Chair of Software Engineering OOSC Summer Semester 2004 1 Object-Oriented Software Construction Bertrand Meyer.
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OOSC Summer Semester 2004
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Chair of Software Engineering
Object-Oriented Software Construction
Bertrand Meyer
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Lecture 8: Inheritance
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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
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Example hierarchy
AIRCRAFT*
PLANE*
COPTER*
BOEING AIRBUS
A_320B_747B_737
B_747_400
* deferred+ effected++ redefined
lower_landing_gear*
lower_landing_gear++
lower_landing_gear+
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(After Barry W. Boehm)
Requirements Design Code Development Acceptance Operationtest test
SMALL PROJECTS
LARGE PROJECTS
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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.
TEACHER STUDENT
TEACHING_ASSISTANT
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The teaching assistant example
This is in fact a case of repeated inheritance:
TEACHER STUDENT
TEACHING_ASSISTANT
UNIVERSITY_MEMBER
id
????
????
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Common examples of multiple inheritance
Combining separate abstractions: Restaurant, train car Calculator, watch Plane, asset
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Multiple inheritance: Combining abstractions
COMPARABLE NUMERIC
STRING COMPLEX
INTEGER
REAL
DOUBLE
* *
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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
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Multiple inheritance: Composite figures
A composite figure
Simple figures
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Defining the notion of composite figure
FIGURELIST
[FIGURE]
COMPOSITE_FIGURE
*display*
hiderotatemove…
countputremove…
display+
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Defining the notion of composite figure through multiple inheritance
FIGURELIST
[FIGURE]
COMPOSITE_FIGURE
*
OPEN_ FIGURE
CLOSED_ FIGURE
SEGMENT POLYLINE
POLYGON ELLIPSE
RECTANGLE
SQUARE
CIRCLE TRIANGLE
…
perimeter+
perimeter*
perimeter++
perimeter++
perimeter++
perimeter+
diagonal …
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A composite figure as a list
start
item
forth after
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Composite figures
class COMPOSITE_FIGURE inherit
FIGUREredefine display, move, rotate, ... end
LIST [FIGURE]
feature
display is-- Display each constituent figure in turn.
dofrom
start
untilafter
loopitem.displayforth
endend
... Similarly for move, rotate etc. ...
end
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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.
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Name clashes under multiple inheritance
A
C
foofoo
B
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Resolving name clashes
A
C
foofoo
B
rename foo as fog rename foo as zoo
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Resolving name clashes (cont’d)
class C inherit
Arename
foo as fogend
Brename
foo as zooend
feature...
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Results of renaming
a1: Ab1: Bc1: C...c1.fogc1.zooa1.foob1.foo
Invalid:a1.fog, a1.zoo, b1.zoo, b1.fog, c1.foo
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Another application of renaming
Provide locally better adapted terminology.
Example: child (TREE); subwindow (WINDOW).
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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
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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-purpose feature move for class FIGURE?
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Deferred classes
deferred class FIGURE feature
move (v: VECTOR) isdeferredend
rotate (a: ANGLE; p: POINT) isdeferredend
... display, hide, ...
end
Not permitted:
create f ...
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Example hierarchy
FIGURE*
OPEN_ FIGURE
*CLOSED_ FIGURE
*
SEGMENT POLYLINE POLYGON ELLIPSE
CIRCLE
RECTANGLETRIANGLE
SQUARE
extent*barycenter*…
display*rotate*…
perimeter*
perimeter+perimeter+
perimeter++diagonal
......
perimeter++perimeter++
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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).
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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).
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Table variants
TABLE*
SEQUENTIAL_TABLE
*
LINKED_TABLE
+ARRAY_TABLE
+FILE_ TABLE
+
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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?
dofrom
start until
after or else equal (item, x) loop
forthendResult := not after
end
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SEQUENTIAL_TABLE (cont’d)
forth is-- Move cursor to the next position.
requirenot after
deferredensure
position = old position + 1end
start is-- Move cursor to the first position.
deferredensure
empty or else position = 1end
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SEQUENTIAL_TABLE (end)
position: INTEGER is deferred end
... empty, found, after, ...
invariant
0 <= positionposition <= size + 1empty implies (after or before)
end
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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*
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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*
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Implementation variants
Array
Linked list
File
start forth after item (x)
c := first_cell
rewind
i := 1
c := c.right
i := i + 1
read end_of_file
c := Void
f
c.item
i > count t [i]
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End of lecture 8
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