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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) 2012 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 java.util.Comparator;
import java.util.Iterator;
import java.util.NavigableSet;
import java.util.Set;

/**
 * A skeleton implementation of a descending multiset.  Only needs
 * {@code forwardMultiset()} and {@code entryIterator()}.
 * 
 * @author Louis Wasserman
 */
@GwtCompatible(emulated = true)
abstract class DescendingMultiset extends ForwardingMultiset
    implements SortedMultiset {
  abstract SortedMultiset forwardMultiset();

  private transient Comparator comparator;

  @Override public Comparator comparator() {
    Comparator result = comparator;
    if (result == null) {
      return comparator =
          Ordering.from(forwardMultiset().comparator()).reverse();
    }
    return result;
  }

  private transient NavigableSet elementSet;

  @Override public NavigableSet elementSet() {
    NavigableSet result = elementSet;
    if (result == null) {
      return elementSet = new SortedMultisets.NavigableElementSet(this);
    }
    return result;
  }

  @Override public Entry pollFirstEntry() {
    return forwardMultiset().pollLastEntry();
  }

  @Override public Entry pollLastEntry() {
    return forwardMultiset().pollFirstEntry();
  }

  @Override public SortedMultiset headMultiset(E toElement,
      BoundType boundType) {
    return forwardMultiset().tailMultiset(toElement, boundType)
        .descendingMultiset();
  }

  @Override public SortedMultiset subMultiset(E fromElement,
      BoundType fromBoundType, E toElement, BoundType toBoundType) {
    return forwardMultiset().subMultiset(toElement, toBoundType, fromElement,
        fromBoundType).descendingMultiset();
  }

  @Override public SortedMultiset tailMultiset(E fromElement,
      BoundType boundType) {
    return forwardMultiset().headMultiset(fromElement, boundType)
        .descendingMultiset();
  }

  @Override protected Multiset delegate() {
    return forwardMultiset();
  }

  @Override public SortedMultiset descendingMultiset() {
    return forwardMultiset();
  }

  @Override public Entry firstEntry() {
    return forwardMultiset().lastEntry();
  }

  @Override public Entry lastEntry() {
    return forwardMultiset().firstEntry();
  }

  abstract Iterator> entryIterator();

  private transient Set> entrySet;

  @Override public Set> entrySet() {
    Set> result = entrySet;
    return (result == null) ? entrySet = createEntrySet() : result;
  }

  Set> createEntrySet() {
    return new Multisets.EntrySet() {
      @Override Multiset multiset() {
        return DescendingMultiset.this;
      }

      @Override public Iterator> iterator() {
        return entryIterator();
      }

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

  @Override public Iterator iterator() {
    return Multisets.iteratorImpl(this);
  }

  @Override public Object[] toArray() {
    return standardToArray();
  }

  @Override public  T[] toArray(T[] array) {
    return standardToArray(array);
  }

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