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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 com.google.common.annotations.GwtCompatible;

import java.util.Comparator;
import java.util.Iterator;
import java.util.NavigableSet;

import javax.annotation.Nullable;

/**
 * This class provides a skeletal implementation of the {@link SortedMultiset} interface.
 *
 * The {@link #count} and {@link #size} implementations all iterate across the set returned by
 * {@link Multiset#entrySet()}, as do many methods acting on the set returned by
 * {@link #elementSet()}. Override those methods for better performance.
 *
 * @author Louis Wasserman
 */
@GwtCompatible(emulated = true)
abstract class AbstractSortedMultiset extends AbstractMultiset implements SortedMultiset {
  @GwtTransient final Comparator comparator;

  // needed for serialization
  @SuppressWarnings("unchecked")
  AbstractSortedMultiset() {
    this((Comparator) Ordering.natural());
  }

  AbstractSortedMultiset(Comparator comparator) {
    this.comparator = checkNotNull(comparator);
  }

  @Override
  public NavigableSet elementSet() {
    return (NavigableSet) super.elementSet();
  }

  @Override
  NavigableSet createElementSet() {
    return new SortedMultisets.NavigableElementSet(this);
  }

  @Override
  public Comparator comparator() {
    return comparator;
  }

  @Override
  public Entry firstEntry() {
    Iterator> entryIterator = entryIterator();
    return entryIterator.hasNext() ? entryIterator.next() : null;
  }

  @Override
  public Entry lastEntry() {
    Iterator> entryIterator = descendingEntryIterator();
    return entryIterator.hasNext() ? entryIterator.next() : null;
  }

  @Override
  public Entry pollFirstEntry() {
    Iterator> entryIterator = entryIterator();
    if (entryIterator.hasNext()) {
      Entry result = entryIterator.next();
      result = Multisets.immutableEntry(result.getElement(), result.getCount());
      entryIterator.remove();
      return result;
    }
    return null;
  }

  @Override
  public Entry pollLastEntry() {
    Iterator> entryIterator = descendingEntryIterator();
    if (entryIterator.hasNext()) {
      Entry result = entryIterator.next();
      result = Multisets.immutableEntry(result.getElement(), result.getCount());
      entryIterator.remove();
      return result;
    }
    return null;
  }

  @Override
  public SortedMultiset subMultiset(@Nullable E fromElement, BoundType fromBoundType,
      @Nullable E toElement, BoundType toBoundType) {
    // These are checked elsewhere, but NullPointerTester wants them checked eagerly.
    checkNotNull(fromBoundType);
    checkNotNull(toBoundType);
    return tailMultiset(fromElement, fromBoundType).headMultiset(toElement, toBoundType);
  }

  abstract Iterator> descendingEntryIterator();

  Iterator descendingIterator() {
    return Multisets.iteratorImpl(descendingMultiset());
  }

  private transient SortedMultiset descendingMultiset;

  @Override
  public SortedMultiset descendingMultiset() {
    SortedMultiset result = descendingMultiset;
    return (result == null) ? descendingMultiset = createDescendingMultiset() : result;
  }

  SortedMultiset createDescendingMultiset() {
    return new DescendingMultiset() {
      @Override
      SortedMultiset forwardMultiset() {
        return AbstractSortedMultiset.this;
      }

      @Override
      Iterator> entryIterator() {
        return descendingEntryIterator();
      }

      @Override
      public Iterator iterator() {
        return descendingIterator();
      }
    };
  }
}