  We examine the negative energy solution in Klein-Gordon equation in terms of
the number of field components. A scalar field has only one component, and
there is no freedom left for an anti-particle since the Klein-Gordon equation
failed to take the negative energy solution into account. This is in contrast
to the Dirac equation which has four components of fields. It is shown that the
current density for a real scalar field is always zero if the field is
classical, but infinite if the field is quantized. This suggests that the
condition of a real field must be physically too strong.
