Constructing a D type flip-flop from JK type flip-flops
As per wikipedia, all you need to do is connect K to the inverse of J. 

Doing this effectively removes the possibility of 00 and 11 inputs. If the J is treated as D then:


D = 1 sets Q
D = 0 resets Q


This behaviour is then identical to the D flip-flop.

For this reason the JK flip-flop is called the universal flip-flop, because you can construct all other flip-flop types with it.