SOEN 343 Software Design Computer Science and Software

SOEN 343 Software Design Computer Science and Software Engineering Department Concordia University Fall 2004 Instructor: Patrice Chalin SOEN 343, © P. Chalin SOEN 344, © P. Chalin

Agenda – Lecture 5 a • • Homework (4 b) Responsibilities, an introduction Quiz 1 review, … OO – Objects and variables. – Inheritance, subtyping. 9/7/2021 SOEN 343, © P. Chalin, 2

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. 9/7/2021 SOEN 343, © P. Chalin, 3

Responsibility Assignment • GRASP – Larman. • Responsibility-Drive Design (RDD) – Wirfs-Brock et. al. 9/7/2021 SOEN 343, © P. Chalin, 4

General Classification of Kinds of Responsibility – To know. – To do. – To decide. 9/7/2021 SOEN 343, © P. Chalin, 5

Responsibilities – A Boat Metaphor • What kind of responsibilities do each of the following “objects” have: … – To know. – To do. – To decide. 9/7/2021 SOEN 343, © P. Chalin, 6

Responsibilities – A Boat Metaphor Kind of responsibility for: • Captain – To know? – To do? – To decide? 9/7/2021 SOEN 343, © P. Chalin, 7

Responsibilities – A Boat Metaphor Kind of responsibility for: • Navigator. – To know? – To do? – To decide? 9/7/2021 SOEN 343, © P. Chalin, 8

Responsibilities – A Boat Metaphor Kind of responsibility for: • Compass. – To know? – To do? – To decide? 9/7/2021 SOEN 343, © P. Chalin, 9

Quiz 1 Focus • OO fundamentals. • Basic UML. • Basic GRASP (applied). 9/7/2021 SOEN 343, © P. Chalin, 10

Quiz 1 Review • You will be given explanation, not necessarily answers. • If you need further clarification: – My office hours. – Ask during tutorial. 9/7/2021 SOEN 343, © P. Chalin, 11

Quiz 1, Qn 1: Objects and Variables • Some ‘theory’ … 9/7/2021 SOEN 343, © P. Chalin, 12

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. 9/7/2021 SOEN 343, © P. Chalin, 13

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(); 9/7/2021 SOEN 343, © P. Chalin, 14

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. 9/7/2021 SOEN 343, © P. Chalin, 15

Stack and Heap: Quiz Question • Given the following declarations int[] String i = b = r; s = t = 2; new int[2]; “abc”; null; • What will be the state of the stack and heap after they have all been processed? 9/7/2021 SOEN 343, © P. Chalin, 16

Stack and Heap Stack i 2 [0, 0] b r ? s t 9/7/2021 Heap “abc” null SOEN 343, © P. Chalin, 17

Quiz 1, Qn 2: Type Hierarchy 9/7/2021 SOEN 343, © P. Chalin, 18

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. 9/7/2021 SOEN 343, © P. Chalin, 19

Type Hierarchy (Lect. 1 b) • 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”); 9/7/2021 SOEN 343, © P. Chalin, 20

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. 9/7/2021 SOEN 343, © P. Chalin, 21

Each Variable: Two Types, Example Object m = new Movie(); • m has two types: – Declared type: Object. – Run-time type: Movie. 9/7/2021 SOEN 343, © P. Chalin, 22

Type Hierarchy: Object, the root of all Object A K B X 9/7/2021 SOEN 343, © P. Chalin, 23

Type Hierarchy: Supertypes, Subtypes Object Supertypes of A A K B Subtypes of A X 9/7/2021 SOEN 343, © P. Chalin, 24

Unrelated Sub-hierarchies are Not Compatible (for assignment, cast). Object • Cannot – Assign, or – Cast A to K A K B X 9/7/2021 SOEN 343, © P. Chalin, 25

‘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 9/7/2021 SOEN 343, © P. Chalin, 26

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 A X is – A, B – Subtype of A. X 9/7/2021 SOEN 343, © P. Chalin, 27

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. 9/7/2021 SOEN 343, © P. Chalin, 28

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 subtype). X K … Z 9/7/2021 SOEN 343, © P. Chalin, 29

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 9/7/2021 SOEN 343, © P. Chalin, 30

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 9/7/2021 SOEN 343, © P. Chalin, 31

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 = 9/7/2021 SOEN 343, © P. Chalin, 32

Question 2 (d)-(f), Another Try (d) String t = x; C/R /N (e) String t = (String)x; C/R/N (f) String t = (Object)s; C/R/N 9/7/2021 SOEN 343, © P. Chalin, 33

Question 3 a: A b: B c: C 1 : m 1( ) c 2 : m 2 ( ) 9/7/2021 SOEN 343, © P. Chalin, 34

Question 3, Code – possible answer class A { B b; public void m() { C c = b. m 1(); c. m 2(); } } class B { C c; . . . public C m 1() {. . . ; return c; } } class C { public void m 2() {. . . } } 9/7/2021 SOEN 343, © P. Chalin, 35

Question 4 • Variant of Exercise Set 1, Question 3. 9/7/2021 SOEN 343, © P. Chalin, 36

Dynamic Dispatching … • Run-time type is only of concern when resolving non-static methods. • Never for fields. • Not for static methods. 9/7/2021 SOEN 343, © P. Chalin, 37

Overridden Non-static Field • There are two fields (one in P, one in C). • Moral: do not do this! P f : int 9/7/2021 C f : int SOEN 343, © P. Chalin, 38

Static Methods • No overriding • P. m(), C. m() are distinct methods, both accessible. C P int m() 9/7/2021 int m() SOEN 343, © P. Chalin, 39

Question 5, GRASP: Polymorphism, … • Solution: – Exercise Set 3 – Tutorial 3 9/7/2021 SOEN 343, © P. Chalin, 40

Computing Movie Charge 9/7/2021 SOEN 343, © P. Chalin, 41