Convert the values to time and subtract them.
You can do this in Oracle or in java as you like most.
Here is an example on how to do it in oracle (it returns seconds between two dates)
select
(TO_DATE('10:11:12', 'HH24:MI:SS') - TO_DATE('10:11:10', 'HH24:MI:SS')) *60*60*24
from dual
Instead if you need the string in HH24:MI:SS format you can do the following, converting the seconds to a date and then formatting it to desired format:
select
TO_CHAR(TO_DATE(
(TO_DATE('10:11:12','HH24:MI:SS') - TO_DATE('10:11:10','HH24:MI:SS')) *60*60*24
, 'SSSSS')
, 'HH24:MI:SS')
from dual
Note that if 10:11:12 is not saved as a VARCHAR but as a DATE in oracle the query is simplified as follow:
select
TO_CHAR(TO_DATE(duration_1 - duration_2) *60*60*24, 'SSSSS'), 'HH24:MI:SS')
from dual