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) 2008 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.common.annotations.GwtIncompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.concurrent.LazyInit;
import com.google.j2objc.annotations.WeakOuter;
import java.io.Serializable;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.function.Function;
import java.util.function.ToIntFunction;
import java.util.stream.Collector;
import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
import org.checkerframework.checker.nullness.qual.Nullable;


A Multiset whose contents will never change, with many other important properties detailed at ImmutableCollection. 
Grouped iteration. In all current implementations, duplicate elements always appear
consecutively when iterating. Elements iterate in order by the first appearance of that
element when the multiset was created.

See the Guava User Guide article on  immutable collections.

Author:Jared Levy, Louis Wasserman
Since:2.0
/**
 * A {@link Multiset} whose contents will never change, with many other important properties
 * detailed at {@link ImmutableCollection}.
 *
 * <p><b>Grouped iteration.</b> In all current implementations, duplicate elements always appear
 * consecutively when iterating. Elements iterate in order by the <i>first</i> appearance of that
 * element when the multiset was created.
 *
 * <p>See the Guava User Guide article on <a href=
 * "https://github.com/google/guava/wiki/ImmutableCollectionsExplained"> immutable collections</a>.
 *
 * @author Jared Levy
 * @author Louis Wasserman
 * @since 2.0
 */
@GwtCompatible(serializable = true, emulated = true)
@SuppressWarnings("serial") // we're overriding default serialization
public abstract class ImmutableMultiset<E> extends ImmutableMultisetGwtSerializationDependencies<E>
    implements Multiset<E> {

  
Returns a Collector that accumulates the input elements into a new 
ImmutableMultiset. Elements iterate in order by the first appearance of that element in
encounter order.

Since:21.0
/**
   * Returns a {@code Collector} that accumulates the input elements into a new {@code
   * ImmutableMultiset}. Elements iterate in order by the <i>first</i> appearance of that element in
   * encounter order.
   *
   * @since 21.0
   */
  @Beta
  public static <E> Collector<E, ?, ImmutableMultiset<E>> toImmutableMultiset() {
    return toImmutableMultiset(Function.identity(), e -> 1);
  }

  
Returns a Collector that accumulates elements into an ImmutableMultiset whose elements are the result of applying elementFunction to the inputs, with counts equal to the result of applying countFunction to the inputs. 
If the mapped elements contain duplicates (according to Object.equals), the first occurrence in encounter order appears in the resulting multiset, with count equal to the sum of the outputs of countFunction.applyAsInt(t) for each t mapped to that element. 
Since:22.0
/**
   * Returns a {@code Collector} that accumulates elements into an {@code ImmutableMultiset} whose
   * elements are the result of applying {@code elementFunction} to the inputs, with counts equal to
   * the result of applying {@code countFunction} to the inputs.
   *
   * <p>If the mapped elements contain duplicates (according to {@link Object#equals}), the first
   * occurrence in encounter order appears in the resulting multiset, with count equal to the sum of
   * the outputs of {@code countFunction.applyAsInt(t)} for each {@code t} mapped to that element.
   *
   * @since 22.0
   */
  public static <T, E> Collector<T, ?, ImmutableMultiset<E>> toImmutableMultiset(
      Function<? super T, ? extends E> elementFunction, ToIntFunction<? super T> countFunction) {
    checkNotNull(elementFunction);
    checkNotNull(countFunction);
    return Collector.of(
        LinkedHashMultiset::create,
        (multiset, t) ->
            multiset.add(checkNotNull(elementFunction.apply(t)), countFunction.applyAsInt(t)),
        (multiset1, multiset2) -> {
          multiset1.addAll(multiset2);
          return multiset1;
        },
        (Multiset<E> multiset) -> copyFromEntries(multiset.entrySet()));
  }

  
Returns the empty immutable multiset. 
/** Returns the empty immutable multiset. */
  @SuppressWarnings("unchecked") // all supported methods are covariant
  public static <E> ImmutableMultiset<E> of() {
    return (ImmutableMultiset<E>) RegularImmutableMultiset.EMPTY;
  }

  
Returns an immutable multiset containing a single element.

Throws:
  • NullPointerException – if element is null
Since:6.0 (source-compatible since 2.0)
/**
   * Returns an immutable multiset containing a single element.
   *
   * @throws NullPointerException if {@code element} is null
   * @since 6.0 (source-compatible since 2.0)
   */
  @SuppressWarnings("unchecked") // generic array created but never written
  public static <E> ImmutableMultiset<E> of(E element) {
    return copyFromElements(element);
  }

  
Returns an immutable multiset containing the given elements, in order.

Throws:
  • NullPointerException – if any element is null
Since:6.0 (source-compatible since 2.0)
/**
   * Returns an immutable multiset containing the given elements, in order.
   *
   * @throws NullPointerException if any element is null
   * @since 6.0 (source-compatible since 2.0)
   */
  @SuppressWarnings("unchecked") //
  public static <E> ImmutableMultiset<E> of(E e1, E e2) {
    return copyFromElements(e1, e2);
  }

  
Returns an immutable multiset containing the given elements, in the "grouped iteration order"
described in the class documentation.

Throws:
  • NullPointerException – if any element is null
Since:6.0 (source-compatible since 2.0)
/**
   * Returns an immutable multiset containing the given elements, in the "grouped iteration order"
   * described in the class documentation.
   *
   * @throws NullPointerException if any element is null
   * @since 6.0 (source-compatible since 2.0)
   */
  @SuppressWarnings("unchecked") //
  public static <E> ImmutableMultiset<E> of(E e1, E e2, E e3) {
    return copyFromElements(e1, e2, e3);
  }

  
Returns an immutable multiset containing the given elements, in the "grouped iteration order"
described in the class documentation.

Throws:
  • NullPointerException – if any element is null
Since:6.0 (source-compatible since 2.0)
/**
   * Returns an immutable multiset containing the given elements, in the "grouped iteration order"
   * described in the class documentation.
   *
   * @throws NullPointerException if any element is null
   * @since 6.0 (source-compatible since 2.0)
   */
  @SuppressWarnings("unchecked") //
  public static <E> ImmutableMultiset<E> of(E e1, E e2, E e3, E e4) {
    return copyFromElements(e1, e2, e3, e4);
  }

  
Returns an immutable multiset containing the given elements, in the "grouped iteration order"
described in the class documentation.

Throws:
  • NullPointerException – if any element is null
Since:6.0 (source-compatible since 2.0)
/**
   * Returns an immutable multiset containing the given elements, in the "grouped iteration order"
   * described in the class documentation.
   *
   * @throws NullPointerException if any element is null
   * @since 6.0 (source-compatible since 2.0)
   */
  @SuppressWarnings("unchecked") //
  public static <E> ImmutableMultiset<E> of(E e1, E e2, E e3, E e4, E e5) {
    return copyFromElements(e1, e2, e3, e4, e5);
  }

  
Returns an immutable multiset containing the given elements, in the "grouped iteration order"
described in the class documentation.

Throws:
  • NullPointerException – if any element is null
Since:6.0 (source-compatible since 2.0)
/**
   * Returns an immutable multiset containing the given elements, in the "grouped iteration order"
   * described in the class documentation.
   *
   * @throws NullPointerException if any element is null
   * @since 6.0 (source-compatible since 2.0)
   */
  @SuppressWarnings("unchecked") //
  public static <E> ImmutableMultiset<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E... others) {
    return new Builder<E>().add(e1).add(e2).add(e3).add(e4).add(e5).add(e6).add(others).build();
  }

  
Returns an immutable multiset containing the given elements, in the "grouped iteration order"
described in the class documentation.

Throws:
  • NullPointerException – if any of elements is null
Since:6.0
/**
   * Returns an immutable multiset containing the given elements, in the "grouped iteration order"
   * described in the class documentation.
   *
   * @throws NullPointerException if any of {@code elements} is null
   * @since 6.0
   */
  public static <E> ImmutableMultiset<E> copyOf(E[] elements) {
    return copyFromElements(elements);
  }

  
Returns an immutable multiset containing the given elements, in the "grouped iteration order"
described in the class documentation.

Throws:
  • NullPointerException – if any of elements is null
/**
   * Returns an immutable multiset containing the given elements, in the "grouped iteration order"
   * described in the class documentation.
   *
   * @throws NullPointerException if any of {@code elements} is null
   */
  public static <E> ImmutableMultiset<E> copyOf(Iterable<? extends E> elements) {
    if (elements instanceof ImmutableMultiset) {
      @SuppressWarnings("unchecked") // all supported methods are covariant
      ImmutableMultiset<E> result = (ImmutableMultiset<E>) elements;
      if (!result.isPartialView()) {
        return result;
      }
    }

    Multiset<? extends E> multiset =
        (elements instanceof Multiset)
            ? Multisets.cast(elements)
            : LinkedHashMultiset.create(elements);

    return copyFromEntries(multiset.entrySet());
  }

  
Returns an immutable multiset containing the given elements, in the "grouped iteration order"
described in the class documentation.

Throws:
  • NullPointerException – if any of elements is null
/**
   * Returns an immutable multiset containing the given elements, in the "grouped iteration order"
   * described in the class documentation.
   *
   * @throws NullPointerException if any of {@code elements} is null
   */
  public static <E> ImmutableMultiset<E> copyOf(Iterator<? extends E> elements) {
    Multiset<E> multiset = LinkedHashMultiset.create();
    Iterators.addAll(multiset, elements);
    return copyFromEntries(multiset.entrySet());
  }

  private static <E> ImmutableMultiset<E> copyFromElements(E... elements) {
    Multiset<E> multiset = LinkedHashMultiset.create();
    Collections.addAll(multiset, elements);
    return copyFromEntries(multiset.entrySet());
  }

  static <E> ImmutableMultiset<E> copyFromEntries(
      Collection<? extends Entry<? extends E>> entries) {
    if (entries.isEmpty()) {
      return of();
    } else {
      return RegularImmutableMultiset.create(entries);
    }
  }

  ImmutableMultiset() {}

  @Override
  public UnmodifiableIterator<E> iterator() {
    final Iterator<Entry<E>> entryIterator = entrySet().iterator();
    return new UnmodifiableIterator<E>() {
      int remaining;
      @MonotonicNonNull E element;

      @Override
      public boolean hasNext() {
        return (remaining > 0) || entryIterator.hasNext();
      }

      @Override
      public E next() {
        if (remaining <= 0) {
          Entry<E> entry = entryIterator.next();
          element = entry.getElement();
          remaining = entry.getCount();
        }
        remaining--;
        return element;
      }
    };
  }

  @LazyInit private transient ImmutableList<E> asList;

  @Override
  public ImmutableList<E> asList() {
    ImmutableList<E> result = asList;
    return (result == null) ? asList = super.asList() : result;
  }

  @Override
  public boolean contains(@Nullable Object object) {
    return count(object) > 0;
  }

  
Guaranteed to throw an exception and leave the collection unmodified.

Throws:
  • UnsupportedOperationException – always
Deprecated:Unsupported operation.
/**
   * Guaranteed to throw an exception and leave the collection unmodified.
   *
   * @throws UnsupportedOperationException always
   * @deprecated Unsupported operation.
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  public final int add(E element, int occurrences) {
    throw new UnsupportedOperationException();
  }

  
Guaranteed to throw an exception and leave the collection unmodified.

Throws:
  • UnsupportedOperationException – always
Deprecated:Unsupported operation.
/**
   * Guaranteed to throw an exception and leave the collection unmodified.
   *
   * @throws UnsupportedOperationException always
   * @deprecated Unsupported operation.
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  public final int remove(Object element, int occurrences) {
    throw new UnsupportedOperationException();
  }

  
Guaranteed to throw an exception and leave the collection unmodified.

Throws:
  • UnsupportedOperationException – always
Deprecated:Unsupported operation.
/**
   * Guaranteed to throw an exception and leave the collection unmodified.
   *
   * @throws UnsupportedOperationException always
   * @deprecated Unsupported operation.
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  public final int setCount(E element, int count) {
    throw new UnsupportedOperationException();
  }

  
Guaranteed to throw an exception and leave the collection unmodified.

Throws:
  • UnsupportedOperationException – always
Deprecated:Unsupported operation.
/**
   * Guaranteed to throw an exception and leave the collection unmodified.
   *
   * @throws UnsupportedOperationException always
   * @deprecated Unsupported operation.
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  public final boolean setCount(E element, int oldCount, int newCount) {
    throw new UnsupportedOperationException();
  }

  @GwtIncompatible // not present in emulated superclass
  @Override
  int copyIntoArray(Object[] dst, int offset) {
    for (Multiset.Entry<E> entry : entrySet()) {
      Arrays.fill(dst, offset, offset + entry.getCount(), entry.getElement());
      offset += entry.getCount();
    }
    return offset;
  }

  @Override
  public boolean equals(@Nullable Object object) {
    return Multisets.equalsImpl(this, object);
  }

  @Override
  public int hashCode() {
    return Sets.hashCodeImpl(entrySet());
  }

  @Override
  public String toString() {
    return entrySet().toString();
  }

  
Since:21.0 (present with return type Set since 2.0)
/** @since 21.0 (present with return type {@code Set} since 2.0) */
  @Override
  public abstract ImmutableSet<E> elementSet();

  @LazyInit private transient ImmutableSet<Entry<E>> entrySet;

  @Override
  public ImmutableSet<Entry<E>> entrySet() {
    ImmutableSet<Entry<E>> es = entrySet;
    return (es == null) ? (entrySet = createEntrySet()) : es;
  }

  private ImmutableSet<Entry<E>> createEntrySet() {
    return isEmpty() ? ImmutableSet.<Entry<E>>of() : new EntrySet();
  }

  abstract Entry<E> getEntry(int index);

  @WeakOuter
  private final class EntrySet extends IndexedImmutableSet<Entry<E>> {
    @Override
    boolean isPartialView() {
      return ImmutableMultiset.this.isPartialView();
    }

    @Override
    Entry<E> get(int index) {
      return getEntry(index);
    }

    @Override
    public int size() {
      return elementSet().size();
    }

    @Override
    public boolean contains(Object o) {
      if (o instanceof Entry) {
        Entry<?> entry = (Entry<?>) o;
        if (entry.getCount() <= 0) {
          return false;
        }
        int count = count(entry.getElement());
        return count == entry.getCount();
      }
      return false;
    }

    @Override
    public int hashCode() {
      return ImmutableMultiset.this.hashCode();
    }

    @GwtIncompatible
    @Override
    Object writeReplace() {
      return new EntrySetSerializedForm<E>(ImmutableMultiset.this);
    }

    private static final long serialVersionUID = 0;
  }

  @GwtIncompatible
  static class EntrySetSerializedForm<E> implements Serializable {
    final ImmutableMultiset<E> multiset;

    EntrySetSerializedForm(ImmutableMultiset<E> multiset) {
      this.multiset = multiset;
    }

    Object readResolve() {
      return multiset.entrySet();
    }
  }

  @GwtIncompatible
  @Override
  Object writeReplace() {
    return new SerializedForm(this);
  }

  
Returns a new builder. The generated builder is equivalent to the builder created by the Builder constructor. 
/**
   * Returns a new builder. The generated builder is equivalent to the builder created by the {@link
   * Builder} constructor.
   */
  public static <E> Builder<E> builder() {
    return new Builder<E>();
  }

  
A builder for creating immutable multiset instances, especially public static final multisets ("constant multisets"). Example: 

public static final ImmutableMultiset<Bean> BEANS =
    new ImmutableMultiset.Builder<Bean>()
        .addCopies(Bean.COCOA, 4)
        .addCopies(Bean.GARDEN, 6)
        .addCopies(Bean.RED, 8)
        .addCopies(Bean.BLACK_EYED, 10)
        .build();



Builder instances can be reused; it is safe to call build multiple times to build multiple multisets in series. 
Since:2.0
/**
   * A builder for creating immutable multiset instances, especially {@code public static final}
   * multisets ("constant multisets"). Example:
   *
   * <pre>{@code
   * public static final ImmutableMultiset<Bean> BEANS =
   *     new ImmutableMultiset.Builder<Bean>()
   *         .addCopies(Bean.COCOA, 4)
   *         .addCopies(Bean.GARDEN, 6)
   *         .addCopies(Bean.RED, 8)
   *         .addCopies(Bean.BLACK_EYED, 10)
   *         .build();
   * }</pre>
   *
   * <p>Builder instances can be reused; it is safe to call {@link #build} multiple times to build
   * multiple multisets in series.
   *
   * @since 2.0
   */
  public static class Builder<E> extends ImmutableCollection.Builder<E> {
    final Multiset<E> contents;

    
Creates a new builder. The returned builder is equivalent to the builder generated by ImmutableMultiset.builder. 
/**
     * Creates a new builder. The returned builder is equivalent to the builder generated by {@link
     * ImmutableMultiset#builder}.
     */
    public Builder() {
      this(LinkedHashMultiset.<E>create());
    }

    Builder(Multiset<E> contents) {
      this.contents = contents;
    }

    
Adds element to the ImmutableMultiset. 
Params:
  • element – the element to add
Throws:
Returns:this Builder object
/**
     * Adds {@code element} to the {@code ImmutableMultiset}.
     *
     * @param element the element to add
     * @return this {@code Builder} object
     * @throws NullPointerException if {@code element} is null
     */
    @CanIgnoreReturnValue
    @Override
    public Builder<E> add(E element) {
      contents.add(checkNotNull(element));
      return this;
    }

    
Adds each element of elements to the ImmutableMultiset. 
Params:
  • elements – the elements to add
Throws:
Returns:this Builder object
/**
     * Adds each element of {@code elements} to the {@code ImmutableMultiset}.
     *
     * @param elements the elements to add
     * @return this {@code Builder} object
     * @throws NullPointerException if {@code elements} is null or contains a null element
     */
    @CanIgnoreReturnValue
    @Override
    public Builder<E> add(E... elements) {
      super.add(elements);
      return this;
    }

    
Adds a number of occurrences of an element to this ImmutableMultiset. 
Params:
  • element – the element to add
  • occurrences – the number of occurrences of the element to add. May be zero, in which
    case no change will be made.
Throws:
Returns:this Builder object
/**
     * Adds a number of occurrences of an element to this {@code ImmutableMultiset}.
     *
     * @param element the element to add
     * @param occurrences the number of occurrences of the element to add. May be zero, in which
     *     case no change will be made.
     * @return this {@code Builder} object
     * @throws NullPointerException if {@code element} is null
     * @throws IllegalArgumentException if {@code occurrences} is negative, or if this operation
     *     would result in more than {@link Integer#MAX_VALUE} occurrences of the element
     */
    @CanIgnoreReturnValue
    public Builder<E> addCopies(E element, int occurrences) {
      contents.add(checkNotNull(element), occurrences);
      return this;
    }

    
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
  • count – the desired count of the element in this multiset
Throws:
Returns:this Builder object
/**
     * 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
     * @param count the desired count of the element in this multiset
     * @return this {@code Builder} object
     * @throws NullPointerException if {@code element} is null
     * @throws IllegalArgumentException if {@code count} is negative
     */
    @CanIgnoreReturnValue
    public Builder<E> setCount(E element, int count) {
      contents.setCount(checkNotNull(element), count);
      return this;
    }

    
Adds each element of elements to the ImmutableMultiset. 
Params:
  • elements – the Iterable to add to the ImmutableMultiset
Throws:
Returns:this Builder object
/**
     * Adds each element of {@code elements} to the {@code ImmutableMultiset}.
     *
     * @param elements the {@code Iterable} to add to the {@code ImmutableMultiset}
     * @return this {@code Builder} object
     * @throws NullPointerException if {@code elements} is null or contains a null element
     */
    @CanIgnoreReturnValue
    @Override
    public Builder<E> addAll(Iterable<? extends E> elements) {
      if (elements instanceof Multiset) {
        Multiset<? extends E> multiset = Multisets.cast(elements);
        multiset.forEachEntry((e, n) -> contents.add(checkNotNull(e), n));
      } else {
        super.addAll(elements);
      }
      return this;
    }

    
Adds each element of elements to the ImmutableMultiset. 
Params:
  • elements – the elements to add to the ImmutableMultiset
Throws:
Returns:this Builder object
/**
     * Adds each element of {@code elements} to the {@code ImmutableMultiset}.
     *
     * @param elements the elements to add to the {@code ImmutableMultiset}
     * @return this {@code Builder} object
     * @throws NullPointerException if {@code elements} is null or contains a null element
     */
    @CanIgnoreReturnValue
    @Override
    public Builder<E> addAll(Iterator<? extends E> elements) {
      super.addAll(elements);
      return this;
    }

    
Returns a newly-created ImmutableMultiset based on the contents of the 
Builder. 
/**
     * Returns a newly-created {@code ImmutableMultiset} based on the contents of the {@code
     * Builder}.
     */
    @Override
    public ImmutableMultiset<E> build() {
      return copyOf(contents);
    }

    @VisibleForTesting
    ImmutableMultiset<E> buildJdkBacked() {
      if (contents.isEmpty()) {
        return of();
      }
      return JdkBackedImmutableMultiset.create(contents.entrySet());
    }
  }

  @WeakOuter
  static final class ElementSet<E> extends ImmutableSet.Indexed<E> {
    private final List<Entry<E>> entries;
    private final Multiset<E> delegate;

    ElementSet(List<Entry<E>> entries, Multiset<E> delegate) {
      this.entries = entries;
      this.delegate = delegate;
    }

    @Override
    E get(int index) {
      return entries.get(index).getElement();
    }

    @Override
    public boolean contains(@Nullable Object object) {
      return delegate.contains(object);
    }

    @Override
    boolean isPartialView() {
      return true;
    }

    @Override
    public int size() {
      return entries.size();
    }
  }

  static final class SerializedForm implements Serializable {
    final Object[] elements;
    final int[] counts;

    SerializedForm(Multiset<?> multiset) {
      int distinct = multiset.entrySet().size();
      elements = new Object[distinct];
      counts = new int[distinct];
      int i = 0;
      for (Entry<?> entry : multiset.entrySet()) {
        elements[i] = entry.getElement();
        counts[i] = entry.getCount();
        i++;
      }
    }

    Object readResolve() {
      LinkedHashMultiset<Object> multiset = LinkedHashMultiset.create(elements.length);
      for (int i = 0; i < elements.length; i++) {
        multiset.add(elements[i], counts[i]);
      }
      return ImmutableMultiset.copyOf(multiset);
    }

    private static final long serialVersionUID = 0;
  }
}