6.001 SICP Object Oriented ProgrammingThe role of abstractionsOne View of DataDispatch on TypeAn Alternative View of Data: Procedures with StateExample: Pair as a Procedure with StateExample: What is our "pair" object?Pair Mutation as Change in StateExample: Mutating a pair objectMessage Passing Style - RefinementsVariable number of argumentsSlide 12Programming Styles – Procedural vs. Object-OrientedObject-Oriented Programming TerminologyClass Diagram Instance DiagramUsing classes and instances to design a systemA Space-Ship ObjectSpace-Ship ClassExample – Instance DiagramExample – Environment DiagramSome Extensions to our WorldSlide 22Planet ImplementationFurther Extensions to our WorldClass DiagramCoordinating with a clockThe Universe and TimeControlling the clockImplementations for our Extended WorldTorpedo ImplementationRunning the SimulationSummary6.001 SICP 1/306.001 SICPObject Oriented Programming•Data Abstraction using Procedures with State•Message-Passing •Object Oriented Modeling•Class diagrams•Instance diagrams•Example: space wars simulation6.001 SICP 2/30The role of abstractions•Procedural abstractions•Data abstractions•Questions:•How easy is it to break system into abstraction modules?•How easy is it to extend the system?•Adding new data types?•Adding new methods?Goal: treat complex things as primitives, and hide details6.001 SICP 3/30One View of Data•Tagged data:•Some complex structure constructed from cons cells•Explicit tags to keep track of data types•Implement a data abstraction as set of procedures that operate on the data •"Generic" operations by looking at types: (define (scale x factor) (cond ((number? x) (* x factor)) ((line? x) (line-scale x factor)) ((shape? x) (shape-scale x factor)) (else (error "unknown type"))))6.001 SICP 4/30Dispatch on Type•Adding new data types:•Must change every generic operation•Must keep names distinct•Adding new methods:•Just create generic operationsData type 1 Data type 2 Data type 3 Data type 4Operation 1 Some proc Some proc Some proc Some procOperation 2 Some proc Some proc Some proc Some procOperation 3 Some proc Some proc Some proc Some procOperation 4 Some proc Some proc Some proc Some procGeneric operationGeneric data object?6.001 SICP 5/30An Alternative View of Data: Procedures with State•A procedure has•parameters and body as specified by expression•environment (which can hold name-value bindings!)•Can use procedure to encapsulate (and hide) data, and provide controlled access to that data•Procedure application creates private environment•Need access to that environment•constructor, accessors, mutators, predicates, operations•mutation: changes in the private state of the procedure6.001 SICP 6/30Example: Pair as a Procedure with State(define (cons x y) (lambda (msg) (cond ((eq? msg ‘CAR) x) ((eq? msg ‘CDR) y) ((eq? msg ‘PAIR?) #t) (else (error "pair cannot" msg)))))(define (car p) (p ‘CAR))(define (cdr p) (p ‘CDR))(define (pair? p) (and (procedure? p) (p ‘PAIR?)))6.001 SICP 7/30Example: What is our "pair" object?(define foo (cons 1 2))GEp: x ybody: ( (msg) (cond ..))cons:1p: msgbody: (cond ...)E1foo:x: 1y: 21(car foo) | GE=> (foo 'CAR) | E2=>2(cond ...) | E3=> x | E3=> 1msg: CARE3323(car foo) becomes (foo 'CAR)(define (cons x y) (lambda (msg) (cond ((eq? msg ‘CAR) x) ((eq? msg ‘CDR) y) ((eq? msg ‘PAIR?) #t) (else (error …)))))(define (car p) (p ‘CAR))6.001 SICP 8/30Pair Mutation as Change in State(define (cons x y) (lambda (msg) (cond ((eq? msg ‘CAR) x) ((eq? msg ‘CDR) y) ((eq? msg ‘PAIR?) #t) ((eq? msg ‘SET-CAR!) (lambda (new-car) (set! x new-car))) ((eq? msg ‘SET-CDR!) (lambda (new-cdr) (set! y new-cdr))) (else (error "pair cannot" msg)))))(define (set-car! p new-car) ((p ‘SET-CAR!) new-car))(define (set-cdr! p new-cdr) ((p ‘SET-CDR!) new-cdr))6.001 SICP 9/30Example: Mutating a pair object(define bar (cons 3 4))GE(cond ...) | E6=> ( (new-car) (set! x new-car)) | E6msg: SET-CAR!E63p: new-carbody: (set! x new-car)4(set! x new-car) | E7new-car: 0E75(set-car! bar 0) | GE=> ((bar 'SET-CAR!) 0) | E52(set-car! bar 0)6changes x value to 0 in E401p: msgbody: (cond ...)E4bar:x: 3y: 416.001 SICP 10/30Message Passing Style - Refinements•lexical scoping for private state and private procedures(define (cons x y) (define (change-car new-car) (set! x new-car)) (define (change-cdr new-cdr) (set! y new-cdr)) (lambda (msg . args) (cond ((eq? msg ‘CAR) x) ((eq? msg ‘CDR) y) ((eq? msg ‘PAIR?) #t) ((eq? msg ‘SET-CAR!) (change-car (first args))) ((eq? msg ‘SET-CDR!) (change-cdr (first args))) (else (error "pair cannot" msg)))))(define (car p) (p 'CAR))(define (set-car! p val) (p 'SET-CAR! val))6.001 SICP 11/30Variable number of argumentsA scheme mechanism to be aware of:•Desire: (add 1 2) (add 1 2 3 4)•How do this? (define (add x y . rest) ...) (add 1 2) => x bound to 1 y bound to 2 rest bound to '() (add 1) => error; requires 2 or more args (add 1 2 3) => rest bound to (3) (add 1 2 3 4 5) => rest bound to (3 4 5)6.001 SICP 12/30Message Passing Style - Refinements•lexical scoping for private state and private procedures(define (cons x y) (define (change-car new-car) (set! x new-car)) (define (change-cdr new-cdr) (set! y new-cdr)) (lambda (msg . args) (cond ((eq? msg ‘CAR) x) ((eq? msg ‘CDR) y) ((eq? msg ‘PAIR?) #t) ((eq? msg ‘SET-CAR!) (change-car (first args))) ((eq? msg ‘SET-CDR!) (change-cdr (first args))) (else (error "pair cannot" msg)))))(define (car p) (p 'CAR))(define (set-car! p val) (p 'SET-CAR! val))6.001 SICP 13/30Programming Styles – Procedural vs. Object-Oriented•Procedural programming:•Organize system around procedures that operate on data (do-something <data> <arg> ...) (do-another-thing <data>)•Object-based programming:•Organize system around objects that receive messages (<object> 'do-something <arg>) (<object> 'do-another-thing)•An object encapsulates data and operations6.001 SICP 14/30Object-Oriented Programming Terminology•Class: •specifies the common behavior of entities•in scheme, a "maker" procedure•Instance:•A particular object or entity of a given
View Full Document
Unlocking...