  We study a model for two-species hard-core bosons in one dimension. In this
model, the same-species bosons have a hard-core condition at the same site,
while different-species bosons are allowed to occupy the same site with a local
interaction $U$. At half-filling, by Jordan-Wigner transformation, the model
can be exactly mapped to a fermionic Hubbard model. Due to this correspondence,
the phase transition from superfluid ($U=0$) to Mott insulator ($U>0$) can be
explained by simple one-band theory at half-filling. By using an exact
diagonalization method adopting a modified Lanczos method, we obtain the ground
states as a function of $U$ for the lattice size upto $L=16$. We calculate
directional current-current correlation functions in this model, which indicate
that there are some remaining counter-flow in the Mott insulating region
($U>0$) and co-flow in the charge-density-wave region ($U<0$) for the finite
lattices.
