HLine paintOn() method (lines 47,48)
for ( int i = 0; i < length; i++ ){ s.paintAt( x+i , y, paintChar ); }
Counts from i = 0 to i = length-1, executing whats in the body each time
i=0: ask Screen s to put paintChar at
(x,y)
i=1: ask Screen s to put paintChar at
(x+1,y)
i=2: ask Screen s to put paintChar at
(x+2,y)
and so on
at (x+length-1,y)