The Scofield Bible Commentary, by Cyrus Ingerson Scofield, , at sacred-texts.com
1 Kings (1 Samuel) 16:21
Compare (Sa1 17:55-56).
The order of events is:
(1) David, whose skill on the harp, and valour in the combat with the lion and bear (Sa1 17:34); (Sa1 17:36) were known to "one of the servants" of Saul, was brought to play before the king (Sa1 16:17); (Sa1 16:18).
(2) David returns to Bethlehem (Sa1 17:15).
(3) David is sent to Saul's camp (Sa1 17:17); (Sa1 17:18) and performs his great exploit.
(4) Saul's question (Sa1 17:55); (Sa1 17:56) implies only that he had forgotten the name of David's father -- not remarkable certainly in an oriental king.