changes:

added m_A and m_B sliders
added Force vs time graph
add floor
add forces vectors
add force equations which is already provided by F1= getA(x1,x2,m1) and F2 = getA(x2,x1,m2)
by Fu Kwun: // awesome codes
public double getA (double xa,double xb,double m) {
  if(Math.abs(xa-xb)    if(xa    else return k*(L-(xa-xb))/m;
  }
  return 0.;
}