Classes

1
Classes
2

• Class Instance (Object)
• Static structure vs Run-time structure
• (Analogy -- a statue Lincoln Memorial)
• Class Abstract Data Type
• A class is an ADT equipped with a possibly
  partial implementation.
• Class Metaclass
• A metaclass is a class whose instances are
  classes. (Useful for manipulating class
  representations at run-time by interpreters,
  browsers, etc.)

3
Abstraction Equivalence Relations

• Computability
• Recursive vs Non-recursive
• Semantics
• Behavioral Equivalence
• Resource-independent interchangeability
• Performance aspect irrelevant for correctness
• E.g., Groups, Fields, Sorting, UNIX, etc
• Complexity (Algorithms)
• Time and Space requirements
• Big-Oh (Worst-case Analysis)
• NP-hard vs Polynomial-time

4
Specification of Data Types Motivation

• Precise and unambiguous description.
• Sound and complete description.
• Avoids over-specification.
• Spec Flexible enough to accommodate all possible
  implementations.
• Impl. Constraining enough to exploit
  representation details for efficiency.

5
Specification of Data Types Details

• Type Values Operations
• Specify
• Syntax Semantics
• Signature of Ops Meaning of Ops
• Model-based
  Axiomatic(Algebraic)
• Description in terms of
  Give axioms satisfied
• standard primitive data types
  by the operations

6
ADT Table (symbol table/directory)

• empty Table
• update Key x Info x Table -gt Table
• lookUp Key x Table -gt Info
• lookUp(K,empty) error
• (Use of variable) (Alternative Use of
  Preconditions)
• lookUp(K,update(Ki, I, T))
• if K Ki then I else
  lookUp(K,T)
• (last update overrides the others)

7
Implementations

• Array-based
• LinearList-based
• Tree-based
• Binary Search Trees, AVL Trees, B-Trees etc
• HashTable-based
• These exhibit a common Table behavior, but differ
  in performance aspects.
• Correctness of a client program is assured even
  when the implementation is changed.

8
Java approach to metaclass benefits

• To make available certain features to all
  classes, Java defines a root class (class
  Object) with those features. E.g., Thread support
  (locks), cloning, equality check, string
  representation, etc
• Certain class operations are given special
  status. E.g., creation and initialization of an
  object is incorporated via constructors.
• Meta-programming is achieved through appropriate
  libraries supporting Reflection and
  Introspection. E.g., In Java 1.1, package
  java.lang.reflect.

9
Class as module and type

• Module (unit of software decomposition)
• Syntactic Only affects the form of software
  text, not what the software can do. Separately
  compilable.
• Type (static description of dynamic objects)
• Semantic Influences execution of a software
  system by defining objects manipulated at
  run-time.
• The facilities provided by a class, viewed
  as a module, are precisely the operations on
  instances of the class.

10
Example class Point

• Features/Members
• Attributes/Fields (represented using memory)
• Routines/Methods (represented using computation)
• real x-coord, y-coord
• real radius, angle
• real distance(Point)
• void translate(real x,y)

11
Feature Classification by Role
No result
Returns result
Procedure
Arguments
No Arguments
Function
Computation
Memory
Routine
Function
Attribute (Field)
12
Feature Classification by Implementation
Computation
Memory
Routine (Method)
Attribute (Field)
Result
No result
Procedure
Function
13
Uniform Access principle

• A client should be able to access a property
  (feature) of an object using a single notation
  whether the property is implemented by memory or
  by computation.
• Eiffel 0-ary routine f and attribute f have
  the same syntax. Attributes cannot be directly
  assigned.
• Java 0-ary method syntax is f() (not f). In
  SmallTalk/Java Beans, a field is accessed via
  getF() and/or setF(valueType) methods.

14
Object-Oriented Style of Computation

• Single Target Principle
• Every operation is relative to a certain object.
• E.g., pt1.distance(pt2)
• Current Instance
• May be implicit, or referred to explicitly using
  a keyword such as this (Java/C), self (Self),
  Current (Eiffel), etc.
• Module-Type Identification

15

• double distance( Point p )
• if (p / this)
• return Math.sqrt(
• (x - p.x) (x - p.x)
• (y - p.y) (y - p.y) )
• else
• return 0

16
Composition Client and Supplier

• A class C which contains a declaration of the
  form S x is said to be a client of S. S is
  said to be a supplier (server) of C.
• x may be an attribute or a function of C, or a
  local entity or argument of a routine of C.

17
Feature Calls

• Qualified calls
• x.f or x.f()
• x.f(a,b,)
• Unqualified calls
• f or f()
• f(a,b,)
• Equivalently,
• this.f or this.f()
• this.f(a,b,)

• Operators
• infix
• associativity
• prefix
• Operator definitions supported in order to be
  consistent with the standard mathematical
  notations.
• A B, - 5, etc.

18
Selective Export and Information Hiding

• Eiffel has a general mechanism to export a
  feature to designated clients and their proper
  descendants.
• Java and C support three levels
• public all clients.
• private (all instances of) current class.
• protected all subclasses.
• Default package wide visibility.

19
Function In-lining in C

• Interface file ( .h extension ).
• Contains prototype declarations, and (short)
  function definitions that may be in-lined.
• Implementation file ( .c extension ).
• Contains function definitions, possibly with
  inline directive.
• In-lining enables programmers to have benefits of
  encapsulation while avoiding some of the run-time
  costs of function invocation.

20
Genericity

• Unconstrained
• Type parameterized classes
• Generic type ListG
• Instantiations Listint, ListString,
  ListWindow, etc
• Constrained
• Types with operations
• Constrained Genericity Ordered, Group, etc
• Java interface types.

21

• type INTMAT is array ( POSITIVE range ltgt,
  POSITIVE range ltgt) of integer
• function int_sum_all( M INTMAT ) return
  integer is
• sum integer 0
• begin
• for I in M'range(1) loop
• for J in M'range(2) loop
• sum sum M(I,J)
• end loop
• end loop
• return (sum)
• end

22

• generic
• type ET is private type INDEX is (ltgt)
• ID ET type AR is
• array ( INDEX range ltgt, INDEX range
  ltgt) of ET
• with function "" ( X,Y ET ) return ET is ltgt
• function sum_all( M AR ) return ET
• function sum_all( M AR ) return ET is
• sum ET ID
• begin
• for I in M'range(1) loop
• for J in M'range(2) loop
• sum sum M(I,J)
• end loop
• end loop
• return (sum)
• end