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Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. Guava has only one code dependency - javax.annotation, per the JSR-305 spec.

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/*
 * Copyright (C) 2011 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 java.util.Collection;
import java.util.Comparator;

import javax.annotation.Nullable;

/**
 * An empty immutable sorted multiset.
 *
 * @author Louis Wasserman
 */
@SuppressWarnings("serial") // Uses writeReplace, not default serialization
final class EmptyImmutableSortedMultiset extends ImmutableSortedMultiset {
  private final ImmutableSortedSet elementSet;

  EmptyImmutableSortedMultiset(Comparator comparator) {
    this.elementSet = ImmutableSortedSet.emptySet(comparator);
  }

  @Override
  public Entry firstEntry() {
    return null;
  }

  @Override
  public Entry lastEntry() {
    return null;
  }

  @Override
  public int count(@Nullable Object element) {
    return 0;
  }

  @Override
  public boolean containsAll(Collection targets) {
    return targets.isEmpty();
  }

  @Override
  public int size() {
    return 0;
  }

  @Override
  public ImmutableSortedSet elementSet() {
    return elementSet;
  }

  @Override
  Entry getEntry(int index) {
    throw new AssertionError("should never be called");
  }

  @Override
  public ImmutableSortedMultiset headMultiset(E upperBound, BoundType boundType) {
    checkNotNull(upperBound);
    checkNotNull(boundType);
    return this;
  }

  @Override
  public ImmutableSortedMultiset tailMultiset(E lowerBound, BoundType boundType) {
    checkNotNull(lowerBound);
    checkNotNull(boundType);
    return this;
  }

  @Override
  public UnmodifiableIterator iterator() {
    return Iterators.emptyIterator();
  }

  @Override
  public boolean equals(@Nullable Object object) {
    if (object instanceof Multiset) {
      Multiset other = (Multiset) object;
      return other.isEmpty();
    }
    return false;
  }

  @Override
  boolean isPartialView() {
    return false;
  }

  @Override
  int copyIntoArray(Object[] dst, int offset) {
    return offset;
  }

  @Override
  public ImmutableList asList() {
    return ImmutableList.of();
  }
}