  The analysis in the present paper is based on the most known concept
introduced by the brilliant physicist Alexander Andreev: Andreev bound states
in a normal metal sandwiched between two superconductors. The paper presents
results of direct calculations of {\em ab initio} expressions for the currents
in a long ballistic SNS junction. The expressions are expanded in $1/L$ ($L$ is
the thickness of the normal layer). The main contribution $\propto 1/L$ to the
current agrees with the results obtained in the past, but the analysis suggests
a new physical picture of the charge transport through the junction free from
the problem with the charge conservation law. The saw-tooth current-phase
relation at $T=0$ directly follows from the Galilean invariance of the
Bogolyubov-de Gennes equations proved in the paper. The proof is valid for any
variation of the energy gap in space if the Andreev reflection is the only
scattering process. The respective roles of the contributions of bound and
continuum states to the current are clarified. They depend on the junction
dimensionality.
