsure if we really want to bother taking such an approach. I could instead just go to TreeMultiset and remind callers of the danger of using a comparator inconsistent with equals().)
Note on element equivalence: Like all collections, a Multiset
implementation often needs to compare two instances to see whether
they are "the same." Multiset
does not specify which equivalence
relation will be used for this purpose; it is left implementation-dependent.
For example, given non-null instances, HashMultiset
uses the typical
choice of relation :
{(x, y) | x.hashCode() == y.hashCode() && x.equals(y)}
... whereas TreeMultiset
instead uses the slightly-less-common
relation: {(x, y) | comparator.compare(x, y) == 0}
... and other implementations may use something else entirely. This approach
may seem novel compared to existing collection specifications such as Set
, however, it matches precisely the de facto specifications of
these interfaces. That is, in practice, it is well-known that JDK
implementation classes such as TreeSet
and IdentityHashMap
freely substitute their own equivalence relations however it suits them.
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. |
boolean |
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. |
boolean |
remove(Object element)
Removes a single occurrence of the specified element from this multiset, if present. |
int |
remove(Object element,
int occurrences)
Conditionally removes a number of occurrences of an element from this multiset, provided that at least this many occurrences are present. |
boolean |
removeAll(Collection<?> c)
|
int |
removeAllOccurrences(Object element)
Removes all occurrences of the specified element from this multiset. |
boolean |
retainAll(Collection<?> c)
|
String |
toString()
|
Methods inherited from interface java.util.Collection |
---|
addAll, clear, isEmpty, iterator, 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
boolean 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 this 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
, no change is made.
Note that if
occurrences == 1
, this is functionally equivalent to the call
remove(element)
.
element
- the element to conditionally remove occurrences ofoccurrences
- the number of occurrences of this element to remove. May
be zero, in which case no change will be made.
true
if the condition for modification was met. Unless
occurrences
is zero, this implies that the multiset was indeed
modified.
IllegalArgumentException
- if occurrences
is negativeint removeAllOccurrences(@Nullable Object element)
remove(Object)
, which removes only
one occurrence at a time.
TODO: Nuke this. Use setCount(e, 0).
element
- the element whose occurrences should all be removed
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.
TODO: caveats about equivalence-relation.
equals
in interface Collection<E>
equals
in class Object
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
String 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, b x 0, e]
toString
in class Object
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
this 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 multisetboolean 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)
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)
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>
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