Space War Class Diagram. Elements of OOP. How to design interactions between objects. Space War Class Diagram with Inheritance

Transcription

1 Elements of OOP Object Smart data structure Set of state variables Set of methods for manipulating state variables Class: Specifies the common behavior of entities Instance: A particular object or entity of a given class Space War Class Diagram TORPEDO velocity: target: proximity-fuse: TORPEDO? VELOCITY MOVE SHIP velocity: num-torps: VELOCITY MOVE SHIP? ATTACK EXPLODE PLANET PLANET? Ships and torpedoes have some behavior that is the same is there are way to capture this commonality? 1/9 2/9 Space War Class Diagram with Inheritance is-a MOBILE -THING velocity: MOBILE-THING? VELOCITY MOVE is-a SHIP class is a specialization or sub-class of the MOBILE -THING class SHIP is-a MOBILE -THING SHIP inherits the state and behavior of MOBILE -THING MOBILE -THING class is a super-class of the SHIP and TORPEDO classes How to design interactions between objects Focus on classes objects Relationships between classes Kinds of interactions that need to be supported between instances of classes For now, assume the following interface to an object: (ask <object> <method> <arguments>) TORPEDO target: proximity-fuse: TORPEDO? has-a target SHIP num-torps: SHIP? ATTACK EXPLODE PLANET PLANET? 3/9 4/9 (define person-1 (make-person Fred Jones)) (ask person-1 (hello there)) hello there (ask person-1 ) Fred (define professor-1 (make-professor sam smith)) (ask professor-1 say (hello there)) hello there 5/9 6/9 1

2 7/9 (ask professor-1 ) Professor Smith (ask professor-1 ) Professor Smith (ask professor-1 (the sky is blue)) Therefore, the sky is blue A professor should delegate part of the lecture method to a person s say method. 8/9 (define ap-1 (make-arrogantprof Ned Infallible)) (define ap-1 (make-arrogantprof Ned Infallible)) (ask ap-1 (nice weather we are having)) nice weather we are having, obviously (ask ap-1 (nice weather we are having)) Therefore, nice weather we are having 9/9 10/9 (define ap-1 (make-arrogantprof Ned Infallible)) (ask ap-1 (nice weather we are having)) Therefore, nice weather we are having, obviously (define stud (make -student bert sesame)) (ask stud (I do not understand)) Excuse me, but I do not understand 11/9 12/9 2

3 13/9 (ask stud question ap-1 (why does this code work)) this should be obvious to you 14/9 (ask professor-1 question ap-1 (why does this code work)) Why are you asking me about why does this code work I thought you published a paper on that topic (ask stud?) #t (ask professor-1?) #t?? 15/9 16/9 Lessons from our simple class hierarchy tagging of instances specifying class hierarchies and ensuring that instances creating superclass instances inheriting of methods from class hierarchies delegation of methods to other instances within a class hierarchy How build an OOP system in Scheme? Objects: as procedures that take messages Instances have Identity: in sense of eq? Object instances are unique Scheme procedures Local State: gives each object (each instance of a class) the ability to perform differently Each instance procedure has own local environment Classes: Scheme make-<object> procedures. Methods returned in response to messages: Scheme procedures (take method-dependent arguments) Inheritance Rule telling what method to use Conventions on messages & methods 17/9 18/9 3

4 Steps toward our Scheme OOPs: 1. Basic Objects A. messages and methods convention B. self variable to refer to oneself 2. Inheritance A. internal superclass instances, and B. match method directly in object, or get-method from internal instance if needed C. delegation: explicitly use methods from internal objects Today's Example World: People, Professors, Arrogant-profs, and Students 3. Multiple Inheritance 19/9 20/9 1. Method convention ; specifies the person class (cond ((eq? message ) (lambda () fname)) ((eq? message CHANGE-MY-NAME) (lambda (new-name) (set! fname new-name))) ((eq? message ) Alternative case syntax for message match: case is more general than this (see Scheme manual), but our convention for message matching will be: ((<msg-1>) <method-1>) ((<msg-2>) <method-2>)... ((<msg-n>) <method-n>) (else <expr>)))) 21/9 22/9 Method convention with case syntax (() (lambda () fname)) (lambda (new-name) (set! fname new-name))) (() Method convention with case syntax (() (lambda () fname)) p: () body: fname (lambda (new-name) (set! fname new-name))) (() 23/9 24/9 4

5 25/9 Method convention with case syntax (() (lambda () fname)) (lambda (new-name) (set! fname new-name)) ) (() p: () body: fname p: (new-name) body: (set!...) How make and use the object? Making an object instance (define g (make-person george orwell)) Using the object instance (painful way) ((g )) ==>(#[proc p:() body:fname]) ;apply to no args ==> george Two things going on: method lookup (g ) ==> <method> method application (<method>) ==> <result> 26/9 Using the object easier way method lookup: (define (get-method message object) (object message)) Example (define g (make-person george orwell)) (ask g ) (get-method G) (g ) ( (g ) ) "ask" an object to do something - combined method retrieval and application to args. (define (ask object message. args) (let ((method (get-method message object))) (if (method? method) (apply method args) (error "No method for message" message)))) (apply op args) (op arg1 arg2 argn) GE make-person:... g: E3 george orwell p: message body: (case...) E4 message: NAME E5 27/9 p: () body: fname name 28/9 E5 Cleaning up some details of our implementation Dealing with missing methods The need for self -reference Dealing with tags Detection of methods (or missing methods): Use (no-method) to indicate that there is no method (define no-method (let ((tag (list NO-METHOD))) (lambda () tag))) Check if something is a method: (define (method? x) (cond ((procedure? x) #T) ((eq? x (no-method)) #F) (else (error "Object returned non-message" x)))) 29/9 30/9 5

6 Limitation self-reference (ask g ' '(the sky is blue)) the sky is blue => nuf-said (ask g 'CHANGE-NAME ishmael) want g to "" his new name whenever it changes We want a person to call its own method, but... Problem: no access to the "object" from inside itself! Solution: add explicit self argument to all methods Better Method Convention (1) --self (() (lambda (self) fname)) (lambda (self new-name) (set! fname new-name))) (() (lambda (self list-of-stuff) 31/9 32/9 Better Method Convention (2) --ask (define (ask object message. args) (let ((method (get-method message object))) (if (method? method) (apply method object args) (error "No method for message" message)))) (ask g CHANGE-NAME ishmael) ==>(apply #[proc p:self,new-name body:...] <g-object> ishmael ) ==> (ask <g-object> say ) call me ishmael nuf-said Typing objects in an OOPS system We want a method that acts differently depending on object type (ask stud question ap-1 (why does this code work)) this should be obvious to you (ask professor-1 question ap-1 (why does this code work)) Why are you asking me about why does this code work I thought yo u published a paper on that topic This means we need to identify stud as a student object, and professor-1 as a professor object. 33/9 34/9 Adding a type method (() (lambda (self) fname)) (lambda (self new-name) (set! fname new-name) (ask self (list call me fname)))) (() (lambda (self list-of-stuff) ((?) (lambda (self) #t)) 35/9 Adding a type method (define someone (make-person bert sesame)) (ask someone person?) #t (ask someone professor?) ;No method for professor? in bert ;Type D to debug error, Q to return back to REP loop (define (is-a object type-pred) (if (not (procedure? Object)) #f (let ((method (get-method type-pred object))) (if (method? Method) (ask object type-pred) #f))))) 36/9 6

7 Summary Basic objects Self reference Tagging object classes Using environments and procedures to capture and change local state 37/9 7