### Question:
TaskSeriesCollection extends AbstractSeriesDataset implements GanttCategoryDataset, Cloneable, PublicCloneable,
                   Serializable { @Override public Comparable getColumnKey(int index) { return this.keys.get(index); } TaskSeriesCollection(); TaskSeries getSeries(Comparable key); TaskSeries getSeries(int series); @Override int getSeriesCount(); @Override Comparable getSeriesKey(int series); @Override int getRowCount(); @Override List getRowKeys(); @Override int getColumnCount(); @Override List<Comparable> getColumnKeys(); @Override Comparable getColumnKey(int index); @Override int getColumnIndex(Comparable columnKey); @Override int getRowIndex(Comparable rowKey); @Override Comparable getRowKey(int index); void add(TaskSeries series); void remove(TaskSeries series); void remove(int series); void removeAll(); @Override Number getValue(Comparable rowKey, Comparable columnKey); @Override Number getValue(int row, int column); @Override Number getStartValue(Comparable rowKey, Comparable columnKey); @Override Number getStartValue(int row, int column); @Override Number getEndValue(Comparable rowKey, Comparable columnKey); @Override Number getEndValue(int row, int column); @Override Number getPercentComplete(int row, int column); @Override Number getPercentComplete(Comparable rowKey, Comparable columnKey); @Override int getSubIntervalCount(int row, int column); @Override int getSubIntervalCount(Comparable rowKey, Comparable columnKey); @Override Number getStartValue(int row, int column, int subinterval); @Override Number getStartValue(Comparable rowKey, Comparable columnKey,
                                int subinterval); @Override Number getEndValue(int row, int column, int subinterval); @Override Number getEndValue(Comparable rowKey, Comparable columnKey,
                              int subinterval); @Override Number getPercentComplete(int row, int column, int subinterval); @Override Number getPercentComplete(Comparable rowKey, Comparable columnKey,
                                     int subinterval); @Override void seriesChanged(SeriesChangeEvent event); @Override boolean equals(Object obj); @Override Object clone();  }### Answer:
@Test public void testGetColumnKey() { TaskSeriesCollection c = createCollection1(); assertEquals("Task 1", c.getColumnKey(0)); assertEquals("Task 2", c.getColumnKey(1)); assertEquals("Task 3", c.getColumnKey(2)); }