com.google.common.collect
Class AbstractMultiset<E>

java.lang.Object
  java.util.AbstractCollection<E>
      com.google.common.collect.AbstractMultiset<E>

All Implemented Interfaces:

Multiset<E>, java.lang.Iterable<E>, java.util.Collection<E>

Direct Known Subclasses:

ConcurrentMultiset, EnumMultiset, HashMultiset, LinkedHashMultiset, TreeMultiset

public abstract class AbstractMultiset<E>
extends java.util.AbstractCollection<E>
implements Multiset<E>

This class provides a skeletal implementation of the Multiset interface. A new multiset implementation can be created easily by extending this class and implementing the Multiset.entrySet() method, plus optionally overriding add(Object, int) and remove(Object, int) to enable modifications to the multiset.

The contains(java.lang.Object), containsAll(java.util.Collection), count(java.lang.Object), and size() implementations all iterate across the set returned by Multiset.entrySet(), as do many methods acting on the set returned by elementSet. Override those methods for better performance.

Author:

Kevin Bourrillion

Nested Class Summary

Nested classes/interfaces inherited from interface com.google.common.collect.Multiset

Multiset.Entry<E>

Constructor Summary

AbstractMultiset()

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	addAll(java.util.Collection<? extends E> elementsToAdd)
void	clear()
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).
protected java.util.Set<E>	createElementSet() Creates a new instance of this multiset's element set, which will be returned by elementSet.
java.util.Set<E>	elementSet() Returns the set of distinct elements contained in this multiset.
abstract 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 other) Compares the specified object with this multiset for equality.
int	hashCode() Returns the hash code for this multiset.
boolean	isEmpty()
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) Conditionally removes a number of occurrences of an element from this multiset, provided that at least this many occurrences are present.
boolean	removeAll(java.util.Collection<?> elementsToRemove)
int	removeAllOccurrences(java.lang.Object element) Removes all occurrences of the specified element from this multiset.
boolean	retainAll(java.util.Collection<?> elementsToRetain)
int	size()
java.lang.String	toString()

Methods inherited from class java.util.AbstractCollection

toArray, toArray

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.Collection

toArray, toArray

Constructor Detail

AbstractMultiset

public AbstractMultiset()

Method Detail

entrySet

public abstract java.util.Set<Multiset.Entry<E>> entrySet()

Description copied from interface: Multiset

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 Multiset.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.

Specified by:

entrySet in interface Multiset<E>

Returns:

a set of entries representing the data of this multiset

size

public int size()

This implementation iterates across Multiset.entrySet() and sums the counts of the entries.

Specified by:

size in interface java.util.Collection<E>

Specified by:

size in class java.util.AbstractCollection<E>

isEmpty

public boolean isEmpty()

Specified by:

isEmpty in interface java.util.Collection<E>

Overrides:

isEmpty in class java.util.AbstractCollection<E>

contains

public boolean contains(@Nullable
                        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.

This implementation checks whether elementSet contains the element.

Specified by:

contains in interface Multiset<E>

Specified by:

contains in interface java.util.Collection<E>

Overrides:

contains in class java.util.AbstractCollection<E>

Parameters:

element - the element to check for

Returns:

true if this multiset contains at least one occurrence of this element

iterator

public java.util.Iterator<E> iterator()

This implementation usually invokes methods of the Multiset.entrySet() iterator. As the only exception, the iterator's remove method sometimes calls the multiset's remove.

Specified by:

iterator in interface java.lang.Iterable<E>

Specified by:

iterator in interface java.util.Collection<E>

Specified by:

iterator in class java.util.AbstractCollection<E>

count

public int count(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.

This implementation iterates across Multiset.entrySet() and sums the count of all entries.

Specified by:

count in interface Multiset<E>

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

public boolean add(@Nullable
                   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.

This implementation calls add(Object, int) with one occurrence.

Specified by:

add in interface Multiset<E>

Specified by:

add in interface java.util.Collection<E>

Overrides:

add in class java.util.AbstractCollection<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

add

public boolean 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 Multiset.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.

This implementation always throws an UnsupportedOperationException. To support adding elements, override it.

Specified by:

add in interface Multiset<E>

Parameters:

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

occurrences - the number of occurrences of this element to add. May be zero, in which case no change will be made.

Returns:

the previous count of this element before the operation; possibly zero - TODO: make this the actual behavior!

remove

public boolean remove(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.

This implementation calls remove(Object, int) with 1 occurrence.

Specified by:

remove in interface Multiset<E>

Specified by:

remove in interface java.util.Collection<E>

Overrides:

remove in class java.util.AbstractCollection<E>

Parameters:

element - the element to remove one occurrence of

Returns:

true if an occurrence was found and removed

remove

public int remove(java.lang.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. If the count of the element is less than occurrences, no change is made. Note that if occurrences == 1, this is functionally equivalent to the call remove(element).

This implementation always throws an UnsupportedOperationException. To support removing elements, override it.

Specified by:

remove in interface Multiset<E>

Parameters:

element - the element to conditionally remove occurrences of

occurrences - the number of occurrences of this element to remove. May be zero, in which case no change will be made.

Returns:

true if the condition for modification was met. Unless occurrences is zero, this implies that the multiset was indeed modified.

removeAllOccurrences

public int removeAllOccurrences(java.lang.Object element)

Removes all occurrences of the specified element from this multiset. This method complements Multiset.remove(Object), which removes only one occurrence at a time. TODO: Nuke this. Use setCount(e, 0).

This implementation calls remove(Object, int) with Integer.MAX_VALUE occurrences.

Specified by:

removeAllOccurrences in interface Multiset<E>

Parameters:

element - the element whose occurrences should all be removed

Returns:

the number of occurrences successfully removed, possibly zero

containsAll

public 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.

This implementation checks whether elementSet contains the elements.

Specified by:

containsAll in interface Multiset<E>

Specified by:

containsAll in interface java.util.Collection<E>

Overrides:

containsAll in class java.util.AbstractCollection<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

addAll

public boolean addAll(java.util.Collection<? extends E> elementsToAdd)

If the collection being added is a multiset, this implementation iterates over that multiset's entry set to add the appropriate number of occurrences of each of its elements to this multiset. Otherwise, it calls AbstractCollection.addAll(java.util.Collection).

Specified by:

addAll in interface java.util.Collection<E>

Overrides:

addAll in class java.util.AbstractCollection<E>

removeAll

public boolean removeAll(java.util.Collection<?> elementsToRemove)

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.

This implementation iterates over the elements in the collection and calls removeAllOccurrences(java.lang.Object) on each element. In some cases, this approach has better performance than AbstractCollection.removeAll(java.util.Collection).

Specified by:

removeAll in interface Multiset<E>

Specified by:

removeAll in interface java.util.Collection<E>

Overrides:

removeAll in class java.util.AbstractCollection<E>

retainAll

public boolean retainAll(java.util.Collection<?> elementsToRetain)

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.

This implementation iterates over entrySet(), checking each entry's element to see if it's contained in the provided collection. If it's not found, the remove method of the entry set's iterator is invoked. In some cases, this approach has better performance than AbstractCollection.removeAll(java.util.Collection).

Specified by:

retainAll in interface Multiset<E>

Specified by:

retainAll in interface java.util.Collection<E>

Overrides:

retainAll in class java.util.AbstractCollection<E>

clear

public void clear()

This implementation calls clear on Multiset.entrySet().

Specified by:

clear in interface java.util.Collection<E>

Overrides:

clear in class java.util.AbstractCollection<E>

elementSet

public 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().

The returned set's methods are implemented by calling Multiset.entrySet() methods.

Specified by:

elementSet in interface Multiset<E>

Returns:

a view of the set of distinct elements in this multiset

createElementSet

protected java.util.Set<E> createElementSet()

Creates a new instance of this multiset's element set, which will be returned by elementSet.

equals

public boolean equals(@Nullable
                      java.lang.Object other)

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. TODO: caveats about equivalence-relation.

This implementation returns true if other is a multiset of the same size and if, for each element, the two multisets have the same count.

Specified by:

equals in interface Multiset<E>

Specified by:

equals in interface java.util.Collection<E>

Overrides:

equals in class java.lang.Object

hashCode

public 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.

This implementation returns the hash code of Multiset.entrySet().

Specified by:

hashCode in interface Multiset<E>

Specified by:

hashCode in interface java.util.Collection<E>

Overrides:

hashCode in class java.lang.Object

toString

public java.lang.String toString()

It is recommended, though not mandatory, that this method return the result of invoking Multiset.toString() on the Multiset.entrySet(), yielding a result such as

     [a x 3, c, d x 2, b x 0, e]
 

This implementation returns the result of invoking toString on Multiset.entrySet().

Specified by:

toString in interface Multiset<E>

Overrides:

toString in class java.util.AbstractCollection<E>