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Object-Oriented Software Construction

Object-Oriented Software Construction. Bertrand Meyer. Lecture 6: Genericity. Genericity. Unconstrained LIST [G] e.g. LIST [INTEGER], LIST [PERSON] Constrained HASH_TABLE [G ―> HASHABLE] VECTOR [G ―> NUMERIC]. SET _OF_STOCKS. LIST_OF_ COMPANIES. LIST_OF_. PERSONS.

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Object-Oriented Software Construction

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  1. Object-Oriented Software Construction Bertrand Meyer Lecture 6: Genericity OOSC - Summer Semester 2005

  2. Genericity Unconstrained LIST [G] e.g. LIST [INTEGER], LIST [PERSON] Constrained HASH_TABLE [G―> HASHABLE] VECTOR [G―> NUMERIC] OOSC - Summer Semester 2005

  3. SET_OF_STOCKS LIST_OF_COMPANIES LIST_OF_ PERSONS LINKED_LIST_OF_STOCKS Extending the basic notion of class Inheritance Abstraction Genericity Type parameterization Type parameterization LIST_OF_STOCKS Specialization OOSC - Summer Semester 2005

  4. SET_OF_PERSONS SET_OF_STOCKS LIST_OF_COMPANIES LIST_OF_ PERSONS LINKED_LIST_OF_COMPANIES LINKED_LIST_OF_STOCKS Extending the basic notion of class Inheritance Genericity LIST_OF_STOCKS OOSC - Summer Semester 2005

  5. Genericity: Ensuring type safety How can we define consistent “container” data structures, e.g. list of accounts, list of points? Dubious use of a container data structure: c : COMPANY a : PERSONcompanies: LIST ... people: LIST ... companies.extend (c) people.extend (a) c := people.last c.change_recommendation (Buy) OOSC - Summer Semester 2005

  6. Possible approaches • Wait until run time; if types don’t match, trigger a run-time failure. (Smalltalk) • Cast to a universal type, such as “pointer to void” in C. • Duplicate code, manually or with help of macro processor. • Parameterize the class, giving an explicit name G to the type of container elements. This is the Eiffel approach. OOSC - Summer Semester 2005

  7. Formal generic parameter Actual generic parameter A generic class classLIST [G] featureextend (x: G)is... last: Gis... end To use the class: obtain a generic derivation, e.g. companies: LIST [COMPANY] OOSC - Summer Semester 2005

  8. Conformance rule • B [U] conforms to A [T]if and only if B is a descendant of A and U conforms to T. OOSC - Summer Semester 2005

  9. Using generic derivations companies: LIST [COMPANY] people: LIST [PERSON] c: COMPANY p: PERSON ... companies.extend (c) people.extend (p) c := companies.last c. change_recommendation (Buy) ... OOSC - Summer Semester 2005

  10. Genericity and static typing Compiler will reject people.extend (c)companies.extend (p) To define more flexible data structures (e.g. stack of figures): use inheritance, polymorphism and dynamic binding. OOSC - Summer Semester 2005

  11. Typing in an O-O context An object-oriented language is statically typed if and only if it is possible to write a “static checker” which, if it accepts a system, guarantees that at run time, for any execution of a feature call x.f, the object attached to x (if any) will have at least one feature corresponding to f. OOSC - Summer Semester 2005

  12. Constrained genericity class VECTOR [G ] feature infix "+" (other: VECTOR [G]): VECTOR [G] is -- Sum of current vector and other require lower = other.lower upper = other.upper local a, b, c: G do ... See next ... end ... Other features ... end OOSC - Summer Semester 2005

  13. = i c a b + Adding two vectors u v = w + OOSC - Summer Semester 2005

  14. Constrained genericity Body of infix "+": createResult.make (lower, upper) from i := lower until i > upper loop a := item (i) b := other.item (i) c := a + b-- Requires “+” operation on G! Result.put (c, i) i := i + 1 end OOSC - Summer Semester 2005

  15. The solution Declare class VECTOR as classVECTOR [G –> NUMERIC] feature ... The rest as before ... end Class NUMERIC (from the Kernel Library) provides features infix"+", infix"*" and so on. OOSC - Summer Semester 2005

  16. Improving the solution Make VECTOR itself a descendant of NUMERIC, effecting the corresponding features: classVECTOR [G –> NUMERIC] inherit NUMERIC feature ... The rest as before, including infix "+"... end Then it is possible to define v : VECTOR [INTEGER] vv : VECTOR [VECTOR [INTEGER]] vvv : VECTOR [VECTOR [VECTOR [INTEGER]]] OOSC - Summer Semester 2005

  17. A generic library class: Arrays Using arrays: a: ARRAY [REAL] ...createa.make (1, 300) a.put (3.5, 25) x := a.item (i) -- Alternatively: x := a @ i -- Using the function infix "@" Also: ARRAY2 [G] etc. OOSC - Summer Semester 2005

  18. Class ARRAY (1) classARRAY [G] create make feature lower, upper: INTEGER count: INTEGER make (min: INTEGER, max: INTEGER) is-- Allocate array with bounds min and max.do ... end OOSC - Summer Semester 2005

  19. Class ARRAY (2) item, infix "@" (i: INTEGER): Gis-- Entry of index irequirelower <= i i <= upperdo ... endput (v: G; i: INTEGER) is-- Assign the value of v to the entry of index i.requirelower <= i i <= upperdo ... end invariantcount = upper – lower + 1 end OOSC - Summer Semester 2005

  20. End of lecture 6 OOSC - Summer Semester 2005

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