Lecture 14: More Inheritance & Genericity

Transcription

1 Chair of Software Engineering Einführung in die Programmierung Introduction to Programming Prof. Dr. Bertrand Meyer October 2006 February 2007 MP1 Lecture 14: More Inheritance & Genericity 2 Genericity: summary 1 Type extension mechanism Reconciles flexibility with type safety Enables us to have parameterized classes Useful for container data structures: lists, arrays, trees, Type now a bit more general than class Intro. to Programming, lecture 14: More Inheritance & Genericity 3 1

2 (Reminder) A generic class Formal generic parameter class LIST [G ] feature ext (x : G ) is last : G is To use the class: obtain a generic derivation, e.g. cities: LIST [CITY ] Actual generic parameter Intro. to Programming, lecture 14: More Inheritance & Genericity 4 (Reminder) Using generic derivations cities : LIST [CITY ] people : LIST [PERSON] c : CITY p : PERSON cities.ext (c) people.ext (p) STATIC TYPING: The compiler will reject: people.ext (c) cities.ext (p) c := cities.last c.add_tram_line (Line8 ) Intro. to Programming, lecture 14: More Inheritance & Genericity 5 (Reminder) Using genericity LIST [CITY ] LIST [LIST [CITY ]] A type is no longer exactly the same thing as a class! (But every type remains based on a class.) Intro. to Programming, lecture 14: More Inheritance & Genericity 6 2

3 Genericity: summary 1 Type extension mechanism Reconciles flexibility with type safety Enables us to have parameterized classes Useful for container data structures: lists, arrays, trees, Type now a bit more general than class Intro. to Programming, lecture 14: More Inheritance & Genericity 7 Definition: Type We use types to declare entities, as in x : SOME_TYPE With the mechanisms defined so far, a type is one of: A non-generic class e.g. METRO_STATION A generic derivation, i.e. the name of a class followed by a list of types, the actual generic parameters, in brackets e.g. LIST [METRO_STATION] LIST [ARRAY [METRO_STATION]] Intro. to Programming, lecture 14: More Inheritance & Genericity 8 Genericity + inheritance 1: Constrained genericity class VECTOR [G ] feature plus alias "+" (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 Other features, in particular item to access an element Intro. to Programming, lecture 14: More Inheritance & Genericity 9 3

4 Adding two vectors u + v = w 2 i1 a + b = c Intro. to Programming, lecture 14: More Inheritance & Genericity 10 Constrained genericity Body of plus alias "+": create Result.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 Intro. to Programming, lecture 14: More Inheritance & Genericity 11 The solution Declare class VECTOR as class VECTOR [G > NUMERIC ] feature The rest as before Class NUMERIC (from the Kernel Library) provides features plus alias "+", minus alias "-"and so on. Intro. to Programming, lecture 14: More Inheritance & Genericity 12 4

5 Improving the solution Make VECTOR itself a descant of NUMERIC, effecting the corresponding features: class VECTOR [G > NUMERIC ] inherit NUMERIC feature Rest as before, including infix "+" Then it is possible to define v : VECTOR [INTEGER ] vv : VECTOR [VECTOR [INTEGER ]] vvv : VECTOR [VECTOR [VECTOR [INTEGER ]]] Intro. to Programming, lecture 14: More Inheritance & Genericity 13 Exting the basic notion of class Inheritance SET_OF_ CARS SET_OF_ PERSONS Genericity LIST_OF_ CITIES LIST_OF_ CARS LIST_OF_ PERSONS LINKED_LIST_ OF_CITIES LINKED_LIST_ OF_CARS Intro. to Programming, lecture 14: More Inheritance & Genericity 14 Genericity + Inheritance 2: Polymorphic data structure class LIST [G ] feature last : G is ext (x : G) is fl (SQUARE) fl: LIST [FIGURE] r: RECTANGLE (TRIANGLE) s: SQUARE t: TRIANGLE p: POLYGON (POLYGON) (RECTANGLE) fl.ext (p); fl.ext (t); fl.ext (s); fl.ext (r) fl.last.display Intro. to Programming, lecture 14: More Inheritance & Genericity 15 5

6 Example hierarchy * OPEN_ FIGURE extent* barycenter* * FIGURE display* rotate perimeter* * CLOSED_ FIGURE * deferred + effective ++ redefined perimeter+ perimeter+ + + SEGMENT POLYLINE POLYGON ELLIPSE TRIANGLE diagonal perimeter++ RECTANGLE side1 side2 perimeter++ SQUARE CIRCLE perimeter++ Intro. to Programming, lecture 14: More Inheritance & Genericity 16 Forcing a type: the problem fl.store ("FILE_NAME") -- Two years later: fl := retrieved ("FILE_NAME") See next x := fl.last -- [1] print (x.diagonal ) -- [2] What s wrong with this? If x is declared of type RECTANGLE, [1] is invalid. If x is declared of type FIGURE, [2] is invalid. Intro. to Programming, lecture 14: More Inheritance & Genericity 17 The solution: Assignment attempt fl.store ("FILE_NAME") -- Two years later: fl?= retrieved ("FILE_NAME") x := fl.last -- [1] print (x.diagonal) -- [2] But: If x is declared of type RECTANGLE, [1] is invalid. If x is declared of type FIGURE, [2] is invalid. Intro. to Programming, lecture 14: More Inheritance & Genericity 18 6

7 Assignment attempt f: FIGURE r: RECTANGLE fl.retrieve ("FILE_NAME") f := fl.last r?= f if r /= Void then print (r.diagonal) else print ("Too bad.") Intro. to Programming, lecture 14: More Inheritance & Genericity 19 Assignment attempt x?= y with x : A If y is attached to an object whose type conforms to A, perform normal reference assignment. Otherwise, make x void. Intro. to Programming, lecture 14: More Inheritance & Genericity 20 The role of deferred classes Express abstract concepts indepently of implementation Express common elements of various implementations Terminology: Effective = non-deferred (i.e. fully implemented) Intro. to Programming, lecture 14: More Inheritance & Genericity 21 7

8 A deferred feature In e.g. LIST: forth is require not after deferred ensure index = old index + 1 Intro. to Programming, lecture 14: More Inheritance & Genericity 22 Mixing deferred and effective features In the same class Effective! search (x: G) is -- Move to first position after current -- where x appears, or after if none. do from until after or else item = x loop forth Deferred! Programs with holes Intro. to Programming, lecture 14: More Inheritance & Genericity 23 Don t call us, we ll call you! A powerful form of reuse: The reusable element defines a general scheme Specific cases fill in the holes in that scheme Combine reuse with adaptation Intro. to Programming, lecture 14: More Inheritance & Genericity 24 8

9 Applications of deferred classes Analysis and design, top-down Taxonomy Capturing common behaviors Intro. to Programming, lecture 14: More Inheritance & Genericity 25 Deferred classes in EiffelBase CONTAINER * BOX * COLLECTION * TRAVERSABLE * FINITE * INFINITE * BAG * SET * HIERARCHICAL * LINEAR * BOUNDED * UNBOUNDED * COUNTABLE * RESIZABLE * INDEXABLE * CURSOR_ TABLE * ACTIVE * INTEGER_ * BILINEAR * INTERVAL * DISPENSER * SEQUENCE * STRUCTURE ARRAY STRING HASH_TABLE STACK * QUEUE * *: deferred Intro. to Programming, lecture 14: More Inheritance & Genericity 26 Java and.net solution Single inheritance only for classes Multiple inheritance from interfaces An interface is like a fully deferred class, with no implementations (do clauses), no attributes (and also no contracts) Intro. to Programming, lecture 14: More Inheritance & Genericity 27 9

10 Inheritance and assertions a1: A a1.r () C A r is require α ensure β Correct call: if a1.α then a1.r () else B r is require γ ensure δ Intro. to Programming, lecture 14: More Inheritance & Genericity 28 Assertion redeclaration rule Redefined version may not have require or ensure. May have nothing (assertions kept by default), or require else new_pre ensure then new_post Resulting assertions are: original_precondition or new_pre original_postcondition and new_post Intro. to Programming, lecture 14: More Inheritance & Genericity 29 Invariant accumulation Every class inherits all the invariant clauses of its parents. These clauses are conceptually and -ed. Intro. to Programming, lecture 14: More Inheritance & Genericity 30 10

11 What we have seen Intro. to Programming, lecture 14: More Inheritance & Genericity 31 What we have seen Genericity Combining inheritance with genericity Constrained genericity Forcing a type: the assignment attempt The relationship of contracts with inheritance Deferred classes and features Intro. to Programming, lecture 14: More Inheritance & Genericity 32 End of lecture 14 Intro. to Programming, lecture 14: More Inheritance & Genericity 33 11