public void test_lower_null() throws Exception {
NavigableMap map = tm.subMap(objArray[100].toString(), true,
objArray[100].toString(), false);
assertNull(map.ceilingKey(objArray[100].toString()));
assertNull(map.floorKey(objArray[100].toString()));
assertNull(map.lowerKey(objArray[100].toString()));
assertNull(map.higherKey(objArray[100].toString()));
assertNull(map.ceilingKey(objArray[111].toString()));
assertNull(map.floorKey(objArray[111].toString()));
assertNull(map.lowerKey(objArray[111].toString()));
assertNull(map.higherKey(objArray[111].toString()));