Traditionally, yes; these days, perhaps not so much.
The author Jack London was originally John London. John F. Kennedy was familiarly known as Jack ("Senator, you're no Jack Kennedy"). The British racing driver John Stewart is far more commonly known as Jackie Stewart. In Patrick O'Brian's naval fiction, Captain John Aubrey is almost always referred to as Jack.
The author Jack London was originally John London. John F. Kennedy was familiarly known as Jack ("Senator, you're no Jack Kennedy"). The British racing driver John Stewart is far more commonly known as Jackie Stewart. In Patrick O'Brian's naval fiction, Captain John Aubrey is almost always referred to as Jack.