  Being stimulated by the recent BESIII observation of a charged charmoniumlike
structure $Z_c(3900)$, in this work we study the distributions of the
$J/\psi\pi^\pm$ and $\pi^+\pi^-$ invariant mass spectra of the $Y(4260)\to
\pi^+\pi^- J/\psi$ decay by the initial-single-pion-emission mechanism, where
the interference effects of the ISPE mechanism with two other decay modes are
also taken into account. The obtained $d\Gamma(Y(4260)\to \pi^+\pi^-
J/\psi)/dm_{J/\psi \pi^\pm}$ and $d\Gamma(Y(4260)\to \pi^+\pi^-
J/\psi)/dm_{\pi^+ \pi^-}$ marvelously agree with the BESIII data to reproduce
the $Z_c(3900)$ structure.
