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Inheritance

Explore the concepts of inheritance and polymorphism in object-oriented programming, including redefining, renaming, and deferred classes. Understand the use of inheritance for capturing high-level classes and defining architecture components.

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Inheritance

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  1. Inheritance

  2. Inheritance: classRECTANGLE inheritPOLYGON • RECTANGLE includes features of POLYGON • r: RECTANGLE; p: POLYGON • Polymorphism • p := r -- legal assignment • Dynamic binding • p := r • p.perimeter -- perimeter of RECTANGLE is called Multiple Inheritance: classTA inheritTEACHER STUDENT

  3. Special syntax (1) Redefine: classRECTANGLE inheritPOLYGON redefine perimeter end feature perimeter : REAL is … end … end • Redefinition allows the feature clause of RECTANGLE to contain a new version of method perimeter

  4. Special syntax (2) Rename: classTA inherit TEACHER rename id as teacher_idend STUDENT rename id as student_idend feature … end • Several or none features can be renamed for each parent

  5. Example class TA inherit TEACHER -- first inherited class rename -- renaming features of TEACHER id as teacher_id, mark as get_mark redefine – redefining methods of TEACHER get_mark, -- need to use last defined name add_factor end STUDENT – second inherited class rename -- renaming of features of STUDENT id as student_id, mark as set_mark end …

  6. Abstract class deferred classDIGIT feature value : INTEGER -- current value lowest: INTEGER is-- lowest legal value deferred end end • A deferred class is a class that has at least one deferred feature • There is no need in redefinition for features inherited in deferred form

  7. The role of deferred class • Capturing high-level classes, with common behaviors. • Defining the components of an architecture during system design, without commitment to a final implementation. • Describing domain-specific concepts in analysis and modeling.

  8. Example deferred class STACK [G] feature -- Access count :INTEGERis-- Number of inserted elements deferred end item: Gis-- Last pushed element require not_empty: not empty deferred end;

  9. Example (cont) feature -- Status report empty :BOOLEAN is-- Is stack empty do Result :=(count = 0) end full :BOOLEAN is-- Is stack full deferred end feature – Element change put(x : G ) is-- Push x on top require not_full: not full deferred ensure not_empty: not empty pushed_in_top: item = x one_more: count = old count + 1 end

  10. Assertion Redeclaration rule In the redeclared version of a routine, it is not permitted to use a require or ensure clause. Instead you may: • Introduce a new condition with require else, for booleanorwith the original precondition. • Introduce a new condition with ensure then, for booleanandwith the original postcondition. In the absence of such a clause, the original assertions are retained.

  11. Example (1) deferred class A … foo (x :INTEGER ) is require r1 deferred … end end; class B inherit A … foo (x :INTEGER ) is require r2 do … end end; • The actual requirement is

  12. Example (2) deferred class A … foo (x :INTEGER ) is deferred … ensure e1 end end; class B inherit A … foo (x :INTEGER ) is do… ensure e2 end end; • The actual promise is

  13. Invariants Redeclaration rule • The invariant property of class is the boolean and of the assertions appearing in its invariant clause and of the invariant properties of its parents if any.

  14. Example class A … invariant i1 end; class B inherit A … invariant i2 end; • The actual invariant is

  15. Example (1) deferred class COURSE feature -- mark treatment average: INTEGER-- average of marks num_marks : INTEGER-- marks number add_mark(mark : INTEGER) is require legal_new_mark: mark >= 0 and mark <= 100 deferred ensure num_marks = old num_marks + 1 end

  16. Example (2) calc_factor (new_avg : INTEGER) : INTEGER is require legal_new_avg: new_avg >= 0 and new_avg <= 100 deferred ensure legal_calc_factor: average + Result = new_avg end invariant legal_avarage: average >= 0 and average <= 100 legar_num_marks: num_marks >= 0 end;

  17. Example (3) class LOGIC inherit COURSE creation feature add_mark(mark : INTEGER) is require legal_new_mark: mark >= 0 and mark <= 55 do if mark = 0 then average := mark else average := (average*num_marks + mark)//(num_marks + 1) end num_marks := num_marks + 1 end

  18. Example (4) calc_factor (new_avg : INTEGER) : INTEGER is require legal_new_avg: new_avg >= 0 and new_avg <= 55 do if average > 50 then Result := new_avg - average else Result := 0 end ensure legal_factor: Result <= 0 end invariant legal_avarage: average >= 0 and average <= 55 end;

  19. Example (5) class MAIN logic: LOGIC -- logic.add_mark(56) -- for now average >= 55 so LOGIC invariant violation logic.add_mark(46) logic.add_mark(56) io.put_line("Average is " + logic.average.out) -- average = 51 < 55 -- io.put_line("factor for average 55 is " + logic.calc_factor(55).out) -- factor = 4 so LOGIC ensure legal_factor violation io.put_line("factor for average 50 is " + logic.calc_factor(50).out) -- factor -1 logic.add_mark(40) -- average 47 -- io.put_line("factor for average 50 is " + logic.calc_factor(50).out) -- factor = 0 so COURSE ensure legal_calc_factor violation

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