common/collect/Multiset.java

/*
 * 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 javax.annotation.CheckForNull;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * 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><b>Warning:</b> as with normal {@link Set}s, it is almost always a bad idea to modify an
 * element (in a way that affects its {@link Object#equals} behavior) while it is contained in a
 * multiset. Undefined behavior and bugs will result.
 *
 * <h3>Implementations</h3>
 *
 * <ul>
 *   <li>{@link ImmutableMultiset}
 *   <li>{@link ImmutableSortedMultiset}
 *   <li>{@link HashMultiset}
 *   <li>{@link LinkedHashMultiset}
 *   <li>{@link TreeMultiset}
 *   <li>{@link EnumMultiset}
 *   <li>{@link ConcurrentHashMultiset}
 * </ul>
 *
 * <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
@ElementTypesAreNonnullByDefault
public interface Multiset<E extends @Nullable Object> extends Collection<E> {
  // Query Operations

  /**
   * 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 <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(@CompatibleWith("E") @CheckForNull 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(@ParametricNullness E element, int occurrences);

  /**
   * 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(@ParametricNullness 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(@CompatibleWith("E") @CheckForNull Object element, int occurrences);

  /**
   * 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(@CheckForNull 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(@ParametricNullness 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(@ParametricNullness 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.
   *
   * <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 {@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 {@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 extends @Nullable Object> {

    /**
     * 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
     */
    @ParametricNullness
    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}
     *
     * <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(@CheckForNull Object o);

    /**
     * {@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
     * {@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(@CheckForNull Object object);

  /**
   * 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}
   *
   * <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}
   *
   * <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.
   *
   * <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(@CheckForNull Object element);

  /**
   * 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}
   *
   * <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}
   *
   * <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);
}