Why current density of a point charge satisfies $\vec{J}\rm{dV}=q\vec{v}$?

Why current density of a point charge satisfies $\vec{J}\rm{dV}=q\vec{v}$?

I read in a book that if a point charge $q$ at the position $\vec{x}$ is moving with the velocity $\vec{v}=\rm{d}\vec{x}/\rm{d}t$ and if the current density generated by the charge is $\vec{J}$, then ...