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
Famous quotes containing the words challenge the, challenge, premise and/or problem:
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