Data Abstraction and Encapsulation Key research Dennis Van Horn CACM March 66 first notion of abstract types and capabilities Parnas On the Criteria to be Used in Decomposing Systems into Modules CACM Dec 72 Liskov CLU Wulf Shaw Alphard Lampson Euclid Good GYPSY Xerox Mesa Hoare Brinch Hansen monitors Wirth Modula Modula 2 Honeywell Bull Ada Parnas s Principles One must provide the intended user with all the information needed to use the module and nothing more One must provide the implementer with all the information needed to complete the module and nothing more Data Abstraction Emphasis on behavior operations capture behavior Implementation is not important to user of abstraction In order to support data abstraction language must provide linguistic construct permits abstraction to be implemented as a unit supports representation specification ways to limit access to object access should only be to defined operations no access to representation clusters modules forms packages classes Simple Data Only package earth is type continent is Africa Antarctica Asia Australia Europe NorthAmerica SouthAmerica radius constant float 6 4e6 area constant array continent of float 30 3e9 13 0e9 43 3e9 7 7e9 10 4e9 24 9e9 17 8e9 population array continent of integer end earth Logical clustering Information Hiding package trig is function sin x float return Float function cos x float return Float end trig package body trig is pi constant Float 3 14159 function norm x float return float is return x modulo 2 pi function sin x float return float is return the sine of norm x function cosine x float return float is return the cosine of norm x end trig Exporting functions only Implementation is hidden Simple Objects package directory object is procedure insert newname in Name newnumber in Number procedure lookup oldname in Name oldnumber out Number found out Boolean end directory object package body directory object is type Dirnode type Dirptr is access Dirnode type Dirnode is record entryname Name entrynumber Number left right Dirptr end record root dirptr Creates one directory exports two procs that can operate on directory directory itself hidden from access Implements a directory cont Simple Objects procedure insert newname in Name newnumber in Number is add new newname newnumber to the directory procedure lookup oldname in Name oldnumber out Number found out Boolean find oldname entry in directory begin initialize the directory end directory object directory object insert me 41039 directory object lookup me mynumber ok Operations user can perform on directory Object Classes generic package directory class is procedure insert newname in Name newnumber in Number procedure lookup oldname in Name oldnumber out Number found out Boolean end directory class package body directory class is type Dirnode type Dirptr is access Dirnode type Dirnode is record entryname Name entrynumber Number left right Dirptr end record root dirptr Describes an instance of a directory cont Object Classes procedure insert newname in Name newnumber in Number is add new newname newnumber to the directory procedure lookup oldname in Name oldnumber out Number found out Boolean find oldname entry in directory begin initialize the directory end directory class package homedir is new directory class package workdir is new directory class workdir insert me 41039 homedir insert me 30570 workdir lookup me mynumber ok Derived objects No notion of inheritance Abstract Types Motivation datatype rational rat of int int val zero rat 0 1 and one rat 1 1 fun op rat m1 n1 rational rat m2 n2 rational rat m1 n2 m2 n1 n1 n2 rat 3 2 creates rational number 3 2 rat 6 4 creates rational number 6 4 Abs Types cont Motivation if one rat 1 2 rat 6 4 then else Won t test although it should rat 0 0 no corresponding rat 1 0 rational numbers Have Rational rat m n m n integer Desire Rational rat m n m n integer n 0 m n have no common factor a type based on a representation type that does not exist in common languages abstract type based on a set of operations constants functions and procedures values defined only indirectly Abstract Types Example abstype rational rat of int int with val zero rat 0 1 and one rat 1 1 fun op m int n int if n 0 then rat m n else invalid rational number and op rat m1 n1 rational rat m2 n2 rational rat m1 n2 m2 n1 n1 n2 Abs Types cont Example and op rat m1 n1 rational rat m2 n2 rational m1 n2 m2 n1 and float rat m n rational m n end val h 1 2 creates rational number 1 2 val i 6 4 creates rational number 6 4 if one h i then else now this works Another Abstract Type package directory type is type Directory is limited private procedure insert dir in out Directory newname in Name newnumber in Number procedure lookup dir in Directory oldname in Name oldnumber out Number found out Boolean private type Dirnode type Directory is access Dirnode type Dirnode is record end record end directory type Implementation details in a specification Another Abstract Type cont package body directory type is procedure insert dir in out Directory newname in Name newnumber in Number is add new newname newnumber to dir procedure lookup dir in Directory oldname in Name oldnumber out Number found out Boolean find oldname entry in dir end directory type Using directory type use directory type allows dropping dot notation homedir Directory workdir Directory insert workdir me 41039 insert homedir me 97548 lookup workdir me mynumber ok Objects and Abstract Types Abstract types are similar to built in types and have firstclass status values of the type are first class Objects in general are not In object example homedir and workdir couldn t be passed as arguments for e g Abstract type notation is more natural values and variables of the type are arguments rather than dot notated prefixes Abstract types work in all programming paradigms Objects being updatable entities fit the imperative programming style Generics generic capacity in positive type Item is private package queue class is procedure append newitem in Item procedure remove olditem out Item end queue class More Generics package body queue class is items array 1 capacity of Item size front rear Integer range 0 capacity procedure append newitem in Item is begin items rear newitem end procedure remove olditem out Item is begin olditem items front end end begin front 1 rear 0 end queue class Using queue class package line buffer is new queue class 120 Character type Transaction is record end record