Csci 490 / Engr 596 Special Topics / Special Projects Software Design and Scala Programming Spring Semester 2010 Lecture Notes

Replacement and Refinement This is a set of slides to accompany sections 16.1-16.2 of Timothy Budd's book An Introduction to Object-Oriented Programming, Third Edition (Addison-Wesley, 2002) Created: 14 August 2004 Revised: 6 April 2010

Replacement and Refinement What happens when child classes have methods with the same names as parent? Two general models describing this situation Replacement "American" school of OOP – child method replaces parent method Refinement "Scandinavian" school of OOP – behavior of parent and child classes are merged to form new behavior If replacement used, then some mechanism usually provided for refinement If refinement default, then replacement not possible 1

Preserving is-a Replacement makes preserving principle of substitutability difficult No guarantee that child class will do anything similar to the parent Other methods that depend upon behavior from parent class may fail Great havoc can ensue if programmer is not careful 2

Documenting Replacement or Refinement Some languages require parent class to indicate replacement is permitted e.g., virtual in C++ Some languages require child class to indicate replacement e.g., override in Object Pascal and Scala Some languages do not automatically document replacement or refinement e.g., Java, Smalltalk 3

Replacement in Java and Scala Constructors use refinement By default, all other methods can be overridden and replaced No keyword to denote overriding in Java; override used in Scala virtual in C++ , override in Object Pascal Binding of "message" (call) to method done based on dynamic type of receiver object Can even replace data fields in Java, although these are static and not dynamic (need to check Scala) Keyword final on method prohibits replacement Keyword final on class prohibits subclassing altogether 4

Refinement Method in parent and method in child merged to form new behavior Parent class method always executed Users guaranteed at least behavior of parent Performed automatically in some languages E.g., Simula, Beta Performed by directly invoking parent method from within child method in other languages E.g., Java, C++, Scala 5

Refinement in Simula and Beta Execute code from parent first -- when INNER statement encountered, execute child code Parent classes wrap around child almost reverse of American school languages Guarantees functionality provided by parent classes 6

Refinement in Java Default constructor of superclass called implicitly as first action by subclass constructor Subclass constructor may explicitly call specific superclass constructor as super(...) in first statement Subclass method may explicitly call overridden superclass method using classname super anywhere 7

Refinement in Java Example public class CardPile { public CardPile (int xl, int yl) { x = xl; y = yl; thePile = new Stack(); } public void addCard (Card aCard) thePile.push(aCard); ... 8

Refinement in Java Example (continued) class DiscardPile extends CardPile { public DiscardPile (int x, int y) { super (x, y); } public void addCard (Card aCard) if (! aCard.faceUp()) aCard.flip(); super.addCard(aCard); ... 9

Acknowledgement This work was supported by a grant from Acxiom Corporation titled “The Acxiom Laboratory for Software Architecture and Component Engineering (ALSACE).” 10