private void isUsingConcurrentHashSet(String key) {
    if (index instanceof CompactRangeIndex) {
      assertEquals(
          "Expected concurrent hash set but instanceForKey is "
              + getValuesFromMap(key).getClass().getName(),
          getValuesFromMap(key) instanceof IndexConcurrentHashSet, true);
    } else {
      fail("Should have used CompactRangeIndex");
    }
  }
Tests adding entries to compact range index where there are undefined and null values
for the key
geode-core/src/integrationTest/java/org/apache/geode/cache/query/internal/index/CompactRangeIndexJUnitTest.java