1. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 1 Advanced Topics in Object Technology Bertrand Meyer

2. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 2 Lecture 9: Inheritance

3. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 3 Typing vs. binding  What do we know about the feature to be called?  Static typing: At least one  Dynamic binding: The right one  Example: my_aircraft.lower_landing_gear

4. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 4 Example hierarchy AIRCRAFT * PLANE * COPTER * BOEINGAIRBUS A_320 B_747B_737 B_747_400 * deferred + effected ++ redefined lower_landing_gear* lower_landing_gear++ lower_landing_gear+

5. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 5 (After Barry W. Boehm) RequirementsDesignCodeDevelopmentAcceptanceOperation test SMALL PROJECTS LARGE PROJECTS

6. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 6 Multiple inheritance  A class may have two or more parents.  What not to use as an elementary example: TEACHING_ASSISTANT inherits from TEACHER and STUDENT. TEACHERSTUDENT TEACHING_ ASSISTANT

7. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 7 The teaching assistant example  This is in fact a case of repeated inheritance: TEACHERSTUDENT TEACHING_ ASSISTANT UNIVERSITY_ PERSON id ?? ????

8. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 8 Common examples of multiple inheritance  Combining separate abstractions:  Restaurant, train car  Calculator, watch  Plane, asset

9. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 9 Multiple inheritance: Combining abstractions COMPARABLENUMERIC STRINGCOMPLEX INTEGER REAL DOUBLE **

10. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 10 Multiple inheritance: Nested windows  ‘‘Graphical’’ features: height, width, change_height, change_width, xpos, ypos, move...  ‘‘Hierarchical’’ features: superwindow, subwindows, change_subwindow, add_subwindow... class WINDOW inherit RECTANGLE TREE [WINDOW] feature... end

11. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 11 Multiple inheritance: Composite figures A composite figure Simple figures

12. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 12 Defining the notion of composite figure FIGURE LIST [FIGURE] COMPOSITE_ FIGURE * display hide rotate move … count put remove …

13. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 13 Defining the notion of composite figure through multiple inheritance FIGURE LIST [FIGURE] COMPOSITE_ FIGURE * OPEN_ FIGURE CLOSED_ FIGURE SEGMENT POLYLINE POLYGON ELLIPSE RECTANGLE SQUARE CIRCLE TRIANGLE … perimeter+ perimeter* perimeter++ perimeter+ diagonal …

14. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 14 A composite figure as a list start item forthafter

15. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 15 Composite figures class COMPOSITE_FIGURE inherit FIGURE redefine display, move, rotate,... end LIST [FIGURE] feature display is -- Display each constituent figure in turn. do from start until after loop item.display forth end end... Similarly for move, rotate etc.... end

16. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 16 Complex figures  Note: a simpler form of procedures display, move etc. can be obtained through the use of iterators.  Exercise: Use agents for that purpose.

17. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 17 Name clashes under multiple inheritance A C foo B

18. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 18 Resolving name clashes A C foo B rename foo as fogrename foo as zoo

19. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 19 Resolving name clashes (cont’d) class C inherit A rename foo as fog end B rename foo as zoo end feature...

20. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 20 Results of renaming a1: A b1: B c1: C... c1.fog c1.zoo a1.foo b1.foo Invalid: a1.fog, a1.zoo, b1.zoo, b1.fog, c1.foo

21. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 21 Another application of renaming  Provide locally better adapted terminology.  Example: child (TREE); subwindow (WINDOW).

22. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 22 The marriage of convenience class ARRAYED_STACK [G] inherit STACK [G] ARRAY [G] feature... end class LINKED_STACK [G] inherit STACK [G] LINKED_LIST [G] feature... end

23. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 23 The need for deferred classes  In the scheme seen earlier: f: FIGURE; c: CIRCLE; p: POLYGON... create c.make (...); create p.make (...)... if... then f := c else f := p end... f.move (...); f.rotate (...); f.display (...);...  How do we ensure that a call such as f.move (...) is valid even though there is no way to implement a general-ee- purpose feature move for class FIGURE?

24. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 24 Deferred classes deferred class FIGURE feature move (v: VECTOR) is deferred end rotate (a: ANGLE; p: POINT) is deferred end... display, hide,... end Not permitted: create f...

25. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 25 Example hierarchy FIGURE * OPEN_ FIGURE * CLOSED_ FIGURE * SEGMENTPOLYLINEPOLYGONELLIPSE CIRCLE RECTANGLETRIANGLE SQUARE extent* barycenter* … display* rotate … perimeter* perimeter+ perimeter++ diagonal... perimeter++

26. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 26 Deferred classes and features  A feature is either deferred or effective.  To effect a inherited feature (deferred in the parent) is to make it effective. No need for redefine clause.  Like a feature, a class is either deferred or effective.  A class is deferred if it has at least one deferred feature (possibly coming from an ancestor) that it does not effect. It is effective otherwise.  A deferred class may not be instantiated.  BUT: A deferred class may have assertions (in particular, a deferred routine may have a precondition and a postcondition, and the class may have a class invariant).

27. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 27 Deferred classes  Compare with Ada-Modula 2-Java interface/body separation:  May contain both deferred and non-deferred elements.  More than one implementation is possible.  Formal semantic specification (assertions).

28. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 28 Table variants TABLE * SEQUENTIAL_ TABLE * LINKED_ TABLE + ARRAY_ TABLE + FILE_ TABLE +

29. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 29 Don’t call us, we’ll call you deferred class SEQUENTIAL_TABLE [G] inherit TABLE [G] feature has (x: G): BOOLEAN is -- Does x appear in table? do from start until after or else equal (item, x) loop forth end Result := not after end

30. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 30 SEQUENTIAL_TABLE (cont’d) forth is require not after deferred ensure position = old position + 1 end start is deferred ensure empty or else position = 1 end

31. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 31 SEQUENTIAL_TABLE (end) position: INTEGER is deferred end... empty, found, after,... invariant 0 <= position position <= size + 1 empty implies (after or before) end

32. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 32 Descendant implementations TABLE * SEQUENTIAL_ TABLE * LINKED_ TABLE + ARRAY_ TABLE + FILE_ TABLE + has+ forth* item* start* forth+ item+ start+ after+ forth+ item+ start+ after+ forth+ item+ start+ after+ after* has*

33. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 33 Descendant implementations (cont’d) SEQUENTIAL_ TABLE * LINKED_ TABLE + ARRAY_ TABLE + FILE_ TABLE + has+ forth* item* start* forth+ item+ start+ after+ forth+ item+ start+ after+ forth+ item+ start+ after+ after*

34. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 34 Implementation variants Array Linked list File startforthafterfound (x) c := first_cell rewind i := 1 c := c.right i := i + 1 readend_of_file c := Void f = x c.item = x i > countt [i] = x

35. Chair of Software Engineering ATOT - Lecture 9, 30 April 2003 35 End of lecture 9