Replacement and Refinement

1. Engr 691Special Topics in Engineering ScienceSoftware ArchitectureSpring Semester 2004Lecture Notes

2. Replacement and Refinement This Web document was based on a set of slides created by the instructor for use in the Fall 1997 and Spring 1999 offerings of the object-oriented design and programming course. That set was, in turn, based on a set of slides created by Timothy Budd to supplement chapter 11 of his textbook An Introduction to Object-Oriented Programming, Second Edition (Addison-Wesley, 1997)

3. Replacement and Refinement What happens when child classes have methods with the same names as parent? There are 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

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

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) • Some languages do not automatically document replacement or refinement (e.g., Java, Smalltalk)

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

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++)

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

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

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

11. Refinement in Java (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); } ... }