Programming Languages

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ProgrammingLanguages
CSCI-4430 CSCI-6969April 1, 2008

• David Goldschmidt, Ph.D.
• Computer Science
• The College of Saint Rose

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Abstraction

• Abstraction Xis a view or representationof
  entity X that includes only themost significant
  attributes of X
• Abstractions combat program complexity
• Abstraction X may be a subset of X oralso
  introduce a new interface

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Process Abstraction

• Nearly all programming languagessupport process
  abstraction with subprograms,functions, methods,
  etc.
• Hide implementation details

// e.g. Java double result calculate( x1, x2
) ...
public static double calculate( double x1, double
x2 ) // ... calculate and return a double
// ... implementation details here ...
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Data Abstraction

• An abstract data type is a construct that
  provides
• A type definition that allows program units to
  declare variables of the type, but hides the
  actual representation
• A set of operations for manipulating objects of
  the type
• Abstract data types can be built-in,
  user-defined, or available via libraries or
  packages
• Nearly all programming languages designedsince
  1980 support data abstraction

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Data Abstraction

• Abstract data types satisfy the following
  conditions
• 1. The representation of (and operations on)
  objects ofthe type are defined in a single
  syntactic unit
• Actual implementation (i.e. code) may be
  elsewhere...
• Advantages?
• 2. The representation of objects of the type is
  hidden from program units that use these
  objects, exposingonly those operations provided
  in the types definition
• Advantages?

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Data Abstraction Example

• Consider an abstract data type that representsa
  collection of elements with operations
• create(collection, maxSize)
• destroy(collection)
• add(collection, element)
• remove(collection, element)
• isEmpty(collection)
• isFull(collection)
• size(collection)

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Data Abstraction Example

• Clients use a types abstract interface tomanage
  collections ofpotentially varied data
• Is coll an array?
• Is coll a linked list?
• Is coll a hash table?

create(coll, 20) while (not isFull(coll)) //
input student info ... add(coll,
student) if (isFull(coll)) display("Course
is full!")
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Data Abstraction Example

• We can define one abstract data type based on
  another to focus or restrict its use e.g. a
  stack
• create(stack, maxSize)
• destroy(stack)
• push(stack, element)
• pop(stack)
• isEmpty(stack)
• isFull(stack)
• size(stack)

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Data Abstraction in C

• In C and C, use struct for data abstraction
  requiring only data
• In C, a class defines an abstract data type
  typically with both data and operations
• Data members are the data
• Member functions are the operations

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Data Abstraction in C

• Define data members and member functions as
  either class members or instance members
• A class member is associated with the class
• An instance member is associated with objects
• All class instances share a single copyof the
  member functions
• Each instance of a class has its own copyof the
  class data members
• Instances can be static, stack dynamic, or heap
  dynamic

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Data Abstraction in C

• Instances can be stack dynamic or heap dynamic
• If stack dynamic, class instances are
  referenceddirectly with value variables
• If heap dynamic, classinstances are
  referencedvia pointers
• Advantages and disadvantages?

main() stack stk stk.push(21)
stk.push(18) cout ltlt stk.pop() ...
main() stack stk stk new stack
stk-gtpush(21) stk-gtpush(18) cout ltlt
stk-gtpop() ...
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Data Abstraction in C

• Constructors are functions that initialize
  thedata members of class instances
• Initialization might allocate storage (via
  new)if data members are heap-dynamic
• Implicitly called when an instance is created
• Can also be explicitly called
• Name is the same as the class name

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Data Abstraction in C

• Destructors are functions that clean up aftera
  class instance is destroyed
• Typically used to reclaim heap storage (via
  delete)
• Implicitly called when the objects lifetime ends
• Can also be explicitly called
• Name is the class name, preceded by a tilde ()

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Data Abstraction in C

• Information hiding in C is supportedvia
  visibility clauses
• public defines visible entities
• private defines hidden entities
• protected defines hidden entities available for
  use in child classes

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Data Abstraction in C
class stack private int stackPtr
int maxLen int topPtr public
stack() // default constructor stackPtr
new int100 maxLen 99 topPtr
-1 stack () delete stackPtr
void push(int number) ... int pop()
... int peek() ... int isEmpty()
...
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Data Abstraction in C

• Friend functions and friend classes
  provideaccess to private members to
  unrelatedprogram units or functions
• Allows for multiple classes to view and
  modifyone anothers private data members and
  functions

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Data Abstraction in Java

• Java is similar to C, except
• All user-defined types are classes
• i.e. no structs
• Java also supports pure abstractions called
  interfaces
• All objects are allocated from the heap
  andaccessed through reference variables
• Individual entities in classes have access
  control modifiers (private or public) rather than
  clauses

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Data Abstraction in Java
class Stack private int stack private
int maxLen, topIndex public Stack() //
default constructor stack new int100
maxLen 99 topIndex -1 public
void push(int number) ... public int
pop() ... public int peek() ...
public boolean isEmpty() ...
Stack s // reference variable s new
Stack() s.push(21) s.push(18) System.out.print(
s.pop()) ...
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Data Abstraction in Java

• Use static to define methods that are available
  without creating an instance of an object
• e.g. Math.random(), System.out.println(), etc.
• Omit static to define instance methods that are
  available only after an object is instantiated
• Circle c1 new Circle(12.5)
• double area c1.getArea()

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Java Garbage Collection

• Objects that are no longer referenced are garbage
  
• The Java Virtual Machine ( JVM ) automatically
  performs garbage collection
• Frees up memory used by garbage
• Usually sufficient to allow JVM to
  performgarbage collection at its own pace
• To force garbage collection on an object,assign
  its reference variable to null
• Can also try calling System.gc()

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Visibility Modifiers in Java

• Visibility modifiers can be applied toany class,
  variable, or method
• Using private, the class, variable, ormethod can
  only be accessed by the declaring class
• Using public, the class, variable, or method
  canbe accessed by any class in any package
• Omitting the modifier, the class, variable, or
  methodcan be accessed by any class within the
  same package

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Object-Oriented Programming

• Object-oriented programming is aprogramming
  paradigm that combines
• Abstract data types
• Inheritance (a.k.a. generalization)
• Dynamic binding (a.k.a. polymorphism)
• Languages often provide OOP in addition to
  procedural and other language constructs

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Inheritance

• Much of our world is organized(or organizes
  itself ) into hierarchies
• Inheritance is used to modelparent-child
  relationships
• Key benefits are organization and reuse
• e.g. Bank accounts, geometric shapes, people,
  etc.
• BankAccount ? CheckingAccount
• Polygon ? Rectangle ? Square
• Person ? Student ? GraduateStudent

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Inheritance

• UML diagram representing Generalization
• Children classes are more specific
• Parent classes are more general

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Inheritance

• A class that inherits methods and data membersis
  called a derived class, a subclass, or a child
  class
• The class from which another class inheritsis a
  parent class or a superclass
• Methods define operations on data members
• Calls to methods are often called messages
• A class inherits all or some of the entitiesof
  its superclass
• e.g. in Java, constructors are not inherited

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Inheritance

• A subclass can modify an inherited method
• The new method overrides the inherited one
• Overriding inherited methods provides formore
  specific operations in the subclass
• Similar to determining scope of variables,when a
  method is called,its specific data type (i.e.
  its class)must be determined

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Polymorphism

• The Substitutability Principle
• Given an inheritance relationship, we can use
  achild element anywhere the parent is expected
• We can use the morespecific element anywherethe
  more general elementis expected

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Polymorphism

• Polymorphic operations have the same
  signature,but different implementations

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Polymorphism

• Polymorphism allows you to send the same message
  to different classes (and the objects respond
  appropriately)

Canvas
r1.draw()
r1Rectangle
c2.draw()
c2Circle
c3.draw()
c3Circle
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Dynamic Binding

• Polymorphism is achieved via dynamic binding
• More specifically, dynamically bind messagesto
  method definitions at run time
• Based on class hierarchies
• Advantages and disadvantages
• Has the advantage of being easily extensible
• Improves writability of reusable methods
• Key disadvantage is run time performance

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Dynamic Binding in C
public class shape public // pure virtual
function virtual void draw() 0
... public class circle public shape
public void draw() ... ... public
class rectangle public shape public
void draw() ... ... public class square
public rectangle public void draw()
... ...
square sq new square rectangle r new
rectangle shape sptr // Cannot create a shape
object sptr new shape // Point sptr to the
square object sptr sq // Dynamically bind to
draw() // method of the square
class sptr-gtdraw() // Statically bind to
draw() // method of the rectangle
class r-gtdraw()
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Dynamic Binding in Java
public class Shape public void draw() // do
nothing ... public class Circle extends
Shape public void draw() ...
... public class Rectangle extends Shape
public void draw() ... ... public class
Square extends Rectangle public void draw()
... ...
Square sq new Square() Rectangle r new
Rectangle() Shape shape // Cannot create a
shape object shape new Shape() // Point shape
to the square object shape sq // Dynamically
bind to draw() // method of the square
class shape.draw() // Statically bind to
draw() // method of the rectangle class r.draw()
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Interfaces in Java
public interface Shape public void draw()
... public class Circle implements Shape
public void draw() ... ... public class
Rectangle implements Shape public void draw()
... ... public class Square extends
Rectangle public void draw() ... ...
Square sq new Square() Rectangle r new
Rectangle() Shape shape // Cannot create a
shape object shape new Shape() // Point shape
to the square object shape sq // Dynamically
bind to draw() // method of the square
class shape.draw() // Statically bind to
draw() // method of the rectangle class r.draw()
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Interfaces (Abstract Base Classes)

• An interface consists of a set of public
  operations that have no implementation

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Interfaces (Abstract Base Classes)

• Why use interfaces?
• Readability Interfaces are typically very
  concise
• Reusability Interfaces provide a contract that
  many classes may implement to realize a service
• Flexibility of Implementation Interfaces define
  only operation signatures and hint at semantics
• Architectural Construct Interfaces represent
  architectural seams, separating specification
  from implementation
• Ease of Integration Sharing interfaces with
  other systems

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Example Interfaces

• Interface names often end in the -able suffix

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Example Interfaces

• More interfaces that end in the -able suffix

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Example Interfaces

• Interfaces that dont end in the -able suffix

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Example Interfaces

• More interfaces that dont end in the -able
  suffix

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HW 4/1

• Suppose the abstract data type Stack provideda
  peek() function that returned an access path
  (i.e. a pointer or reference) to the top
  elementof the stack
• Why is this not a true data abstraction?
• What is the purpose of Java (and Python) classes
  having a common ancestor (i.e. the Object class)?

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HW 4/1

• Use Java, C, C, or another languagewith OOP
  support to create a Queue classfor integers
• Be sure that the class only allows for elements
  to be added to the end of the queue
• Further, elements may only be viewed or removed
  from the beginning of the queue
• Create an entirely static version of the
  class,then create an entirely dynamic version of
  the class
• Modify your Queue class to support elements of
  any data type

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Reading Assignments

• Read for this week
• Chapters 11-13