Start line:  
End line:  

Snippet Preview

Snippet HTML Code

Stack Overflow Questions
  /*
   * Copyright (C) 2007 The Guava Authors
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   * http://www.apache.org/licenses/LICENSE-2.0
   *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
 
 package com.google.common.collect;
 
 
 import java.util.List;
 import java.util.Set;
 
 import  javax.annotation.Nullable;

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, 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 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 com.google.common.util.concurrent.AtomicLongMap instead. Note, however, that unlike Multiset, AtomicLongMap does not automatically remove zeros.

See the Guava User Guide article on Multiset.

Author(s):
Kevin Bourrillion
Since:
2.0 (imported from Google Collections Library)
 
 public interface Multiset<E> extends Collection<E> {
   // Query Operations
 
  
Returns the number of occurrences of an element in this multiset (the count of the element). Note that for an Object.equals-based multiset, this gives the same result as Collections.frequency (which would presumably perform more poorly).

Note: the utility method Iterables.frequency 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
  int count(@Nullable Object element);
  // Bulk Operations

  
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.
  int add(@Nullable E elementint 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
  int remove(@Nullable Object elementint occurrences);

  
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.
  int setCount(E elementint count);

  
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.
  boolean setCount(E elementint oldCountint newCount);
  // Views

  
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
  Set<E> elementSet();

  
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
  Set<Entry<E>> entrySet();

  
An unmodifiable element-count pair for a multiset. The Multiset.entrySet method returns a view of the multiset whose elements are of this class. A multiset implementation may return Entry instances that are either live "read-through" views to the Multiset, or immutable snapshots. Note that this type is unrelated to the similarly-named type Map.Entry.

Since:
2.0 (imported from Google Collections Library)
  interface Entry<E> {

    
Returns the multiset element corresponding to this entry. Multiple calls to this method always return the same instance.

Returns:
the element corresponding to this entry
    E getElement();

    
Returns the count of the associated element in the underlying multiset. This count may either be an unchanging snapshot of the count at the time the entry was retrieved, or a live view of the current count of the element in the multiset, depending on the implementation. Note that in the former case, this method can never return zero, while in the latter, it will return zero if all occurrences of the element were since removed from the multiset.

Returns:
the count of the element; never negative
    int getCount();

    

Returns true if the given object is also a multiset entry and the two entries represent the same element and count. That is, two entries a and b are equal if:

   Objects.equal(a.getElement(), b.getElement())
       && a.getCount() == b.getCount()
    @Override
    // TODO(kevinb): check this wrt TreeMultiset?
    boolean equals(Object o);

    

The hash code of a multiset entry for element element and count count is defined as:

   ((element == null) ? 0 : element.hashCode()) ^ count
    @Override
    int hashCode();

    
Returns the canonical string representation of this entry, defined as follows. If the count for this entry is one, this is simply the string representation of the corresponding element. Otherwise, it is the string representation of the element, followed by the three characters " x " (space, letter x, space), followed by the count.
    @Override
    String toString();
  }
  // Comparison and hashing

  
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(kevinb): caveats about equivalence-relation?
  boolean equals(@Nullable Object object);

  
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.

  int 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].

  // Refined Collection Methods

  

Elements that occur multiple times in the multiset will appear multiple times in this iterator, though not necessarily sequentially.

  Iterator<E> iterator();

  
Determines whether this multiset contains the specified element.

This method refines Collection.contains to further specify that it may not throw an exception in response to element being null or of the wrong type.

Parameters:
element the element to check for
Returns:
true if this multiset contains at least one occurrence of the element
  boolean contains(@Nullable Object element);

  
Returns true if this multiset contains at least one occurrence of each element in the specified collection.

This method refines Collection.containsAll 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.

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
  boolean containsAll(Collection<?> elements);

  
Adds a single occurrence of the specified element to this multiset.

This method refines Collection.add, 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.

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
  boolean add(E element);

  
Removes a single occurrence of the specified element from this multiset, if present.

This method refines Collection.remove to further specify that it may not throw an exception in response to element being null or of the wrong type.

Parameters:
element the element to remove one occurrence of
Returns:
true if an occurrence was found and removed
  boolean remove(@Nullable Object element);

  

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 to further specify that it may not throw an exception in response to any of elements being null or of the wrong type.

  boolean 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.retainOccurrences(Multiset, Multiset).

This method refines Collection.retainAll to further specify that it may not throw an exception in response to any of elements being null or of the wrong type.

See also:
Multisets.retainOccurrences(Multiset, Multiset)
  boolean retainAll(Collection<?> c);
New to GrepCode? Check out our FAQ X