Integer

The type to use for an int that might be null is Integer.

void main() {
Integer i = null;
System.out.println(i);
i = 5;
System.out.println(i);
}

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

void main() {
Integer i = null;
System.out.println(i * 5);
}