001/*
002 * Copyright (C) 2009 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import static com.google.common.base.Preconditions.checkArgument;
020import static com.google.common.base.Preconditions.checkElementIndex;
021import static com.google.common.base.Preconditions.checkNotNull;
022
023import com.google.common.annotations.Beta;
024import com.google.common.annotations.GwtCompatible;
025import com.google.common.annotations.GwtIncompatible;
026import com.google.common.base.Objects;
027import com.google.common.collect.Maps.IteratorBasedAbstractMap;
028import com.google.errorprone.annotations.CanIgnoreReturnValue;
029import com.google.j2objc.annotations.WeakOuter;
030import java.io.Serializable;
031import java.lang.reflect.Array;
032import java.util.Arrays;
033import java.util.Collection;
034import java.util.Iterator;
035import java.util.Map;
036import java.util.Map.Entry;
037import java.util.Set;
038import javax.annotation.Nullable;
039
040/**
041 * Fixed-size {@link Table} implementation backed by a two-dimensional array.
042 *
043 * <p>The allowed row and column keys must be supplied when the table is
044 * created. The table always contains a mapping for every row key / column pair.
045 * The value corresponding to a given row and column is null unless another
046 * value is provided.
047 *
048 * <p>The table's size is constant: the product of the number of supplied row
049 * keys and the number of supplied column keys. The {@code remove} and {@code
050 * clear} methods are not supported by the table or its views. The {@link
051 * #erase} and {@link #eraseAll} methods may be used instead.
052 *
053 * <p>The ordering of the row and column keys provided when the table is
054 * constructed determines the iteration ordering across rows and columns in the
055 * table's views. None of the view iterators support {@link Iterator#remove}.
056 * If the table is modified after an iterator is created, the iterator remains
057 * valid.
058 *
059 * <p>This class requires less memory than the {@link HashBasedTable} and {@link
060 * TreeBasedTable} implementations, except when the table is sparse.
061 *
062 * <p>Null row keys or column keys are not permitted.
063 *
064 * <p>This class provides methods involving the underlying array structure,
065 * where the array indices correspond to the position of a row or column in the
066 * lists of allowed keys and values. See the {@link #at}, {@link #set}, {@link
067 * #toArray}, {@link #rowKeyList}, and {@link #columnKeyList} methods for more
068 * details.
069 *
070 * <p>Note that this implementation is not synchronized. If multiple threads
071 * access the same cell of an {@code ArrayTable} concurrently and one of the
072 * threads modifies its value, there is no guarantee that the new value will be
073 * fully visible to the other threads. To guarantee that modifications are
074 * visible, synchronize access to the table. Unlike other {@code Table}
075 * implementations, synchronization is unnecessary between a thread that writes
076 * to one cell and a thread that reads from another.
077 *
078 * <p>See the Guava User Guide article on <a href=
079 * "https://github.com/google/guava/wiki/NewCollectionTypesExplained#table">
080 * {@code Table}</a>.
081 *
082 * @author Jared Levy
083 * @since 10.0
084 */
085@Beta
086@GwtCompatible(emulated = true)
087public final class ArrayTable<R, C, V> extends AbstractTable<R, C, V> implements Serializable {
088
089  /**
090   * Creates an empty {@code ArrayTable}.
091   *
092   * @param rowKeys row keys that may be stored in the generated table
093   * @param columnKeys column keys that may be stored in the generated table
094   * @throws NullPointerException if any of the provided keys is null
095   * @throws IllegalArgumentException if {@code rowKeys} or {@code columnKeys}
096   *     contains duplicates or is empty
097   */
098  public static <R, C, V> ArrayTable<R, C, V> create(
099      Iterable<? extends R> rowKeys, Iterable<? extends C> columnKeys) {
100    return new ArrayTable<R, C, V>(rowKeys, columnKeys);
101  }
102
103  /*
104   * TODO(jlevy): Add factory methods taking an Enum class, instead of an
105   * iterable, to specify the allowed row keys and/or column keys. Note that
106   * custom serialization logic is needed to support different enum sizes during
107   * serialization and deserialization.
108   */
109
110  /**
111   * Creates an {@code ArrayTable} with the mappings in the provided table.
112   *
113   * <p>If {@code table} includes a mapping with row key {@code r} and a
114   * separate mapping with column key {@code c}, the returned table contains a
115   * mapping with row key {@code r} and column key {@code c}. If that row key /
116   * column key pair in not in {@code table}, the pair maps to {@code null} in
117   * the generated table.
118   *
119   * <p>The returned table allows subsequent {@code put} calls with the row keys
120   * in {@code table.rowKeySet()} and the column keys in {@code
121   * table.columnKeySet()}. Calling {@link #put} with other keys leads to an
122   * {@code IllegalArgumentException}.
123   *
124   * <p>The ordering of {@code table.rowKeySet()} and {@code
125   * table.columnKeySet()} determines the row and column iteration ordering of
126   * the returned table.
127   *
128   * @throws NullPointerException if {@code table} has a null key
129   * @throws IllegalArgumentException if the provided table is empty
130   */
131  public static <R, C, V> ArrayTable<R, C, V> create(Table<R, C, V> table) {
132    return (table instanceof ArrayTable<?, ?, ?>)
133        ? new ArrayTable<R, C, V>((ArrayTable<R, C, V>) table)
134        : new ArrayTable<R, C, V>(table);
135  }
136
137  private final ImmutableList<R> rowList;
138  private final ImmutableList<C> columnList;
139
140  // TODO(jlevy): Add getters returning rowKeyToIndex and columnKeyToIndex?
141  private final ImmutableMap<R, Integer> rowKeyToIndex;
142  private final ImmutableMap<C, Integer> columnKeyToIndex;
143  private final V[][] array;
144
145  private ArrayTable(Iterable<? extends R> rowKeys, Iterable<? extends C> columnKeys) {
146    this.rowList = ImmutableList.copyOf(rowKeys);
147    this.columnList = ImmutableList.copyOf(columnKeys);
148    checkArgument(!rowList.isEmpty());
149    checkArgument(!columnList.isEmpty());
150
151    /*
152     * TODO(jlevy): Support empty rowKeys or columnKeys? If we do, when
153     * columnKeys is empty but rowKeys isn't, the table is empty but
154     * containsRow() can return true and rowKeySet() isn't empty.
155     */
156    rowKeyToIndex = Maps.indexMap(rowList);
157    columnKeyToIndex = Maps.indexMap(columnList);
158
159    @SuppressWarnings("unchecked")
160    V[][] tmpArray = (V[][]) new Object[rowList.size()][columnList.size()];
161    array = tmpArray;
162    // Necessary because in GWT the arrays are initialized with "undefined" instead of null.
163    eraseAll();
164  }
165
166  private ArrayTable(Table<R, C, V> table) {
167    this(table.rowKeySet(), table.columnKeySet());
168    putAll(table);
169  }
170
171  private ArrayTable(ArrayTable<R, C, V> table) {
172    rowList = table.rowList;
173    columnList = table.columnList;
174    rowKeyToIndex = table.rowKeyToIndex;
175    columnKeyToIndex = table.columnKeyToIndex;
176    @SuppressWarnings("unchecked")
177    V[][] copy = (V[][]) new Object[rowList.size()][columnList.size()];
178    array = copy;
179    // Necessary because in GWT the arrays are initialized with "undefined" instead of null.
180    eraseAll();
181    for (int i = 0; i < rowList.size(); i++) {
182      System.arraycopy(table.array[i], 0, copy[i], 0, table.array[i].length);
183    }
184  }
185
186  private abstract static class ArrayMap<K, V> extends IteratorBasedAbstractMap<K, V> {
187    private final ImmutableMap<K, Integer> keyIndex;
188
189    private ArrayMap(ImmutableMap<K, Integer> keyIndex) {
190      this.keyIndex = keyIndex;
191    }
192
193    @Override
194    public Set<K> keySet() {
195      return keyIndex.keySet();
196    }
197
198    K getKey(int index) {
199      return keyIndex.keySet().asList().get(index);
200    }
201
202    abstract String getKeyRole();
203
204    @Nullable
205    abstract V getValue(int index);
206
207    @Nullable
208    abstract V setValue(int index, V newValue);
209
210    @Override
211    public int size() {
212      return keyIndex.size();
213    }
214
215    @Override
216    public boolean isEmpty() {
217      return keyIndex.isEmpty();
218    }
219
220    @Override
221    Iterator<Entry<K, V>> entryIterator() {
222      return new AbstractIndexedListIterator<Entry<K, V>>(size()) {
223        @Override
224        protected Entry<K, V> get(final int index) {
225          return new AbstractMapEntry<K, V>() {
226            @Override
227            public K getKey() {
228              return ArrayMap.this.getKey(index);
229            }
230
231            @Override
232            public V getValue() {
233              return ArrayMap.this.getValue(index);
234            }
235
236            @Override
237            public V setValue(V value) {
238              return ArrayMap.this.setValue(index, value);
239            }
240          };
241        }
242      };
243    }
244
245    // TODO(lowasser): consider an optimized values() implementation
246
247    @Override
248    public boolean containsKey(@Nullable Object key) {
249      return keyIndex.containsKey(key);
250    }
251
252    @Override
253    public V get(@Nullable Object key) {
254      Integer index = keyIndex.get(key);
255      if (index == null) {
256        return null;
257      } else {
258        return getValue(index);
259      }
260    }
261
262    @Override
263    public V put(K key, V value) {
264      Integer index = keyIndex.get(key);
265      if (index == null) {
266        throw new IllegalArgumentException(
267            getKeyRole() + " " + key + " not in " + keyIndex.keySet());
268      }
269      return setValue(index, value);
270    }
271
272    @Override
273    public V remove(Object key) {
274      throw new UnsupportedOperationException();
275    }
276
277    @Override
278    public void clear() {
279      throw new UnsupportedOperationException();
280    }
281  }
282
283  /**
284   * Returns, as an immutable list, the row keys provided when the table was
285   * constructed, including those that are mapped to null values only.
286   */
287  public ImmutableList<R> rowKeyList() {
288    return rowList;
289  }
290
291  /**
292   * Returns, as an immutable list, the column keys provided when the table was
293   * constructed, including those that are mapped to null values only.
294   */
295  public ImmutableList<C> columnKeyList() {
296    return columnList;
297  }
298
299  /**
300   * Returns the value corresponding to the specified row and column indices.
301   * The same value is returned by {@code
302   * get(rowKeyList().get(rowIndex), columnKeyList().get(columnIndex))}, but
303   * this method runs more quickly.
304   *
305   * @param rowIndex position of the row key in {@link #rowKeyList()}
306   * @param columnIndex position of the row key in {@link #columnKeyList()}
307   * @return the value with the specified row and column
308   * @throws IndexOutOfBoundsException if either index is negative, {@code
309   *     rowIndex} is greater then or equal to the number of allowed row keys,
310   *     or {@code columnIndex} is greater then or equal to the number of
311   *     allowed column keys
312   */
313  public V at(int rowIndex, int columnIndex) {
314    // In GWT array access never throws IndexOutOfBoundsException.
315    checkElementIndex(rowIndex, rowList.size());
316    checkElementIndex(columnIndex, columnList.size());
317    return array[rowIndex][columnIndex];
318  }
319
320  /**
321   * Associates {@code value} with the specified row and column indices. The
322   * logic {@code
323   * put(rowKeyList().get(rowIndex), columnKeyList().get(columnIndex), value)}
324   * has the same behavior, but this method runs more quickly.
325   *
326   * @param rowIndex position of the row key in {@link #rowKeyList()}
327   * @param columnIndex position of the row key in {@link #columnKeyList()}
328   * @param value value to store in the table
329   * @return the previous value with the specified row and column
330   * @throws IndexOutOfBoundsException if either index is negative, {@code
331   *     rowIndex} is greater then or equal to the number of allowed row keys,
332   *     or {@code columnIndex} is greater then or equal to the number of
333   *     allowed column keys
334   */
335  @CanIgnoreReturnValue
336  public V set(int rowIndex, int columnIndex, @Nullable V value) {
337    // In GWT array access never throws IndexOutOfBoundsException.
338    checkElementIndex(rowIndex, rowList.size());
339    checkElementIndex(columnIndex, columnList.size());
340    V oldValue = array[rowIndex][columnIndex];
341    array[rowIndex][columnIndex] = value;
342    return oldValue;
343  }
344
345  /**
346   * Returns a two-dimensional array with the table contents. The row and column
347   * indices correspond to the positions of the row and column in the iterables
348   * provided during table construction. If the table lacks a mapping for a
349   * given row and column, the corresponding array element is null.
350   *
351   * <p>Subsequent table changes will not modify the array, and vice versa.
352   *
353   * @param valueClass class of values stored in the returned array
354   */
355  @GwtIncompatible // reflection
356  public V[][] toArray(Class<V> valueClass) {
357    @SuppressWarnings("unchecked") // TODO: safe?
358    V[][] copy = (V[][]) Array.newInstance(valueClass, rowList.size(), columnList.size());
359    for (int i = 0; i < rowList.size(); i++) {
360      System.arraycopy(array[i], 0, copy[i], 0, array[i].length);
361    }
362    return copy;
363  }
364
365  /**
366   * Not supported. Use {@link #eraseAll} instead.
367   *
368   * @throws UnsupportedOperationException always
369   * @deprecated Use {@link #eraseAll}
370   */
371  @Override
372  @Deprecated
373  public void clear() {
374    throw new UnsupportedOperationException();
375  }
376
377  /**
378   * Associates the value {@code null} with every pair of allowed row and column
379   * keys.
380   */
381  public void eraseAll() {
382    for (V[] row : array) {
383      Arrays.fill(row, null);
384    }
385  }
386
387  /**
388   * Returns {@code true} if the provided keys are among the keys provided when
389   * the table was constructed.
390   */
391  @Override
392  public boolean contains(@Nullable Object rowKey, @Nullable Object columnKey) {
393    return containsRow(rowKey) && containsColumn(columnKey);
394  }
395
396  /**
397   * Returns {@code true} if the provided column key is among the column keys
398   * provided when the table was constructed.
399   */
400  @Override
401  public boolean containsColumn(@Nullable Object columnKey) {
402    return columnKeyToIndex.containsKey(columnKey);
403  }
404
405  /**
406   * Returns {@code true} if the provided row key is among the row keys
407   * provided when the table was constructed.
408   */
409  @Override
410  public boolean containsRow(@Nullable Object rowKey) {
411    return rowKeyToIndex.containsKey(rowKey);
412  }
413
414  @Override
415  public boolean containsValue(@Nullable Object value) {
416    for (V[] row : array) {
417      for (V element : row) {
418        if (Objects.equal(value, element)) {
419          return true;
420        }
421      }
422    }
423    return false;
424  }
425
426  @Override
427  public V get(@Nullable Object rowKey, @Nullable Object columnKey) {
428    Integer rowIndex = rowKeyToIndex.get(rowKey);
429    Integer columnIndex = columnKeyToIndex.get(columnKey);
430    return (rowIndex == null || columnIndex == null) ? null : at(rowIndex, columnIndex);
431  }
432
433  /**
434   * Always returns {@code false}.
435   */
436  @Override
437  public boolean isEmpty() {
438    return false;
439  }
440
441  /**
442   * {@inheritDoc}
443   *
444   * @throws IllegalArgumentException if {@code rowKey} is not in {@link
445   *     #rowKeySet()} or {@code columnKey} is not in {@link #columnKeySet()}.
446   */
447  @CanIgnoreReturnValue
448  @Override
449  public V put(R rowKey, C columnKey, @Nullable V value) {
450    checkNotNull(rowKey);
451    checkNotNull(columnKey);
452    Integer rowIndex = rowKeyToIndex.get(rowKey);
453    checkArgument(rowIndex != null, "Row %s not in %s", rowKey, rowList);
454    Integer columnIndex = columnKeyToIndex.get(columnKey);
455    checkArgument(columnIndex != null, "Column %s not in %s", columnKey, columnList);
456    return set(rowIndex, columnIndex, value);
457  }
458
459  /*
460   * TODO(jlevy): Consider creating a merge() method, similar to putAll() but
461   * copying non-null values only.
462   */
463
464  /**
465   * {@inheritDoc}
466   *
467   * <p>If {@code table} is an {@code ArrayTable}, its null values will be
468   * stored in this table, possibly replacing values that were previously
469   * non-null.
470   *
471   * @throws NullPointerException if {@code table} has a null key
472   * @throws IllegalArgumentException if any of the provided table's row keys or
473   *     column keys is not in {@link #rowKeySet()} or {@link #columnKeySet()}
474   */
475  @Override
476  public void putAll(Table<? extends R, ? extends C, ? extends V> table) {
477    super.putAll(table);
478  }
479
480  /**
481   * Not supported. Use {@link #erase} instead.
482   *
483   * @throws UnsupportedOperationException always
484   * @deprecated Use {@link #erase}
485   */
486  @CanIgnoreReturnValue
487  @Override
488  @Deprecated
489  public V remove(Object rowKey, Object columnKey) {
490    throw new UnsupportedOperationException();
491  }
492
493  /**
494   * Associates the value {@code null} with the specified keys, assuming both
495   * keys are valid. If either key is null or isn't among the keys provided
496   * during construction, this method has no effect.
497   *
498   * <p>This method is equivalent to {@code put(rowKey, columnKey, null)} when
499   * both provided keys are valid.
500   *
501   * @param rowKey row key of mapping to be erased
502   * @param columnKey column key of mapping to be erased
503   * @return the value previously associated with the keys, or {@code null} if
504   *     no mapping existed for the keys
505   */
506  @CanIgnoreReturnValue
507  public V erase(@Nullable Object rowKey, @Nullable Object columnKey) {
508    Integer rowIndex = rowKeyToIndex.get(rowKey);
509    Integer columnIndex = columnKeyToIndex.get(columnKey);
510    if (rowIndex == null || columnIndex == null) {
511      return null;
512    }
513    return set(rowIndex, columnIndex, null);
514  }
515
516  // TODO(jlevy): Add eraseRow and eraseColumn methods?
517
518  @Override
519  public int size() {
520    return rowList.size() * columnList.size();
521  }
522
523  /**
524   * Returns an unmodifiable set of all row key / column key / value
525   * triplets. Changes to the table will update the returned set.
526   *
527   * <p>The returned set's iterator traverses the mappings with the first row
528   * key, the mappings with the second row key, and so on.
529   *
530   * <p>The value in the returned cells may change if the table subsequently
531   * changes.
532   *
533   * @return set of table cells consisting of row key / column key / value
534   *     triplets
535   */
536  @Override
537  public Set<Cell<R, C, V>> cellSet() {
538    return super.cellSet();
539  }
540
541  @Override
542  Iterator<Cell<R, C, V>> cellIterator() {
543    return new AbstractIndexedListIterator<Cell<R, C, V>>(size()) {
544      @Override
545      protected Cell<R, C, V> get(final int index) {
546        return new Tables.AbstractCell<R, C, V>() {
547          final int rowIndex = index / columnList.size();
548          final int columnIndex = index % columnList.size();
549
550          @Override
551          public R getRowKey() {
552            return rowList.get(rowIndex);
553          }
554
555          @Override
556          public C getColumnKey() {
557            return columnList.get(columnIndex);
558          }
559
560          @Override
561          public V getValue() {
562            return at(rowIndex, columnIndex);
563          }
564        };
565      }
566    };
567  }
568
569  /**
570   * Returns a view of all mappings that have the given column key. If the
571   * column key isn't in {@link #columnKeySet()}, an empty immutable map is
572   * returned.
573   *
574   * <p>Otherwise, for each row key in {@link #rowKeySet()}, the returned map
575   * associates the row key with the corresponding value in the table. Changes
576   * to the returned map will update the underlying table, and vice versa.
577   *
578   * @param columnKey key of column to search for in the table
579   * @return the corresponding map from row keys to values
580   */
581  @Override
582  public Map<R, V> column(C columnKey) {
583    checkNotNull(columnKey);
584    Integer columnIndex = columnKeyToIndex.get(columnKey);
585    return (columnIndex == null) ? ImmutableMap.<R, V>of() : new Column(columnIndex);
586  }
587
588  private class Column extends ArrayMap<R, V> {
589    final int columnIndex;
590
591    Column(int columnIndex) {
592      super(rowKeyToIndex);
593      this.columnIndex = columnIndex;
594    }
595
596    @Override
597    String getKeyRole() {
598      return "Row";
599    }
600
601    @Override
602    V getValue(int index) {
603      return at(index, columnIndex);
604    }
605
606    @Override
607    V setValue(int index, V newValue) {
608      return set(index, columnIndex, newValue);
609    }
610  }
611
612  /**
613   * Returns an immutable set of the valid column keys, including those that
614   * are associated with null values only.
615   *
616   * @return immutable set of column keys
617   */
618  @Override
619  public ImmutableSet<C> columnKeySet() {
620    return columnKeyToIndex.keySet();
621  }
622
623  private transient ColumnMap columnMap;
624
625  @Override
626  public Map<C, Map<R, V>> columnMap() {
627    ColumnMap map = columnMap;
628    return (map == null) ? columnMap = new ColumnMap() : map;
629  }
630
631  @WeakOuter
632  private class ColumnMap extends ArrayMap<C, Map<R, V>> {
633    private ColumnMap() {
634      super(columnKeyToIndex);
635    }
636
637    @Override
638    String getKeyRole() {
639      return "Column";
640    }
641
642    @Override
643    Map<R, V> getValue(int index) {
644      return new Column(index);
645    }
646
647    @Override
648    Map<R, V> setValue(int index, Map<R, V> newValue) {
649      throw new UnsupportedOperationException();
650    }
651
652    @Override
653    public Map<R, V> put(C key, Map<R, V> value) {
654      throw new UnsupportedOperationException();
655    }
656  }
657
658  /**
659   * Returns a view of all mappings that have the given row key. If the
660   * row key isn't in {@link #rowKeySet()}, an empty immutable map is
661   * returned.
662   *
663   * <p>Otherwise, for each column key in {@link #columnKeySet()}, the returned
664   * map associates the column key with the corresponding value in the
665   * table. Changes to the returned map will update the underlying table, and
666   * vice versa.
667   *
668   * @param rowKey key of row to search for in the table
669   * @return the corresponding map from column keys to values
670   */
671  @Override
672  public Map<C, V> row(R rowKey) {
673    checkNotNull(rowKey);
674    Integer rowIndex = rowKeyToIndex.get(rowKey);
675    return (rowIndex == null) ? ImmutableMap.<C, V>of() : new Row(rowIndex);
676  }
677
678  private class Row extends ArrayMap<C, V> {
679    final int rowIndex;
680
681    Row(int rowIndex) {
682      super(columnKeyToIndex);
683      this.rowIndex = rowIndex;
684    }
685
686    @Override
687    String getKeyRole() {
688      return "Column";
689    }
690
691    @Override
692    V getValue(int index) {
693      return at(rowIndex, index);
694    }
695
696    @Override
697    V setValue(int index, V newValue) {
698      return set(rowIndex, index, newValue);
699    }
700  }
701
702  /**
703   * Returns an immutable set of the valid row keys, including those that are
704   * associated with null values only.
705   *
706   * @return immutable set of row keys
707   */
708  @Override
709  public ImmutableSet<R> rowKeySet() {
710    return rowKeyToIndex.keySet();
711  }
712
713  private transient RowMap rowMap;
714
715  @Override
716  public Map<R, Map<C, V>> rowMap() {
717    RowMap map = rowMap;
718    return (map == null) ? rowMap = new RowMap() : map;
719  }
720
721  @WeakOuter
722  private class RowMap extends ArrayMap<R, Map<C, V>> {
723    private RowMap() {
724      super(rowKeyToIndex);
725    }
726
727    @Override
728    String getKeyRole() {
729      return "Row";
730    }
731
732    @Override
733    Map<C, V> getValue(int index) {
734      return new Row(index);
735    }
736
737    @Override
738    Map<C, V> setValue(int index, Map<C, V> newValue) {
739      throw new UnsupportedOperationException();
740    }
741
742    @Override
743    public Map<C, V> put(R key, Map<C, V> value) {
744      throw new UnsupportedOperationException();
745    }
746  }
747
748  /**
749   * Returns an unmodifiable collection of all values, which may contain
750   * duplicates. Changes to the table will update the returned collection.
751   *
752   * <p>The returned collection's iterator traverses the values of the first row
753   * key, the values of the second row key, and so on.
754   *
755   * @return collection of values
756   */
757  @Override
758  public Collection<V> values() {
759    return super.values();
760  }
761
762  private static final long serialVersionUID = 0;
763}