How do you know if an explicit object casting is needed (CoreJava)

Question :How do you know if an explicit object casting is needed? (CoreJava)
Answer :If you assign a superclass object to a variable of a subclass's data type,
you need to do explicit casting. For example:
Object a; Customer b; b = (Customer) a;
When you assign a subclass to a variable having a supeclass type, the
casting is performed automatically.