com.google.common.collect
Interface Multiset<E>

All Superinterfaces:
Collection<E>, Iterable<E>
All Known Implementing Classes:
ConcurrentHashMultiset, EnumMultiset, ForwardingMultiset, HashMultiset, ImmutableMultiset, LinkedHashMultiset, TreeMultiset

public interface Multiset<E>
extends Collection<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: 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.

Author:
Kevin Bourrillion

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()
          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 in setCount(Object, int), provided that the element has the expected current count.
 String toString()
          Returns a string representation of the object.
 
Methods inherited from interface java.util.Collection
addAll, clear, isEmpty, size, toArray, toArray
 

Method Detail

count

int count(@Nullable
          Object element)
Returns the number of occurrences of an element in this multiset (the count of the element). Note that for an 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.

Parameters:
element - the element to count occurrences of
Returns:
the number of occurrences of the element in this multiset; possibly zero but never negative

add

int add(@Nullable
        E element,
        int occurrences)
Adds a number of occurrences of an element to this multiset. Note that if 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.

Parameters:
element - the element to add occurrences of; may be null only if explicitly allowed by the implementation
occurrences - 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 - 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.

remove

int remove(@Nullable
           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 if occurrences == 1, this is functionally equivalent to the call remove(element).

Parameters:
element - the element to conditionally remove occurrences of
occurrences - 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 - if occurrences is negative

setCount

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 implementation
count - the desired count of the element in this multiset
Returns:
the count of the element before the operation; possibly zero
Throws:
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.

setCount

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. If the current count is not oldCount, no change is made.

Parameters:
element - the element to conditionally set the count of; may be null only if explicitly allowed by the implementation
oldCount - the expected present count of the element in this multiset
newCount - 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, unless oldCount == newCount.
Throws:
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.

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

entrySet

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. 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.

Returns:
a set of entries representing the data of this multiset

equals

boolean equals(@Nullable
               Object object)
Compares the specified object with this multiset for equality. Returns true if the given object is also a multiset and contains equal elements with equal counts, regardless of order.

Specified by:
equals in interface Collection<E>
Overrides:
equals in class Object
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(), Hashtable

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 interface Collection<E>
Overrides:
hashCode in class Object
Returns:
a hash code value for this object.
See Also:
Object.equals(java.lang.Object), Hashtable

toString

String toString()
Returns a string representation of the object. In general, the 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]
 

Overrides:
toString in class Object
Returns:
a string representation of the object.

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.

Specified by:
iterator in interface Collection<E>
Specified by:
iterator in interface Iterable<E>
Returns:
an Iterator over the elements in this collection

contains

boolean contains(@Nullable
                 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 to element being null or of the wrong type.

Specified by:
contains in interface Collection<E>
Parameters:
element - the element to check for
Returns:
true if this multiset contains at least one occurrence of the element

containsAll

boolean containsAll(Collection<?> elements)
Returns 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.

Specified by:
containsAll in interface Collection<E>
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 in elements
Throws:
NullPointerException - if elements is null
See Also:
Collection.contains(Object)

add

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.

Specified by:
add in interface Collection<E>
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 other Collection types
Throws:
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

remove

boolean remove(@Nullable
               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 to element being null or of the wrong type.

Specified by:
remove in interface Collection<E>
Parameters:
element - the element to remove one occurrence of
Returns:
true if an occurrence was found and removed

removeAll

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.

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.

Specified by:
removeAll in interface Collection<E>
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)

retainAll

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.

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.

Specified by:
retainAll in interface Collection<E>
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:
Collection.remove(Object), Collection.contains(Object)