question: private void makeTree ( int nstep ) { for ( Enumeration < Body > e = bodiesRev ( ) ; e . hasMoreElements ( ) ; ) { Body q = e . nextElement ( ) ; if ( q . mass != 0.0 ) { q . expandBox ( this , nstep ) ; MathVector xqic = intcoord ( q . pos ) ; if ( root == null ) { root = q ; } else { root = root . loadTree ( q , xqic , Node . IMAX > > 1 , this ) ; } } } root . hackcofm ( ) ; }
target:  Initialize the tree structure for hack force calculation.