Transcript Object-Oriented Analysis and Design

Object-Oriented
Analysis and Design
Lecture 8
State Space, Behavior, and
Type Conformance
Goals
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We’re looking for desirable properties of
classes and class hierarchies.
Violations of these good properties
certainly may compile, but…
Think of these properties as being like
“normal forms” in database theory.
A Class Should
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“…represent a uniform abstraction of
the properties of the individual objects
that belong to the class.”
The keys are state space and behavior.
State Space of a Class
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The state space of a class C is the set of all
permitted states of any object of class C.
The dimensions of a state space are the
coordinates needed to specify the state of an
object.
Example: Intersections at
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Forbes and Morewood
Forbes and Craig
Forbes and Morewood parking lot
Another Example
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A CD player; what are the legal
transitions?
Current
Track
10
9
8
7
6
5
4
3
2
1
“Point” in
state space
0123456…
Seconds Remaining
State Space
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The dimensions of a class may be the same
as its attributes.
But attributes may not be simple types (ints,
etc.) so state may be much more
complicated.
Also, there may be many ways to specify
dimensions, e.g.,
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secondsRemaining & totalLength
percentFinished & totalLength
State Space of a Subclass
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If S is a subclass of T, then the state
space of S must be contained in the
state space of T.
S’s state space is “confined” by T’s.
Ex:
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0
T = Book, attribute numPages
S = ChildrensBook
numPages
600
State Space of a Subclass
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IF S is a subclass of T, then S’s state
space must include at least all the
dimensions of T’s.
S’s state space can “extend” from T’s.
ChildrensBook
Book
0
numPages
600
Behavior of Classes &
Subclasses
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A class’s behavior is the set of allowed
transitions within its state space.
A subclass can have its behavior
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Confined (can’t add 150 pages to a
ChildrensBook), and
Extended (we can add illustrations).
Class Invariants
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A class invariant is a condition that must be
satisfied by every object of the class.
Ex: Triangle, w/ side lengths a, b, and c.
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a+bc
b+ca
c+ab
c
b
a
In the 3-D state space, not all points are
legitimate.
In good designs, invariants are inherited
(e.g., isosceles triangles, right triangles, etc.).
Pre and Post Conditions
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These are for methods.
Think of a contract between a user and
supplier:
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Precondition: must be true (of the object) when
the method is invoked. If not, the method has no
obligation to perform correctly.
Postcondition: must be true (of the object) when
the method finishes. If not, the method is broken.
Responsibilities
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The object sending the message is obliged to check
the precondition.
If it’s satisfied, the object being called must
guarantee the postcondition will be true after
execution
If the sender can’t guarantee the precondition, there
is no guarantee of the postcondition.
Class invariant and operation precondition
Operation executes
Class invariant and operation postcondition
Example: Database Insertion
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Invariant: Unique primary key in table A
and foreign key points to existing
record in table B.
Precondition on insert into table A:
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Primary key of new record is unique in A
Foreign key of new record exists in B
Postcondition:
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New record exists in table A
When Designing New Methods
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Know the class invariants.
Document the method’s pre and
postconditions.
Decide what to do if precondtions aren’t
met.
Test all three parts.
What is the True Dimension?
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Consider the positioning of Rectangle class
objects (that can stretch, rotate).
The class could hold
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Four corner points, or
Top-left, bottom-right, and orientation, or
Center, height, width, orientation.
What is the correct number of dimensions?
What does this have to do with invariants?
Types
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Again, the goal is to characterize good classes
and class hierarchies.
The type of a class includes its
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purpose
invariants
attributes
operations, and their
pre and postconditions, and
signatures
Type vs. Class
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A type is independent of the
implementation.
There may be several implementations
(classes) for a given type.
In Together, We Could Say:
So Far, So Good…
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But what if we try this?
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subclass = subtype?? No!
Type Conformance
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If S is a true subtype of T, then S must
conform to T.
That is, an object of type S can be
provided wherever an object of type T
is expected.
Now technically, in Java and C++ this
will always be so, but finding the
diagonal of an Elephant is nonsensical.
Ensuring Type Conformance
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The invariants of the subclass must be at
least as strong (restrictive) as those of the
superclass.
Every method of the superclass has a
corresponding method in the subclass (with
the same name and signature).
Every subclass method’s precondition is no
stronger than that in the superclass.
Every subclass method’s postcondition is at
least as strong as that in the superclass.
A Classic Example
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Employee
 gradeLevel > 0
 perfEval  [0, 5]
 bonusPct  [0, 10]
 calcBonus(perfEval, bonusPct)  [0, 10]
Manager
 calcBonus(perfEval, bonusPct) overridden
What are legitimate ranges for Manager’s
 gradeLevel?
 perfEval?
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bonusPct?
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calcBonus()?
A Classic Example (cont.)
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Manager class invariant:
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calcBonus() precondition:
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gradeLevel > 10 is OK
gradeLevel  [-1, 15] not OK
perfEval  [0, 6] is OK
perfEval  [1, 4] not OK
calcBonus() output (postcondition):
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calcBonus()  [0,8] is OK
calcBonus()  [-1, 13] not OK
Why?
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Once again, an object of subtype S
must be usable anywhere an object of
the supertype T is.
Closed Behavior
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A final principle:
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Any operation on an object from class C –
including inherited operations – should obey C’s
class invariants.
Example: the intersection at Forbes and
Morewood parking lot.
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Intersection invariant: “Walk” signal can’t be
displayed when any traffic light is green.
Maybe we want a “turn right” arrow when the
“Walk” signal is displayed?
A Question
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A Rectangle class, with methods
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rotate(angle) and
scaleHoriz(scaleFactor)
Now add subclass Square.
Any principles violated?
What are the possible solutions?
Another Question
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What’s wrong here? How to fix it?