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Einführung in Die Programmierung Introduction to Programming

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Einführung in Die Programmierung Introduction to Programming Chair of Software Engineering Einführung in die Programmierung Introduction to Programming Prof. Dr. Bertrand Meyer Lecture 9: Abstraction Topics for today Abstraction, especially fflunctional abstraction The notion of routine The final word on features: all feature categories The Uniform Access ppprinciple Abstraction and client privileges Information hiding 2 Routine: algorithm abstraction To abstract is to capture the essence behind the details and the specifics Implies giving a name to the result In programming: ¾ Data abstraction: class ¾ Algorithm (operational) abstraction: routine 3 A routine is one of the two kinds of feature... ... the other is attribute We have encountered lots of routines already, without the name. 4 A routine r (arg : TYPE ; ...) is -- Header comment require Precondition (boolean expression) do Body (instructions) ensure Postcondition (boolean expression) end 5 Uses of routines Bottom-up: capture existing algorithm, possibly for reuse Top-down: placeholder routines — attractive alternative to pseudddocode. build_ a_ line is create_fancy _line is -- Build imaginary line. -- Create line and fill do -- stations. Paris.disppylay do Metro.highlight -- To be completed create_fancy_line -- BM, 26 Oct 08 end end 6 Two kinds of routine Procedure: d’doesn’t return a result Yields a command CllCalls are itinstruc tions Function: returns a result f (arg : TYPE; ...): RESULT_TYPE is ... (The rest as before) ... Yields a query Calls are expressions 7 Features: the full story A class is characterized by its features Each feature is an operation on the corresponding objects: query or command Features are grouped into categories for readability Class clauses: ¾ Indexing ¾ Inheritance ¾ Creation ¾ Feature (any number) ¾ Invariant Anatomy of a class: Demo 8 Features: the full story Client view Internal view (specification) (implementation) Command Procedure No result Routine Computation Feature Memory Feature Returns result Function Computation Query Memory Attribute 9 Uniform access principle It doesn‘t matter to the client whthhether you lklook up or compute A call such as yoontour_account.blnebalance could use an attribute or a function 10 Uniform Access: an example blbalance = lfdlist_of_deposits.total – lfhdllist_of_withdrawals.total list_of_deposits (()A1) list_of_withdrawals balance list_of_deposits (A2) list_of_withdrawals 11 Uniform Access Principle Expressed more technically: Features should be accessible to clients the same way whether implemented by storage or by computation 12 An object has an interface set x y set_ x set_y 13 An object has an implementation set firstx y set_ x set_y count 14 Information hiding set x y set_ x set_y 15 What clients may do class METRO_ STATION feature x, y: REAL -- Coordinates of metro station size: REAL -- Size of bounding square upper_lftleft: POSITION -- Upper-left position of bounding square adjust_ positions is -- Set positions of bounding square do upper_left . set (x – size/2, y + size/2) ... end end 16 What clients may not do class METRO_ STATION feature x, x, y: REAL -- Coordinates of metro station size: REAL -- Size of bounding square upper_lftleft: POSITION -- Upper-left position of bounding square-- Upperposition of bounding square adjj_pust_positions is ---- Set positions of bounding square do upper_left . x := 3 ... end NOT PERMITTED! end 17 Use procedures: upper_left.set (3, upper_left.y ) upper_left.set_x (3 ) upper_left.move (3, h) 18 Possible client privileges If class A has an attribute att : SOME_TYPE, what may a client class C with att a : A CA do with aatt ? a : A The attribute may be: Read, Read-only Secret restricted write Full write Example: modify x Modify through with move in “set_...” procedure POINT 19 Possible client privileges If class A has an attribute att : SOME_TYPE, what may a client class C with a : A C A a : A att do with aatt ? The attribute may be: Read, Read-only Secret restricted write Full write Modify through Modify through a some_procedure a set_att (v) a att a att permitted in C (for access) invalid 20 Abstraction and client privileges If class A has an attribute att : SOME_TYPE, what may a client class C with att a : A CA do with aatt ? a: A Read access if attribute is exported ¾a.att is an expression. ¾An assignment a.att := v would be syntactically illegal! (It would assign to an expression, like x + y := v.) 21 Applying abstraction principles Beyon d read access: fu ll or restr ic te d write, throug h exported procedures. Full write privileges: set_attribute procedure, e.g. set_temperature (u : REAL) is -- Set temperature value to u. do temperature := u ensure temperature_set: temperature = u end Client will use e.g. x.set_temperature (21.5). 22 Other uses of a setter procedure set_temperature (u : REAL) is -- Set temperature value to u. require not_under_minimum: u >= -273 not_abibove_maximum: u <= 2000 do temperature := u update_database ensure temperature_set: temperature = u end 23 Having it both ways Make it possibblle to call a setter procedure temperature: RE AL assign set_ temperature Then the syntax x.temperature := 21.5 is accepted as a shorthand for x.set_temperature (21.5) Retains contracts etc. 24 Information hiding class A Status of calls in a client with a1: A: feature f ... ¾ a1.f, a1.g: valid in any client g ... ¾ a1 h: invalid everywhere feature {NONE} . (including in A’s own text!) h, i ... feature {B, C} ¾ a1.j: valid only in B, C and their descendants j, k, l ... (not valid in A!) feature {A, B, C} ¾ a1.m: valid in B, C and their descendants, m, n… as well as in A and its descendants end 25 An example of selective export LINKABLE exports its features to LINKED_LIST ¾ Does not export them to the rest of the world ¾ Clients of LINKED_LIST don’ t need to know about LINKABLE cells. count 3 first_element active Haldenegg Central Haupt- bahnhof item right item right item right 26 Exporting selectively These feat ures are selecti vel y exported to LINKED_LIST and its class descendants (and no other classes) LINKABLE [G] feature {LINKED_LIST } put_right (...) is do ... end right: G is do ... end ... end 27 Information hiding Information hiding only applies to use by clients, using dot notation or infix notation, as with a1.f (Qualified calls). Unqualified calls (within class) not subject to information hiding: class A feature {NONE } h is ... do ... end feature f is do ...; h ; ... end end 28 What we have seen The full categorization of features Routines, procedures, functions Uniform access Information hiding Selective exports Setters and getters Eiffel: assig ner commands 29.
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