Replacement and Refinement

1. Csci 490 / Engr 596Special Topics / Special ProjectsSoftware Design and Scala ProgrammingSpring Semester 2010Lecture Notes

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

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