Jack is a common English male given name. It is a nickname for the name John. Although it may appear at first glance to be derived from the French Jacques—which is cognate with the names James and Jacob—native speakers of English would almost certainly associate the name "Jack" with "John," and rarely with "Jacob" and probably never with "James." Rather, it is believed that "Jack" is the result of adding the French diminutive suffix "-kin" (much like the German "-chen") to the name "Jean," which is the French form of the English "John", or "Jan," which is a shortening of the German "Johann." The result would have pronounced as "ZHANG-kin," with the "n" being nasalized, and thus later omitted by Anglophones without much difficulty.
The etymology of the name "Jack" and its association to the name "John" is addressed in the book Why Do Clocks Run Clockwise?, the second book in the Imponderables book series by David Feldman.
OMG MY NAME IS OSCAR AND I THINK THAT JACK OR JOHN OR SOMeTHING IS A HOTTIE