Include decimal point in floating point values?

I know this is pretty basic, but is there any difference between:

	double x = 3.0;
	double y = x/4.0;

and

	double x = 3;
	double y = x/4;
?

In other words, if I don't include a decimal point when using a whole 
number in a type double expression, as in the second case, does the 
compiler assume it's an integer and do a type cast when  evaluating it? 
  If so, does that mean it's more efficient to always include the 
decimal point in this situation?

Thanks for the explanation.  -Pat