Every instance of a subclass is an instance of the superclass, but not vice-versa. As a result, inheritance allows substitutability : the ability to substitute a child class object where a parent class object is expected.
an Academic
is an instance of a Staff
, but a Staff
is not necessarily an instance of an Academic
. i.e. wherever an object of the superclass
is expected, it can be substituted by an object of any of its subclasses.
The following code is valid because an AcademicStaff
object is substitutable as a Staff
object.
Staff staff = new AcademicStaff (); // OK
But the following code is not valid because staff
is declared as a Staff
type and therefore its value may or may not be of type AcademicStaff
, which is the type expected by variable academicStaff
.
Staff staff;
...
AcademicStaff academicStaff = staff; // Not OK