1. SOEN 343 Software Design
Computer Science and Software Engineering Department
Concordia University
Fall 2004
Instructor: Patrice Chalin

2. Agenda – Lecture 5a Homework (4b) Responsibilities, an introduction
Quiz 1 review, … OO
Objects and variables.
Inheritance, subtyping.
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SOEN 343, © P.Chalin,

3. Homework was … Planned (e.g. MDD) vs. Evolutionary (e.g. XP)
How does each approach address:
No doc. produced.
Increased gap: design vs. implementation.
Add-on syndrome.
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SOEN 343, © P.Chalin,

4. Responsibility Assignment
GRASP – Larman.
Responsibility-Drive Design (RDD) – Wirfs-Brock et. al.
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SOEN 343, © P.Chalin,

5. General Classification of Kinds of Responsibility
To know.
To do.
To decide.
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6. Responsibilities – A Boat Metaphor
What kind of responsibilities do each of the following “objects” have: …
To know.
To do.
To decide.
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SOEN 343, © P.Chalin,

7. Responsibilities – A Boat Metaphor
Kind of responsibility for:
Captain
To know?
To do?
To decide?
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SOEN 343, © P.Chalin,

8. Responsibilities – A Boat Metaphor
Kind of responsibility for:
Navigator.
To know?
To do?
To decide?
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SOEN 343, © P.Chalin,

9. Responsibilities – A Boat Metaphor
Kind of responsibility for:
Compass.
To know?
To do?
To decide?
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SOEN 343, © P.Chalin,

10. Quiz 1 Focus OO fundamentals. Basic UML. Basic GRASP (applied).
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SOEN 343, © P.Chalin,

11. Quiz 1 Review You will be given explanation, not necessarily answers.
If you need further clarification:
My office hours.
Ask during tutorial.
2/4/2019
SOEN 343, © P.Chalin,

12. Quiz 1, Qn 1: Objects and Variables
Some ‘theory’ …
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SOEN 343, © P.Chalin,

13. Object and Variables: Run-time Initialization
Local variables:
Are not implicitly initialized.
Must be explicitly initialized.
Object fields always initialized by default to
Integral types (including char): 0.
Floating point types: 0.0.
Reference types: null.
2/4/2019
SOEN 343, © P.Chalin,

14. Object Creation and Variable Declarations
Basic declaration (no object creation):
Animal a;
null initialized declaration (no object creation):
Animal a = null;
Only using new will create (instantiate) objects
a = new Duck();
2/4/2019
SOEN 343, © P.Chalin,

15. Objects and Variables: Key Points
What you should know:
Run-time memory model
Has two parts: Stack, Heap.
You should know what gets allocated where.
Object creation and variable declarations.
Declarations do not cause objects to be created.
Know the role of null.
Object creation / instantiation: via new.
2/4/2019
SOEN 343, © P.Chalin,

16. Stack and Heap: Quiz Question
Given the following declarations
int i = 2;
int[] b = new int[2];
String r;
String s = “abc”;
String t = null;
What will be the state of the stack and heap after they have all been processed?
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SOEN 343, © P.Chalin,

17. Stack and Heap Stack Heap i 2 b r ? [0,0] s t null “abc”
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SOEN 343, © P.Chalin,

18. Quiz 1, Qn 2: Type Hierarchy
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SOEN 343, © P.Chalin,

19. Disjointedness of Primitive and Reference Types
You cannot assign a primitive type to a variable of a reference type.
You cannot assign a reference to a variable of a primitive type.
The remaining few slides discuss only reference types.
2/4/2019
SOEN 343, © P.Chalin,

20. Type Hierarchy (Lect. 1b)
Every class is a subclass of Object.
If S is a subclass of T then we can use an instance of S where ever a T is expected:
T t = new S();
Object o = new S();
// Do not do this (it is wasteful):
Object o = new String(“abc”);
2/4/2019
SOEN 343, © P.Chalin,

21. Each Reference Variable: Two Types
Each variable of a reference type has
Declared type – fixed.
Run-time type – can change at run-time.
Synonyms:
Declared type: apparent type.
Run-time type: dynamic, or actual type.
2/4/2019
SOEN 343, © P.Chalin,

22. Each Variable: Two Types, Example
Object m = new Movie();
m has two types:
Declared type: Object.
Run-time type: Movie.
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SOEN 343, © P.Chalin,

23. Type Hierarchy: Object, the root of allK B X
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24. Type Hierarchy: Supertypes, Subtypes
Supertypes of A
Object
Subtypes of A A K B X
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SOEN 343, © P.Chalin,

25. Unrelated Sub-hierarchies are Not Compatible (for assignment, cast).
Object
Cannot
Assign, or
Cast
A to K A B X K
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SOEN 343, © P.Chalin,

26. ‘Declared Type’ Fixes Bounds
Declared type – fixed, e.g. A.
Run-time type – can change at run-time ...
… within bounds of declared type subhierarchy. A B X
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SOEN 343, © P.Chalin,

27. Type Checking: Assignment
Always done relative to declared type.
A a = some-expression-of-type-X
Legal? (or will cause a compile-time error?)
Assignment is legal iff X is A,
Subtype of A. A B X
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SOEN 343, © P.Chalin,

28. Type Casting and Type Checking I
A a = (X) some-expression-of-type-K;
Legal? (will cause a compile-time error?)
Same answer as given on previous slide because this is just a special case of the previous slide.
2/4/2019
SOEN 343, © P.Chalin,

29. Type Casting and Type Checking II
A a = (X) some-expression-of-type-K;
Legal? (will cause a compile-time error?)
Error unless X and K are related (super- or sub-type). X … Z K
2/4/2019
SOEN 343, © P.Chalin,

30. Type Casting and Run-time Checks? I
A a = (X) some-expression-of-type-K;
If X and K are related then:
X is a supertype of K: nothing is needed. X … K
2/4/2019
SOEN 343, © P.Chalin,

31. Type Casting and Run-time Checks? II
A a = (X) some-expression-of-type-K;
If X and K are related then:
X is a subtype of K: a run-time check is needed. K … X
2/4/2019
SOEN 343, © P.Chalin,

32. Question 2 (a)-(c), Another Try
Find the answer (but also tell me why):
(a) Object o = 1; C / R / N
(b) Object o = s; C / R / N
(c) Object o = (Object) s; C / R / N
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SOEN 343, © P.Chalin,

33. Question 2 (d)-(f), Another Try
String t = x; C / R / N
String t = (String)x; C / R / N
String t = (Object)s; C / R / N
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SOEN 343, © P.Chalin,

34. Question 3 a : A c : C b : B 1 : m1( ) 2 : m2 ( ) c
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35. Question 3, Code – possible answer
class A {
B b;
public void m() {
C c = b.m1();
c.m2(); }
class B {
C c; ...
public C m1() { ...; return c; }
class C {
public void m2() { ... }
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SOEN 343, © P.Chalin,

36. Question 4 Variant of Exercise Set 1, Question 3.
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37. Dynamic Dispatching …
Run-time type is only of concern when resolving non-static methods.
Never for fields.
Not for static methods.
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SOEN 343, © P.Chalin,

38. Overridden Non-static Field
There are two fields (one in P, one in C).
Moral: do not do this! P
f : int C
f : int
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SOEN 343, © P.Chalin,

39. Static Methods No overriding
P.m(), C.m() are distinct methods, both accessible. P
int m() C
int m()
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SOEN 343, © P.Chalin,

40. Question 5, GRASP: Polymorphism, …
Solution:
Exercise Set 3
Tutorial 3
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SOEN 343, © P.Chalin,

41. Computing Movie Charge
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SOEN 343, © P.Chalin,