@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
  public void testGetString() {
    // Run the test
    final String result = _groupByResultSetUnderTest.getString(0, 0);

    // Verify the results
    assertEquals("1", result);
  }