COMPUTER 2430 Object Oriented Programming and Data Structures I

Inheritance and Polymorphism Prog 6

Liskov Substitution Principle (LSP) In object-oriented programming, the Liskov substitution principle is a particular definition of subtype that was introduced by Barbara Liskov in a 1987 conference keynote address entitled Data abstraction and hierarchy Liskov's notion of "subtype" is based on the notion of substitutability

Liskov Substitution Principle (LSP) The Liskov substitution principle is closely related to the design by contract methodology, leading to some restrictions on how contracts can interact with inheritance: Preconditions cannot be strengthened in a subclass. Require no more! Postconditions cannot be weakened in a subclass. Promise no less! A function using a class hierarchy violating the principle uses a reference to a base class, yet must have knowledge of the subclasses. Such a function violates the open/closed principle because it must be modified whenever a new derivative of the base class is created.

Liskov Substitution Principle (LSP) Two paraphrases that capture the essence A child can be used wherever a parent is expected For every overridden method in a child class: Require no more Promise no less It will be on Test 3 and the final

public class Animal { private String _id; private int numLegs; private float weight; public void grow() . . . } public class Frog extends Animal @Override // weight determines numLegs

public class AnimalList { private Animal[] list; private int count; public boolean add( Animal animal ) . . . } // Can FrogList be a sub-class of AnimalList? public class FrogList extends AnimalList // Can a fish be added to the list? @Override

public class AnimalList { private Animal[] list; private int count; // Any animal can be added to the lis. public boolean add( Animal animal ) . . . } public class FrogList extends AnimalList // Require More! @Override public boolean add( Frog frog )

public class AnimalList { private Animal[] list; private int count; // animal will be added to the list if not full public boolean add( Animal animal ) . . . } public class FrogList extends AnimalList // Promise Less! @Override // do not add if not Frog

public class BagOfFruit { private Fruit items[];   public void add ( Fruit x ) ... } public class BagOfApple extends BagOfFruit private Apple items[]; // How to override method add? Is a BagOfApple a BagOfFruit? Should BagOfApple be a subclass of BagOfFruit? NO!

Don't Overdo Inheritance Is your inheritance good ? the notion of "is-a“ LSP Circle can't inherit from Ellipse even though it "is-a” have to restrict overridden 2D "resize" BagOfApples is not a BagOfFruit restrict overridden add or allow bananas in the apple bag

Quiz 6