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/*
* 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 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 org.checkerframework.checker.nullness.compatqual.NullableDecl;
/**
* A collection that supports order-independent equality, like {@link 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 {@code int}, a multiset
* may never contain more than {@link Integer#MAX_VALUE} occurrences of any one element.
*
*
{@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.
*
*
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)}, 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.
*
*
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.
*
*
A multiset uses {@link Object#equals} to determine whether two instances should be considered
* "the same," unless specified otherwise by the implementation.
*
*
Common implementations include {@link ImmutableMultiset}, {@link HashMultiset}, and {@link
* ConcurrentHashMultiset}.
*
*
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.
*
*
See the Guava User Guide article on {@code
* Multiset}.
*
* @author Kevin Bourrillion
* @since 2.0
*/
@GwtCompatible
public interface Multiset extends Collection {
// 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 {@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 {@link Object#equals}-based multiset, this gives the same result as
* {@link Collections#frequency} (which would presumably perform more poorly).
*
*
Note: 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(@NullableDecl @CompatibleWith("E") Object element);
// Bulk Operations
/**
* 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(@NullableDecl E element, int occurrences);
/**
* Adds a single occurrence of the specified element to this multiset.
*
*
This method refines {@link 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 {@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 {@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(@NullableDecl @CompatibleWith("E") Object element, int occurrences);
/**
* Removes a single occurrence of the specified element from this multiset, if present.
*
*
This method refines {@link Collection#remove} to further specify that it may not
* throw an exception in response to {@code element} being null or of the wrong type.
*
*
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(@NullableDecl Object element);
/**
* 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 {@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:
* {@code elementSet().size()}.
*
* @return a view of the set of distinct elements in this multiset
*/
Set elementSet();
/**
* 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.
*
* 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> entrySet();
/**
* 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 {
/**
* 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.
*
* @return the count of the element; never negative
*/
int getCount();
/**
* {@inheritDoc}
*
* 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:
*
*
{@code
* Objects.equal(a.getElement(), b.getElement())
* && a.getCount() == b.getCount()
* }
*/
@Override
// TODO(kevinb): check this wrt TreeMultiset?
boolean equals(Object o);
/**
* {@inheritDoc}
*
* The hash code of a multiset entry for element {@code element} and count {@code count} is
* defined as:
*
*
{@code
* ((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
* {@code " 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 {@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(@NullableDecl Object object);
/**
* Returns the hash code for this multiset. This is defined as the sum of
*
* {@code
* ((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.
*/
@Override
int hashCode();
/**
* {@inheritDoc}
*
*
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.
*/
@Override
Iterator iterator();
/**
* Determines whether this multiset contains the specified element.
*
* This method refines {@link Collection#contains} to further specify that it may not
* 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(@NullableDecl Object element);
/**
* Returns {@code true} if this multiset contains at least one occurrence of each element in the
* specified collection.
*
*
This method refines {@link Collection#containsAll} to further specify that it may not
* throw an exception in response to any of {@code 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 {@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 {@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)}.
*
*
This method refines {@link Collection#removeAll} to further specify that it may not
* 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 {@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)}.
*
*
This method refines {@link Collection#retainAll} to further specify that it may not
* 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);
}