Challenge The Premise of The Problem
While at first glance it may seem obvious that a Circle is-an Ellipse, consider the following alternate representation of essentially the same problem, stated in terms of Java code.
class Person { void walkNorth( int meters ) {...} // No failure or exception allowed void walkEast( int meters ) {...} // No failure or exception allowed }Now, a prisoner is obviously a person. So we could logically create a sub-class:
class Prisoner extends Person { void walkNorth( int meters ) {...} // No failure or exception allowed void walkEast( int meters ) {...} // No failure or exception allowed }Just as obviously, this leads us into trouble, since a prisoner is not free to move an arbitrary distance in any direction, yet the contract of the Person
class states that a Person can.
So, in fact, the class Person
could better be named FreePerson
. If that were the case, then the idea that class Prisoner extends FreePerson
is clearly wrong.
By analogy, then, a Circle, is not an Ellipse, because it lacks the same degrees of freedom as an Ellipse.
This strongly suggests that inheritance should never be used when the sub-class restricts the freedom implicit in the base class, but should only be used when the sub-class adds extra detail to the concept represented by the base class as in 'Monkey' is-a 'Animal'.
Read more about this topic: Circle-ellipse Problem, Possible Solutions
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