How can we create an empty varriable for datatype double?

Answer 1

If you want a double to be null , use Wrapper class Double.

double a; // a = 0.0
Double b; // b = null

https://www.javatpoint.com/wrapper-class-in-java

Answer 2

I dont understand what is your question, but you can create a variable without initialize that:
double a;

This was your question?

Answer 3

not for local variable, then you have to initialize before use it
double d = 0;
double d2= 0.0;