a typical Jazz bass has two single-coil pickups that in and of themselves are not hum cancelling. these pickups are reverse wound and have reverse polarity from each other (commonly referred to as RWRP or RPRW) - and the beauty is that when both pickups are at an equal volume level they become humcancelling just like a standard P-bass pickup. when you have one pickup with more or less volume, the degree that you hear the EMI is equal to the amount of difference between volumes (less volume difference = quieter, more volume difference = noisier)
contrary to what most guitarists believe, we bassists do not normally play with both pickups at the same volume because more bassists appreciate the subtlety of blending the pickups with differing amounts ... for some songs we favor the neck pickup, and for others we favor the bridge (for example, Jaco and Marcus usually favor the bridge for their solos)
it is also becoming increasingly popular to see a Jazz basses (even authentic Fender Jazz basses!) wired with Volume + Blend + Passive tone
note that if your bridge is grounded, you will not hear hum when playing just a single pickup if your fingers are touching your strings. so if a single-coil pickup's EMI is gastly to you you can either keep your fingers on the strings (like most bassists do, even when playing an open string ... you need to damp off the other strings from ringing)
all the best,
R
