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COEN 171 - Data Abstraction and OOP. Data Abstraction Problems with subprogram abstraction Encapsulation Data abstraction Language issues for ADTs Examples Ada C++ Java Parameterized ADTs. COEN 171 - Data Abstraction and OOP. Object-oriented programming

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coen 171 data abstraction and oop
COEN 171 - Data Abstraction and OOP
  • Data Abstraction
    • Problems with subprogram abstraction
    • Encapsulation
    • Data abstraction
    • Language issues for ADTs
    • Examples
      • Ada
      • C++
      • Java
    • Parameterized ADTs
coen 171 data abstraction and oop1
COEN 171 - Data Abstraction and OOP
  • Object-oriented programming
    • Components of object-oriented programming languages
    • Fundamental properties of the object-oriented model
    • Relation to data abstraction
    • Design issues for OOPL
    • Examples
      • Smalltalk 80
      • C++
      • Ada 95
      • Java
    • Comparisons
      • C++ and Smalltalk
      • C++ and Ada 95
      • C++ and Java
    • Implementation issues
subprogram problems
Subprogram Problems
  • No way to selectively provide visibility for subprograms
  • No convenient ways to collect subprograms together to perform a set of services
  • Program that uses subprogram (client program) must know details of all data structures used by subprogram
    • client can “work around” services provided by subprogram
    • hard to make client independent of implementation techniques for data structures
      • discourages reuse
  • Difficult to build on and modify the services provided by subprogram
  • Many languages don’t provide for separately compiled subprograms
encapsulation
Encapsulation
  • One solution
    • a grouping of subprograms that are logically related that can be separately compiled
    • called encapsulations
  • Examples of encapsulation mechanisms
    • nested subprograms in some ALGOL-like languages
      • Pascal
    • FORTRAN 77 and C
      • files containing one or more subprograms can be independently compiled
    • FORTRAN 90, Modula-2, Modula-3, C++, Ada (and other contemporary languages)
      • separately compilable modules
data abstraction
Data Abstraction
  • A better solution than just encapsulation
  • Can write programs that depend on abstract properties of a type, rather than implementation
  • Informally, an Abstract Data Type (ADT) is a [collection of] data structures and operations on those data structures
    • example is floating point number
      • can define variables of that type
      • operations are predefined
      • representation is hidden and can’t manipulate except through built-in operations
  • ADT
    • isolates programs from the representation
    • maintains integrity of data structure by preventing direct manipulation
data abstraction continued
Data Abstraction (continued)
  • Formally, an ADT is a user-defined data type where
    • the representation of and operations on objects of the type are defined in a single syntactic unit; also, other units can create objects of the type.
    • the representation of objects of the type is hidden from the program units that use these objects, so the only operations possible are those provided in the type\'s definition.
  • Advantages of first restriction are same as those for encapsulation
    • program organization
    • modifiability (everything associated with a data structure is together)
    • separate compilation
data abstraction continued1
Data Abstraction (continued)
  • Advantage of second restriction is reliability
    • by hiding the data representations, user code cannot directly access objects of the type
    • user code cannot depend on the representation, allowing the representation to be changed without affecting user code
  • By this definition, built-in types are ADTs
    • e.g., int type in C
      • the representation is hidden
      • operations are all built-in
      • user programs can define objects of int type
  • User-defined abstract data types must have the same characteristics as built-in abstract data types
data abstraction continued2
Data Abstraction (continued)
  • ADTs provide mechanisms to limit visibility
    • public part indicates what can be seen (and used from) outside
      • what is exported
    • private part describes what will be hidden from clients
      • made available to allow compiler to determine needed information
      • C++ allows specified program units access to the private information
        • friend functions and classes
language issues for adts
Language Issues for ADTs
  • Language requirements for data abstraction
    • a syntactic unit in which to encapsulate the type definition.
    • a method of making type names and subprogram headers visible to clients, while hiding actual definitions
      • public/private
    • some primitive operations must be built into the language processor (usually just assignment and comparisons for equality and inequality)
      • some operations are commonly needed, but must be defined by the type designer
      • e.g., iterators, constructors, destructors
  • Can put ADTs in PL
    • as a type definition extended to include operations (C++)
      • use directly to declare variables
    • as a collection of objects and operations (Ada)
      • may need to be instantiated before declaring variables
language issues for adts continued
Language Issues for ADTs (continued)
  • Language design issues
    • encapsulate a single type, or something more?
    • what types can be abstract?
    • can abstract types be parameterized?
    • how are imported types and operations qualified?
  • Simula-67 was first language to address this issue
    • classes provided encapsulation, but no information hiding
data abstraction in ada
Data Abstraction in Ada
  • Abstraction mechanism is the package
  • Each package has two pieces (can be in same or separate files)
    • specification
      • public part
      • private part
    • body
      • implementation of all operations exported in public part
      • may include other procedures, functions, type and variable declarations, which are hidden from clients
        • all variables are static
      • may provide initialization section
        • executed when declaration involving package is elaborated
  • Any type can be exported
  • Operations on exported types may be restricted
    • private (:=, =, /=, plus operations exported)
    • limited private (only operations exported)
data abstraction in ada continued
Data Abstraction in Ada (continued)
  • Evaluation
    • exporting any type as private is good
      • cost is recompilation of clients when the representation is changed
    • can’t import specific entities from other packages
    • good facilities for separate compilation
data abstraction in c
Data Abstraction in C++
  • Based on C struct type and Simula 67 classes
  • Class is the encapsulation device
    • all of the class instances of a class share a single copy of the member functions
    • each instance of a class has its own copy of the class data members
    • instances can be static, semidynamic, or explicit dynamic
  • Information Hiding
    • private clause for hidden entities
    • public clause for interface entities
    • protected clause - for inheritance
data abstraction in c continued
Data Abstraction in C++ (continued)
  • Constructors
    • functions to initialize the data members of instances
    • may also allocate storage if part of the object is heap-dynamic
    • can include parameters to provide parameterization of the objects
    • implicitly called when an instance is created
      • can be explicitly called
    • name is the same as the class name
  • Destructors
    • functions to cleanup after an instance is destroyed; usually just to reclaim heap storage
    • implicitly called when the object’s lifetime ends
      • can be explicitly called
    • name is the class name, preceded by a tilda (~)
data abstraction in c continued1
Data Abstraction in C++ (continued)
  • Friend functions
    • allow access to private members to some unrelated units or functions
  • Evaluation
    • classes are similar to Ada packages for providing abstract data type
    • difference is packages are encapsulations, whereas classes are types
data abstraction in java
Data Abstraction in Java
  • Similar to C++ except
    • all user-defined types are classes
    • all objects are allocated from the heap and accessed through reference variables
    • individual entities in classes (methods and variables) have access control modifiers (public or private), rather than C++ clauses
    • functions can only be defined in classes
    • Java has a second scoping mechanism, package scope, that is used instead of friends
      • all entities in all classes in a package that don’t have access control modifiers are visible throughout the package
parameterized adts
Parameterized ADTs
  • Ada generic packages may be parameterized with
    • type of element stored in data structure
    • operators among those elements
  • Must be instantiated before declaring variables
    • instantiation of generic behaves like text substitution
    • package BST_Integer is new binary_search _tree(INTEGER)
      • like text of generic package substituted here, with parameters substituted
    • EXCEPT references to non-local variables, etc. occur as if happen at point where generic was declared
  • If have multiple instantiations, need to disambiguate when declare exported types
    • package BST_Real is new binary_search_tree(REAL)
    • tree1: BST_Integer.bst;
    • tree2: BST_Real.bst;
parameterized adts continued
Parameterized ADTs (continued)
  • C++
    • classes can be somewhat generic by writing parameterized constructor functions
    • class itself may be parameterized as a templated class
    • Java doesn’t support generic abstract data types

stack (int size) {

stk_ptr = new int [size];

max_len = size - 1;

top = -1;

}

stack (100) stk;

object oriented programming
Object-Oriented Programming
  • The problem with Abstract Data Types is that they are static
    • can’t modify types or operations
      • except for generics/templates
    • means extra work to modify existing ADTs
  • Object-oriented programming (OOP) languages extend data abstraction ideas to
    • allow hierarchies of abstractions
    • make modifying existing abstractions for other uses very easy
  • Leads to new approach to programming
    • identify real world objects of problem domain and processing required of them
    • create simulations of those objects, processes, and the communication between them by modifying existing objects whenever possible
object oriented programming continued
Object-Oriented Programming (continued)
  • Two approaches to designing OOPL
    • start from scratch (Smalltalk 1972!!)
      • allows cleaner design
      • better integration of object features
      • no installed base
    • modify an existing PL (C++, Ada 95)
      • can build on body of existing code
      • OO features usually not as smoothly integrated
      • backward compatibility issues of warts from initial language design
oopl components
OOPL Components
  • Object: encapsulated operations plus local variables that define an object’s state
    • state is retained between executions
    • objects send and receive messages
  • Messages: requests from sender to receiver to perform work
    • can be parameterized
    • in pure OOL are also objects
    • return results
  • Methods: descriptions of operations to be done when a message is received
  • Classes: templates for objects, with methods and state variables
    • objects are instantiations of classes
    • classes are also objects (have instantiation method to create new objects)
fundamental properties of oo model
Fundamental Properties of OO Model
  • Abstract Data Types
    • encapsulation into a single syntactic unit that includes operations and variables
    • also information hiding capabilities
  • Inheritance
    • fundamental defining characteristic of OOPL
    • classes are hierarchical
      • subclass/superclass or parent/derived
      • lower in structure inherit variables and methods of ancestor classes
      • can redefine those, or add additional, or eliminate some
    • single inheritance (tree structure) or multiple inheritance (acyclic graph)
      • if single inheritance can talk about a root class
fundamental properties of oo model continued
Fundamental Properties of OO Model (continued)
  • Polymorphism
    • special kind of dynamic binding
      • message to method
    • same message can be sent to different objects, and the object will respond properly
    • similar to function overloading except
      • overloading is static (known at compile time)
      • polymorphism is dynamic (class of object known at run time)
comparison with data abstraction
Comparison with Data Abstraction
  • Class == generic package
  • Object == instantiation of generic
    • actually, closer to instance of exported type
  • Messages == calls to operations exported by ADT
  • Methods == bodies (code) for operations exported by ADT
  • EXCEPT
    • data abstraction mechanism allows only one level of generic/instantiation
    • OO model allows multiple levels of inheritance
    • no dynamic binding of method invocation in ADTs
oop language design issues
OOP Language Design Issues
  • Exclusivity of objects
    • everything is an object
      • elegant and pure, but slow for primitive types
    • add objects to complete typing system
      • fast for primitive types, but confusing
    • include an imperative style typing system for primitive types, but everything else is an object
      • relatively fast, and less confusion
  • Are subclasses subtypes?
    • does an “is a” relationship hold between parent and child classes?
oop language design issues continued
OOP Language Design Issues (continued)
  • Interface or implementation inheritance?
    • if only interface of parent class is visible to subclass, interface inheritance
      • may be inefficient
    • if interface and implementation visible to subclass, implementation inheritance
  • Type checking and polymorphism
    • if overridding methods must have the same parameter types and return type, checking may be static
    • Otherwise need dynamic type checking, which is slow and delays error detection
  • Single or multiple inheritance
    • multiple is extremely convenient
    • multiple also makes the language and implementation more complex, and is less efficient
oop language design issues continued1
OOP Language Design Issues (continued)
  • Allocation and deallocation of objects
    • if all objects are allocated from heap, references to them are uniform (as in Java)
    • is deallocation explicit (heap-dynamic objects in C++) or implicit (Java)
  • Should all binding of messages to methods be dynamic?
    • if yes, inefficient
    • if none are, great loss of flexibility
smalltalk 80
Smalltalk 80
  • Smalltalk is the prototypical pure OOPL
  • All entities in a program are objects
    • referenced by pointers
  • All computation is done by sending messages (perhaps parameterized by object names) to objects
    • message invokes a method
    • reply returns result to sender, or notifies that action has been done
  • Also incorporates graphical programming environment
    • program editor
    • compiler
    • class library browser
      • with associated classes
    • also written in Smalltalk
      • can be modified
smalltalk 80 continued
Smalltalk 80 (continued)
  • Messages
    • object to receive message
    • message
      • method to invoke
      • possibly parameters
  • Unary messages
    • specify only object and method
    • firstAngle sin
      • invokes sin method of firstAngle object
  • Binary messages
    • infix order
    • total / 100
      • sends message / 100 to object total
      • which invokes / method of total with parameter 100
smalltalk 80 continued1
Smalltalk 80 (continued)
  • Keyword messages
    • indicate parameter values by specifying keywords
    • keywords also identify the method
    • firstArray at: 1 put: 5
      • invokes at:put: method of firstArray with parameters 1 and 5
  • Message expressions
    • messages may be combined in expressions
      • unary have highest precedence, then binary, then keyword
      • associate left to right
      • order may be specified by parentheses
    • messages may be cascaded

ourPen home.

ourPen up.

ourPen goto: [email protected]

smalltalk 80 continued2
Smalltalk 80 (continued)
  • Assignment
    • object <- object
    • index <- index + 5
  • Blocks
    • unnamed objects specified by [ <expressions> ]
      • expressions are separated by .
    • evaluated when they are sent the value message
      • always in the context of their definition
    • may be assigned to variables
      • foo <- [ ... ]
  • Logical loops
    • blocks may contain conditions
    • all blocks have whileTrue methods
    • sends value to condition block
    • evaluates body block if result is true

[ <logical condition> ]

whileTrue:

[ <body of loop> ]

smalltalk 80 continued3
Smalltalk 80 (continued)
  • Iterative loops
    • all integer objects have a timesRepeat method
    • also have
      • to:do:
      • to:by:do:
    • a block is the loop body
  • Selection
    • true and false are also objects
    • each has ifTrue:, ifFalse:, ifTrue:ifFalse:, and IfFalse:ifTrue: methods

12 timesRepeat: [ ... ]

6 to: 10 do: [ ... ]

total = 0 “returns true or false object”

ifTrue: [ ... ] “true object executes this; false ignores”

ifFalse: [ ... ] “false object executes this; true ignores”

smalltalk 80 continued4
Smalltalk 80 (continued)
  • Dynamic binding
    • when a message arrives at an object, the class of which the object is an instance is searched for a corresponding method
    • if not there, search superclass, etc.
  • Only single inheritance
    • every class is an offspring of the root class Object
  • Evaluation
    • simple, consistent syntax
    • relatively slow
      • message passing overhead for all control constructs
      • dynamic binding of message to method
    • dynamic binding allows type errors to be detected only at run-time
slide34
C++
  • Essentially all of variable declaration, types, and control structures are those of C
  • C++ classes represent an addition to type structure of C
  • Inheritance
    • multiple inheritance allowed
    • classes may be stand-alone
    • three information hiding modes
      • public: everyone may access
      • private: no one else may access
      • protected: class and subclasses may access
    • when deriving a class from a base class, specify a protection mode
      • public mode: public, protected, and private are retained in subclass
      • private mode: everything in base class is private
        • may reexport public members of base class
c continued
C++ (continued)
  • Dynamic binding
    • C++ member functions are statically bound unless the function definition is identified as virtual
    • if virtual function name is called with a pointer or reference variable with the base class type, which member function to execute must be determined at run-time
    • pure virtual functions are set to 0 in class header
      • must be redefined in derived classes
    • classes containing a pure virtual function can never be instantiated directly
      • must be derived
slide36
Java
  • General characteristics
    • all data are objects except the primitive types
    • all primitive types have wrapper classes that store one data value
    • all objects are heap-dynamic, referenced through reference variables, and most are explicitly allocated
  • Inheritance
    • single inheritance only
      • but implementing interface can provide some of the benefits of multiple inheritance
      • an interface can include only method declarations and named constants
    • methods can be final (can’t be overridden)

public class Clock extends Applet

implements Runnable

java continued
Java (continued)
  • Dynamic binding is the default
    • except for final methods
  • Package provides additional encapsulation mechanism
    • packages are a container for related classes
    • entries defined without access modifier (private, protected, public) has package scope
      • visible throughout package but not outside
    • similarly, protected entries are visible throughout package
ada 95
Ada 95
  • Type extension builds on derived types with tagged types
    • tag associated with type identifies particular type
  • Classes are packages with tagged types

Package Object_Package is

type Object is tagged private;

procedure Draw (O: in out Object);

private

type Object is tagged record

X_Coord, Y_Coord: Real;

end record;

end Object_Package;

ada 95 continued
Ada 95 (continued)
  • Then may derive a new class by using new reserved word and modifying tagged type exported
  • Overloading defines new methods

with Object_Package; use Object_package;

Package Circle_Package is

type Circle is new Object with record

radius: Real;

end record;

procedure Draw (C: in out Circle);

end Circle_Package

ada 95 continued1
Ada 95 (continued)
  • Derived packages form tree of classes
  • Can refer to type and all types beneath it in tree by type’class
    • Object’class
    • Square’class
  • Then use these as parameters to procedures to provide dynamic binding of procedure invocation

Object

Circle

Square

Ellipse

Rectangle

procedure foo (OC:Object’class) is

begin

Area(OC); -- which Area

-- determined at

-- run time

end foo;

ada 95 continued2
Ada 95 (continued)
  • Pure abstract base types are defined using the word abstract in type and subprogram definitions

Package World is

type Thing is abstract tagged null record;

function Area(T: in Thing) return Real is abstract;

end World;

With World;

package My_World is

type Object is new Thing with record ... end record;

procedure Area(O: in Object) return Real is ... end Area;

type Circle is new Object with record ... end record;

procedure Area(C: in Circle) return Real is ... end Area;

end My_World;

comparing c and smalltalk
Comparing C++ and Smalltalk
  • Inheritance
    • C++ provides greater flexibility of access control
    • C++ provides multiple inheritance
      • good or bad?
  • Dynamic vs. static binding
    • Smalltalk full dynamic binding with great flexibility
    • C++ allows programmer to control binding time
      • virtual functions, which all must return same type
  • Control
    • Smalltalk does everything through message passing
    • C++ provides conventional control structures
comparing c and smalltalk continued
Comparing C++ and Smalltalk (continued)
  • Classes as types
    • C++ classes are types
      • all instances of a class are the same type, and one can legally access the instance variables of another
    • Smalltalk classes are not types, and the language is essentially typeless
    • C++ provides static type checking, Smalltalk does not
  • Efficiency
    • C++ substantially more efficient with run-time CPU and memory requirements
  • Elegance
    • Smalltalk is consistent, fundamentally object-oriented
    • C++ is a hybrid language in which compatibility with C was an essential design consideration
comparing c and ada 95
Comparing C++ and Ada 95
  • Ada 95 has more consistent type mechanism
    • C++ has C type structure, plus classes
  • C++ provides cleaner multiple inheritance
  • C++ must make dynamic/static function invocation decision at time root class is defined
    • must be virtual function
    • Ada 95 allows that decision to be made at time derived class is defined
  • C++ allows dynamic binding only for pointers and reference types
  • Ada 95 doesn’t provide constructor and destructor functions
    • must be explicitly invoked
comparing c and java
Comparing C++ and Java
  • Java more consistent with OO model
    • all classes must descend from Object
  • No friend mechanism in Java
    • packages provide cleaner alternative
  • Dynamic binding “normal” way of binding messages to methods
  • Java allows single inheritance only
    • but interfaces provide some of the same capability as multiple inheritance
implementing oo constructs
Implementing OO Constructs
  • Store state of an object in a class instance record
    • template known at compile time
    • access instance variables by offset
    • subclass instantiates CIR from parent before populating local instance variables
  • CIR also provides a mechanism for accessing code for dynamically bound methods
    • CIR points to table (virtual method table) which contains pointers to code for each dynamically bound method
ad