How to know if an explicit object casting is needed in Java?

Question

Java does not have sizeof operator now my question is How to find out how much memory is used by an object in Java?

Answer 1

If we assign a superclass object to a variable of a subclass’s data type, we need to do explicit casting.

For example:
Object a;
Customer b;
b = (Customer) a;

I hope I understood question correctly...