Double

The type to use for a double that might be null is Double.

void main() {
Double d = null;
System.out.println(d);
d = 3.14;
System.out.println(d);
}

If you try to do any math on a Double which holds null you will get a NullPointerException.

void main() {
Double d = null;
System.out.println(d + 1);
}