@Override
  public String getString(int rowIndex, int columnIndex) {
    if (columnIndex != 0) {
      throw new IllegalArgumentException("Column index must always be 0 for aggregation result sets");
    }
    return _groupByResults.get(rowIndex).get("value").asText();
  }
@Test(expectedExceptions = IllegalArgumentException.class)
  public void testGetStringExceptionState() {
    // Run the test
    final String result = _groupByResultSetUnderTest.getString(0, 1);
  }