ceg860 (Prasad)L7Class1 Classes

ceg860 (Prasad)L7Class2 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.)

ceg860 (Prasad)L7Class3 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

ceg860 (Prasad)L7Class4 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.

ceg860 (Prasad)L7Class5 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

ceg860 (Prasad)L7Class6 ADT Table ( symbol table/directory ) empty : Table update : Key x Info x Table  Table lookUp: Key x Table  nfo 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”)

ceg860 (Prasad)L7Class7 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.

ceg860 (Prasad)L7Class8 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-programmingMeta-programming is achieved through appropriate libraries supporting Reflection and Introspection. E.g., In Java 1.1, package java.lang.reflect. Java approach to metaclass benefits

ceg860 (Prasad)L7Class9 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.

ceg860 (Prasad)L7Class10 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);

ceg860 (Prasad)L7Class11 Feature Feature Classification by Role Procedure Function Function Attribute (Field) (Field) No resultReturns result ArgumentsNo Arguments ComputationMemory Routine

ceg860 (Prasad)L7Class12 Feature Feature Classification by Implementation ComputationMemory ProcedureFunction No result Result Routine(Method) Attribute (Field) (Field)

ceg860 (Prasad)L7Class13 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 –Eiffel : 0-ary routine f and attribute f have the same syntax. Attributes cannot be directly assigned. –Java SmallTalkJava Beans, –Java: 0-ary method syntax is f() (not f ). In SmallTalk/Java Beans, a field is accessed via getF() and/or setF( valueType ) methods.

ceg860 (Prasad)L7Class14 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

ceg860 (Prasad)L7Class15 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; }

ceg860 (Prasad)L7Class16 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.

ceg860 (Prasad)L7Class17 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.

ceg860 (Prasad)L7Class18 Selective Export and Information Hiding EiffelEiffel has a general mechanism to export a feature to designated clients and their proper descendants. JavaC++Java and C++ support three levels: –public : all clients. –private : (all instances of) current class. –protected : all subclasses. –Default : package wide visibility.

ceg860 (Prasad)L7Class19 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.

ceg860 (Prasad)L7Class20 Genericity Unconstrained ©Type parameterized classes  Generic type : List[G]  Instantiations: List[int], List[String], List[Window], etc Constrained ©Types with operations  Constrained Genericity: Ordered, Group, etc  Java interface types.

ceg860 (Prasad)L7Class21 type INTMAT is array ( POSITIVE range <>, POSITIVE range <>) 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; return (sum); end;

ceg860 (Prasad)L7Class22 generic type ET is private; type INDEX is (<>); ID : ET;type AR is array ( INDEX range <>, INDEX range <>) of ET; with function "+" ( X,Y : ET ) return ET is <>; 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; return (sum); end;