Java Database Connectivity (JDBC)

Is neccesary to use commit()..... when you use

stmt.executeUpdate(sentence);

and the sentence is by example:

sentence = "delete from table_X where ELEMENT1 = 'hi'"; ????

This is because...i did a servlet in which i send a sentence (the sentence could be insert or delete).....and apparently it is working very well....i can see that the rows are added or removed in the DB table...but i am a little confused...because in a paper says "modifications in tuples are temporarily stored in the db system. They become permanent only after the statement commit has been issued"

and i am not calling commit in my servlet.....

Thanks in advance,
Mary