https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * 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 static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.CompatibleWith;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Set;
import java.util.Spliterator;
import java.util.function.Consumer;
import java.util.function.ObjIntConsumer;
import org.checkerframework.checker.nullness.qual.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 AtomicLongMap instead. Note, however, that unlike Multiset, AtomicLongMap does not automatically remove zeros. 
See the Guava User Guide article on  
Multiset.

Author:Kevin Bourrillion
Since:2.0
/**
 * A collection that supports order-independent equality, like {@link Set}, but may have duplicate
 * elements. A multiset is also sometimes called a <i>bag</i>.
 *
 * <p>Elements of a multiset that are equal to one another are referred to as <i>occurrences</i> of
 * the same single element. The total number of occurrences of an element in a multiset is called
 * the <i>count</i> 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 {@code int}, a multiset
 * may never contain more than {@link Integer#MAX_VALUE} occurrences of any one element.
 *
 * <p>{@code Multiset} refines the specifications of several methods from {@code Collection}. It
 * also defines an additional query operation, {@link #count}, which returns the count of an
 * element. There are five new bulk-modification operations, for example {@link #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 {@link #add(Object)} or {@link #remove(Object)} are encouraged
 * to implement the related methods as well. Finally, two collection views are provided: {@link
 * #elementSet} contains the distinct elements of the multiset "with duplicates collapsed", and
 * {@link #entrySet} is similar but contains {@link Entry Multiset.Entry} instances, each providing
 * both a distinct element and the count of that element.
 *
 * <p>In addition to these required methods, implementations of {@code Multiset} are expected to
 * provide two {@code static} creation methods: {@code create()}, returning an empty multiset, and
 * {@code create(Iterable<? extends E>)}, returning a multiset containing the given initial
 * elements. This is simply a refinement of {@code Collection}'s constructor recommendations,
 * reflecting the new developments of Java 5.
 *
 * <p>As with other collection types, the modification operations are optional, and should throw
 * {@link 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 {@code setCount} methods as well.
 *
 * <p>A multiset uses {@link Object#equals} to determine whether two instances should be considered
 * "the same," <i>unless specified otherwise</i> by the implementation.
 *
 * <p>Common implementations include {@link ImmutableMultiset}, {@link HashMultiset}, and {@link
 * ConcurrentHashMultiset}.
 *
 * <p>If your values may be zero, negative, or outside the range of an int, you may wish to use
 * {@link com.google.common.util.concurrent.AtomicLongMap} instead. Note, however, that unlike
 * {@code Multiset}, {@code AtomicLongMap} does not automatically remove zeros.
 *
 * <p>See the Guava User Guide article on <a href=
 * "https://github.com/google/guava/wiki/NewCollectionTypesExplained#multiset"> {@code
 * Multiset}</a>.
 *
 * @author Kevin Bourrillion
 * @since 2.0
 */
@GwtCompatible
public interface Multiset<E> extends Collection<E> {
  // Query Operations

  
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(). 
/**
   * Returns the total number of all occurrences of all elements in this multiset.
   *
   * <p><b>Note:</b> this method does not return the number of <i>distinct elements</i> in the
   * multiset, which is given by {@code entrySet().size()}.
   */
  @Override
  int size();

  
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. 
Params:
  • element – the element to count occurrences of
Returns:the number of occurrences of the element in this multiset; possibly zero but never
    negative
/**
   * Returns the number of occurrences of an element in this multiset (the <i>count</i> of the
   * element). Note that for an {@link Object#equals}-based multiset, this gives the same result as
   * {@link Collections#frequency} (which would presumably perform more poorly).
   *
   * <p><b>Note:</b> the utility method {@link 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.
   *
   * @param element the element to count occurrences of
   * @return the number of occurrences of the element in this multiset; possibly zero but never
   *     negative
   */
  int count(@Nullable @CompatibleWith("E") 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. 
Params:
  • 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.
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.
Returns:the count of the element before the operation; possibly zero
/**
   * Adds a number of occurrences of an element to this multiset. Note that if {@code occurrences ==
   * 1}, this method has the identical effect to {@link #add(Object)}. This method is functionally
   * equivalent (except in the case of overflow) to the call {@code
   * addAll(Collections.nCopies(element, occurrences))}, which would presumably perform much more
   * poorly.
   *
   * @param element the element to add occurrences of; may be null only if explicitly allowed by the
   *     implementation
   * @param occurrences the number of occurrences of the element to add. May be zero, in which case
   *     no change will be made.
   * @return the count of the element before the operation; possibly zero
   * @throws IllegalArgumentException if {@code occurrences} is negative, or if this operation would
   *     result in more than {@link Integer#MAX_VALUE} occurrences of the element
   * @throws NullPointerException if {@code element} is null and this implementation does not permit
   *     null elements. Note that if {@code occurrences} is zero, the implementation may opt to
   *     return normally.
   */
  @CanIgnoreReturnValue
  int add(@Nullable E element, int occurrences);

  
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.

To both add the element and obtain the previous count of that element, use add(element, 1) instead. 
Params:
  • element – the element to add one occurrence of; may be null only if explicitly allowed by
    the implementation
Throws:
Returns:true always, since this call is required to modify the multiset, unlike other Collection types
/**
   * Adds a single occurrence of the specified element to this multiset.
   *
   * <p>This method refines {@link Collection#add}, which only <i>ensures</i> 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.
   *
   * <p>To both add the element and obtain the previous count of that element, use {@link
   * #add(Object, int) add}{@code (element, 1)} instead.
   *
   * @param element the element to add one occurrence of; may be null only if explicitly allowed by
   *     the implementation
   * @return {@code true} always, since this call is required to modify the multiset, unlike other
   *     {@link Collection} types
   * @throws NullPointerException if {@code element} is null and this implementation does not permit
   *     null elements
   * @throws IllegalArgumentException if {@link Integer#MAX_VALUE} occurrences of {@code element}
   *     are already contained in this multiset
   */
  @CanIgnoreReturnValue
  @Override
  boolean add(E element);

  
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). 
Params:
  • 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.
Throws:
Returns:the count of the element before the operation; possibly zero
/**
   * 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 {@code occurrences == 1}, this is functionally equivalent to the call {@code
   * remove(element)}.
   *
   * @param element the element to conditionally remove occurrences of
   * @param occurrences the number of occurrences of the element to remove. May be zero, in which
   *     case no change will be made.
   * @return the count of the element before the operation; possibly zero
   * @throws IllegalArgumentException if {@code occurrences} is negative
   */
  @CanIgnoreReturnValue
  int remove(@Nullable @CompatibleWith("E") Object element, int occurrences);

  
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. 
To both remove the element and obtain the previous count of that element, use remove(element, 1) instead. 
Params:
  • element – the element to remove one occurrence of
Returns:true if an occurrence was found and removed
/**
   * Removes a <i>single</i> occurrence of the specified element from this multiset, if present.
   *
   * <p>This method refines {@link Collection#remove} to further specify that it <b>may not</b>
   * throw an exception in response to {@code element} being null or of the wrong type.
   *
   * <p>To both remove the element and obtain the previous count of that element, use {@link
   * #remove(Object, int) remove}{@code (element, 1)} instead.
   *
   * @param element the element to remove one occurrence of
   * @return {@code true} if an occurrence was found and removed
   */
  @CanIgnoreReturnValue
  @Override
  boolean remove(@Nullable Object element);

  
Adds or removes the necessary occurrences of an element such that the element attains the
desired count.

Params:
  • 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
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.
Returns:the count of the element before the operation; possibly zero
/**
   * Adds or removes the necessary occurrences of an element such that the element attains the
   * desired count.
   *
   * @param element the element to add or remove occurrences of; may be null only if explicitly
   *     allowed by the implementation
   * @param count the desired count of the element in this multiset
   * @return the count of the element before the operation; possibly zero
   * @throws IllegalArgumentException if {@code count} is negative
   * @throws NullPointerException if {@code element} is null and this implementation does not permit
   *     null elements. Note that if {@code count} is zero, the implementor may optionally return
   *     zero instead.
   */
  @CanIgnoreReturnValue
  int setCount(E element, int 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. 
Params:
  • 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
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.
Returns:true if the condition for modification was met. This implies that the multiset was indeed modified, unless oldCount == newCount.
/**
   * Conditionally sets the count of an element to a new value, as described in {@link
   * #setCount(Object, int)}, provided that the element has the expected current count. If the
   * current count is not {@code oldCount}, no change is made.
   *
   * @param element the element to conditionally set the count of; may be null only if explicitly
   *     allowed by the implementation
   * @param oldCount the expected present count of the element in this multiset
   * @param newCount the desired count of the element in this multiset
   * @return {@code true} if the condition for modification was met. This implies that the multiset
   *     was indeed modified, unless {@code oldCount == newCount}.
   * @throws IllegalArgumentException if {@code oldCount} or {@code newCount} is negative
   * @throws NullPointerException if {@code element} is null and the implementation does not permit
   *     null elements. Note that if {@code oldCount} and {@code newCount} are both zero, the
   *     implementor may optionally return {@code true} instead.
   */
  @CanIgnoreReturnValue
  boolean setCount(E element, int oldCount, int 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
/**
   * 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.
   *
   * <p>If the element set supports any removal operations, these necessarily cause <b>all</b>
   * 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.
   *
   * <p>A common use for the element set is to find the number of distinct elements in the multiset:
   * {@code elementSet().size()}.
   *
   * @return 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
/**
   * Returns a view of the contents of this multiset, grouped into {@code 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 {@link #elementSet}). The order of the elements in the element set is unspecified.
   *
   * <p>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 {@code 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 {@code 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.
   *
   * @return 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
/**
   * An unmodifiable element-count pair for a multiset. The {@link 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 {@code Map.Entry}.
   *
   * @since 2.0
   */
  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
/**
     * Returns the multiset element corresponding to this entry. Multiple calls to this method
     * always return the same instance.
     *
     * @return 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
/**
     * 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.
     *
     * @return the count of the element; never negative
     */
    int getCount();

    
{@inheritDoc}

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()



/**
     * {@inheritDoc}
     *
     * <p>Returns {@code true} if the given object is also a multiset entry and the two entries
     * represent the same element and count. That is, two entries {@code a} and {@code b} are equal
     * if:
     *
     * <pre>{@code
     * Objects.equal(a.getElement(), b.getElement())
     *     && a.getCount() == b.getCount()
     * }</pre>
     */
    @Override
    // TODO(kevinb): check this wrt TreeMultiset?
    boolean equals(Object o);

    
{@inheritDoc}

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

((element == null) ? 0 : element.hashCode()) ^ count



/**
     * {@inheritDoc}
     *
     * <p>The hash code of a multiset entry for element {@code element} and count {@code count} is
     * defined as:
     *
     * <pre>{@code
     * ((element == null) ? 0 : element.hashCode()) ^ count
     * }</pre>
     */
    @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. 
/**
     * 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
     * {@code " x "} (space, letter x, space), followed by the count.
     */
    @Override
    String toString();
  }

  
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
/**
   * Runs the specified action for each distinct element in this multiset, and the number of
   * occurrences of that element. For some {@code Multiset} implementations, this may be more
   * efficient than iterating over the {@link #entrySet()} either explicitly or with {@code
   * entrySet().forEach(action)}.
   *
   * @since 21.0
   */
  @Beta
  default void forEachEntry(ObjIntConsumer<? super E> action) {
    checkNotNull(action);
    entrySet().forEach(entry -> action.accept(entry.getElement(), entry.getCount()));
  }

  // 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. 
/**
   * Compares the specified object with this multiset for equality. Returns {@code true} if the
   * given object is also a multiset and contains equal elements with equal counts, regardless of
   * order.
   */
  @Override
  // 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.

/**
   * Returns the hash code for this multiset. This is defined as the sum of
   *
   * <pre>{@code
   * ((element == null) ? 0 : element.hashCode()) ^ count(element)
   * }</pre>
   *
   * <p>over all distinct elements in the multiset. It follows that a multiset and its entry set
   * always have the same hash code.
   */
  @Override
  int hashCode();

  
{@inheritDoc}

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]. 
/**
   * {@inheritDoc}
   *
   * <p>It is recommended, though not mandatory, that this method return the result of invoking
   * {@link #toString} on the {@link #entrySet}, yielding a result such as {@code [a x 3, c, d x 2,
   * e]}.
   */
  @Override
  String toString();

  // Refined Collection Methods

  
{@inheritDoc}

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

/**
   * {@inheritDoc}
   *
   * <p>Elements that occur multiple times in the multiset will appear multiple times in this
   * iterator, though not necessarily sequentially.
   */
  @Override
  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. 
Params:
  • element – the element to check for
Returns:true if this multiset contains at least one occurrence of the element
/**
   * Determines whether this multiset contains the specified element.
   *
   * <p>This method refines {@link Collection#contains} to further specify that it <b>may not</b>
   * throw an exception in response to {@code element} being null or of the wrong type.
   *
   * @param element the element to check for
   * @return {@code true} if this multiset contains at least one occurrence of the element
   */
  @Override
  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. 
Params:
  • elements – the collection of elements to be checked for containment in this multiset
Throws:
Returns:true if this multiset contains at least one occurrence of each element contained in elements
/**
   * Returns {@code true} if this multiset contains at least one occurrence of each element in the
   * specified collection.
   *
   * <p>This method refines {@link Collection#containsAll} to further specify that it <b>may not</b>
   * throw an exception in response to any of {@code elements} being null or of the wrong type.
   *
   * <p><b>Note:</b> this method does not take into account the occurrence count of an element in
   * the two collections; it may still return {@code true} even if {@code elements} contains several
   * occurrences of an element and this multiset contains only one. This is no different than any
   * other collection type like {@link List}, but it may be unexpected to the user of a multiset.
   *
   * @param elements the collection of elements to be checked for containment in this multiset
   * @return {@code true} if this multiset contains at least one occurrence of each element
   *     contained in {@code elements}
   * @throws NullPointerException if {@code elements} is null
   */
  @Override
  boolean containsAll(Collection<?> elements);

  
{@inheritDoc}

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. 
/**
   * {@inheritDoc}
   *
   * <p><b>Note:</b> This method ignores how often any element might appear in {@code 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 {@code c}, see {@link Multisets#removeOccurrences(Multiset,
   * Multiset)}.
   *
   * <p>This method refines {@link Collection#removeAll} to further specify that it <b>may not</b>
   * throw an exception in response to any of {@code elements} being null or of the wrong type.
   */
  @CanIgnoreReturnValue
  @Override
  boolean removeAll(Collection<?> c);

  
{@inheritDoc}

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:
/**
   * {@inheritDoc}
   *
   * <p><b>Note:</b> This method ignores how often any element might appear in {@code 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 {@code c}, see {@link Multisets#retainOccurrences(Multiset,
   * Multiset)}.
   *
   * <p>This method refines {@link Collection#retainAll} to further specify that it <b>may not</b>
   * throw an exception in response to any of {@code elements} being null or of the wrong type.
   *
   * @see Multisets#retainOccurrences(Multiset, Multiset)
   */
  @CanIgnoreReturnValue
  @Override
  boolean retainAll(Collection<?> c);

  
{@inheritDoc}

Elements that occur multiple times in the multiset will be passed to the Consumer correspondingly many times, though not necessarily sequentially. 
/**
   * {@inheritDoc}
   *
   * <p>Elements that occur multiple times in the multiset will be passed to the {@code Consumer}
   * correspondingly many times, though not necessarily sequentially.
   */
  @Override
  default void forEach(Consumer<? super E> action) {
    checkNotNull(action);
    entrySet()
        .forEach(
            entry -> {
              E elem = entry.getElement();
              int count = entry.getCount();
              for (int i = 0; i < count; i++) {
                action.accept(elem);
              }
            });
  }

  @Override
  default Spliterator<E> spliterator() {
    return Multisets.spliteratorImpl(this);
  }
}