Will add a few other possibilities of playing chords on a bass:
Classic way to play maj7, min7 and dom7 chords on a bass is:
Over 3 strings: play root, 3rd and 7th of chord. Eg. C (15th fret on A) - E (14th on D) - B (16th on G) will give you a maj7 chord.
Change the B to a Bb (15th) and you'll have a dominant7 chord.
Now change the E in the dom7 chord to an Eb (13th) and you have instant Cmin7.
Over 4 strings: root, 7th and 3rd. Eg. E (12th on E) - D (12th on D) and G# (13th on G) will give you an E7 chord.
So playing a jazz II-V-I in the key of C is:
Dm7 (3string method: D-F-C or 17th on A - 15th on D - 17th on G)
G7 (4string method: G-F-B or 15th on E - 15th on D - 16th on G)
Cmaj7 (3 string method: C-E-B or 15th on A - 14th on D - 16th on G)
See how your 3rd and 7ths practically don't move at all... welcome to the wonderfull world of voice leading
Other interesting chords: dim7: actually a dom7 of the chord a half tone down. Eg.: Fdim7 you play E7 but move the root a half step up, giving you: 4string method: F-D-G# (13th/E - 12th/D - 13th/G)
If your hands are big enough you could go trying 4-tone chords and implement tensions (9ths, 13ths, #11s).... scary stuff, and I've just started to get it...
practice, practice, practice... I think it's so important for a bass player to know how harmony works and sounds like. I'm no piano player so I learned myself to play some harmony on the bass which helped me a whole lot!
Hope anyone appreciates this.
Herwig