I've seen some of this symbols, but I cannot find anything strange with it,
double d = 5D;
float f = 3.0F;
What does the D and F behind 5 exactly means?
Means that these numbers are doubles and floats, respectively. Assume you have
void foo(int x); void foo(float x); void foo(double x);
and then you call
the compiler might be stumped. That's why you can say
5.0 to specify the type.