  In this paper, we present the results on the gravitational collapse of
charged massless scalar field in asymptotically flat spacetime with a perfectly
reflecting wall. Differing from previous works, we study the system in the
double null coordinates, by which we could simulate the system until the black
hole forms with higher precision but less performance time. We investigate the
influence of charge on the black hole formation and the scaling behavior near
the critical collapses. The gapless and gapped critical behaviors for black
hole mass and charge are studied numerically. We find that they satisfy the
scaling laws for critical gravitational collapse but the gapped critical
behavior is different from its AdS counterpart.
