All Superinterfaces:

java.util.Collection<E>, java.lang.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 java.lang.Object>
extends java.util.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 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.
Warning: as with normal Sets, it is almost always a bad idea to modify an element (in a way that affects its Object.equals(java.lang.Object) behavior) while it is contained in a multiset. Undefined behavior and bugs will result.
Implementations
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.
Since:

2.0

Author:

Kevin Bourrillion

Nested Class Summary

Nested Classes
Modifier and Type Interface Description

static interface Multiset.Entry<E extends @Nullable java.lang.Object>
An unmodifiable element-count pair for a multiset.

Method Summary

All Methods Instance Methods Abstract Methods Default 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(java.lang.Object element)`	Determines whether this multiset contains the specified element.
`boolean`	`containsAll(java.util.Collection<?> elements)`	Returns `true` if this multiset contains at least one occurrence of each element in the specified collection.
`int`	`count(java.lang.Object element)`	Returns the number of occurrences of an element in this multiset (the count of the element).
`java.util.Set<E>`	`elementSet()`	Returns the set of distinct elements contained in this multiset.
`java.util.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(java.lang.Object object)`	Compares the specified object with this multiset for equality.
`default void`	`forEach(java.util.function.Consumer<? super E> action)`
`default void`	`forEachEntry(java.util.function.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.
`java.util.Iterator<E>`	`iterator()`
`boolean`	`remove(java.lang.Object element)`	Removes a single occurrence of the specified element from this multiset, if present.
`int`	`remove(java.lang.Object element, int occurrences)`	Removes a number of occurrences of the specified element from this multiset.
`boolean`	`removeAll(java.util.Collection<?> c)`
`boolean`	`retainAll(java.util.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.
`int`	`size()`	Returns the total number of all occurrences of all elements in this multiset.
`default java.util.Spliterator<E>`	`spliterator()`
`java.lang.String`	`toString()`

Methods inherited from interface java.util.Collection
addAll, clear, isEmpty, parallelStream, removeIf, stream, toArray, toArray, toArray

- 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 interface java.util.Collection<E extends @Nullable java.lang.Object>
  - count
```
int count(@CompatibleWith("E") @CheckForNull
          java.lang.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
```
@CanIgnoreReturnValue
int add(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:
    
    java.lang.IllegalArgumentException - if occurrences is negative, or if this operation would result in more than Integer.MAX_VALUE occurrences of the element
    
    java.lang.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.
  - 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 interface java.util.Collection<E extends @Nullable java.lang.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 other Collection types
    
    Throws:
    
    java.lang.NullPointerException - if element is null and this implementation does not permit null elements
    
    java.lang.IllegalArgumentException - if Integer.MAX_VALUE occurrences of element are already contained in this multiset
  - remove
```
@CanIgnoreReturnValue
int remove(@CompatibleWith("E") @CheckForNull
           java.lang.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:
    
    java.lang.IllegalArgumentException - if occurrences is negative
  - remove
```
@CanIgnoreReturnValue
boolean remove(@CheckForNull
               java.lang.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.
    To both remove the element and obtain the previous count of that element, use remove(element, 1) instead.
    
    Specified by:
    
    remove in interface java.util.Collection<E extends @Nullable java.lang.Object>
    
    Parameters:
    
    element - the element to remove one occurrence of
    
    Returns:
    
    true if an occurrence was found and removed
  - 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 implementation
    
    count - the desired count of the element in this multiset
    
    Returns:
    
    the count of the element before the operation; possibly zero
    
    Throws:
    
    java.lang.IllegalArgumentException - if count is negative
    
    java.lang.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
```
@CanIgnoreReturnValue
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:
    
    java.lang.IllegalArgumentException - if oldCount or newCount is negative
    
    java.lang.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
```
java.util.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
```
java.util.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
  - forEachEntry
```
default void forEachEntry(java.util.function.ObjIntConsumer<? super E> action)
```
    Runs the specified action for each distinct element in this multiset, and the number of occurrences of that element. For some Multiset implementations, this may be more efficient than iterating over the entrySet() either explicitly or with entrySet().forEach(action).
    
    Since:
    
    21.0
  - equals
```
boolean equals(@CheckForNull
               java.lang.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 java.util.Collection<E extends @Nullable java.lang.Object>
    
    Overrides:
    
    equals in class java.lang.Object
  - 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 java.util.Collection<E extends @Nullable java.lang.Object>
    
    Overrides:
    
    hashCode in class java.lang.Object
  - toString
```
java.lang.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, e].
    
    Overrides:
    
    toString in class java.lang.Object
  - iterator
```
java.util.Iterator<E> iterator()
```
    Elements that occur multiple times in the multiset will appear multiple times in this iterator, though not necessarily sequentially.
    
    Specified by:
    
    iterator in interface java.util.Collection<E extends @Nullable java.lang.Object>
    
    Specified by:
    
    iterator in interface java.lang.Iterable<E extends @Nullable java.lang.Object>
  - contains
```
boolean contains(@CheckForNull
                 java.lang.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 java.util.Collection<E extends @Nullable java.lang.Object>
    
    Parameters:
    
    element - the element to check for
    
    Returns:
    
    true if this multiset contains at least one occurrence of the element
  - containsAll
```
boolean containsAll(java.util.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 java.util.Collection<E extends @Nullable java.lang.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 in elements
    
    Throws:
    
    java.lang.NullPointerException - if elements is null
  - removeAll
```
@CanIgnoreReturnValue
boolean removeAll(java.util.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.
    
    Specified by:
    
    removeAll in interface java.util.Collection<E extends @Nullable java.lang.Object>
  - retainAll
```
@CanIgnoreReturnValue
boolean retainAll(java.util.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.
    
    Specified by:
    
    retainAll in interface java.util.Collection<E extends @Nullable java.lang.Object>
    
    See Also:
    
    Multisets.retainOccurrences(Multiset, Multiset)
  - forEach
```
default void forEach(java.util.function.Consumer<? super E> action)
```
    Elements that occur multiple times in the multiset will be passed to the Consumer correspondingly many times, though not necessarily sequentially.
    
    Specified by:
    
    forEach in interface java.lang.Iterable<E extends @Nullable java.lang.Object>
  - spliterator
```
default java.util.Spliterator<E> spliterator()
```
    Specified by:
    
    spliterator in interface java.util.Collection<E extends @Nullable java.lang.Object>
    
    Specified by:
    
    spliterator in interface java.lang.Iterable<E extends @Nullable java.lang.Object>

Interface Multiset<E extends @Nullable java.lang.Object>

Implementations

Nested Class Summary

Method Summary

Methods inherited from interface java.util.Collection

Method Detail

size

count

add

add

remove

remove

setCount

setCount

elementSet

entrySet

forEachEntry

equals

hashCode

toString

iterator

contains

containsAll

removeAll

retainAll

forEach

spliterator