OOPs Object oriented programming

Based on ADT principles  Representation of type and operations in a single unit  Available for other units to create variables of the type  Possibility of hiding data structure of type and implementation of operations

Limitations of ADT --> OOP extensions  Software reuse: ADTs too rigid --> inheritance  No structure to collections of ADTs --> inheritance OOP provides class hierarchies

The three* hierarchies of OOP  Inheritance: 1 - n  Instantiation 1 - n  Composition 1 - n superclass subclass class instance object instance variable * Not including interfaces

Inheritance  Superclass – subclass Subclass inherits all methods, variables Subclass can add methods, variables Subclass can overwrite all methods, variables -> increased reusability -> structure collections of classes

Access to variables and methods  ADTs – public (global scope) or private (class scope)  Objects - public or private -plus protected (class plus subclasses)? Design decision: info hiding vs efficiency  Java adds package scope

Scope and objects - java looking for a variable while executing a method: block, local, instance, class extensions: inner class, package, inheritance public, protected, private

superclass w class z instance x Scope & objects in java block t class Nest extends Bird { int x=10; static int z=0; void doIt(int q) { int r = 100; { int t = r+x+z+q+w; } class Nest extends Bird { int x=10; static int z=0; void doIt(int q) { int r = 100; { int t = r+x+z+q+w; } looking for a variable while executing a method: 1.block, 2.local, 3.instance, 4.class, 5.superclass w in superclass, either protected or public, may be static method r,q

Scope and objects - java extensions: inner class - static scoping protected, private, public(looking inward) packages  access to classes, methods, public variables  importing

Inheritance rules  Single inheritance (Smalltalk) – subclass has only one superclass; classes form an inheritance tree  Multiple inheritance (C++) – subclass can have multiple superclasses  Single inheritance plus interfaces (Java)

Need for multiple inheritance  ‘orthogonal’ properties – the ‘diamond’ problem Vehicle GMVehicleKiaVehicle GMCarGMSUVKiaCarKiaSUV Vehicle CarSUV GMCarGMSUVKiaCarKiaSUV Car, SUV properties repeated Manufacturer properties repeated

Using multiple inheritance Vehicle KiaVehicleGMVehicleCarSUV GMCarGMSUVKiaCarKiaSUV

Problems of multiple inheritance  Identifier conflict: Class Super1 int x, y Class Super2 double x, z Class Sub extends Super1, Super2 Which x?

Interfaces solve the multiple inheritance problem  No variables in interfaces  No method implementations in interfaces – only protocols BUT  Weaker inheritance – no gain in code reuse

Type checking  none (Smalltalk typeless) message compatibility  subclasses are subtypes* type compatibility – object of subtype can be assigned to identifier declared of superclass type dynamic type checking provides polymorphism

Polymorphism in method calls class Super method x() class Sub1 method x() class Sub3 method x() class Sub2 Super a,b,c,d; a = new Super(); b = new Sub1(); c = new Sub2(); d = new Sub3(); a.x(); b.x(); c.x(); d.x(); Super a,b,c,d; a = new Super(); b = new Sub1(); c = new Sub2(); d = new Sub3(); a.x(); b.x(); c.x(); d.x();

Polymorphism means dynamic type checking  class of object must be determined at run time methods bound to messages dynamically parameters type checked dynamically BUT  protocol can be verified statically -> purpose of abstract methods

Forced static binding  if a method cannot be overridden, it can be statically bound C++ assumes methods cannot be overridden -> statically bound – virtual keyword causes dynamic binding Java (and Objective C) assumes methods can be overridden -> dynamic binding – final keyword allows static binding

Implementing dynamic method invocation Superclass method table (VMT) -methA -methB Superclass method table (VMT) -methA -methB Subclass method table (VMT) -methA -methC Ref to parent VMT Subclass method table (VMT) -methA -methC Ref to parent VMT Superclass var1; var1 = new Subclass(); var1.methA(); var1.methB(); var1.methC(); // compile error var1 Instance vars; Ref to class VMT Instance vars; Ref to class VMT

Allocation and de-allocation  What kinds of object memory allocation? Static Stack-dynamic Heap-dynamic  Heap reclamation Programmer control Garbage collection C++Smalltalk,Java X X

C++  Access: Classes private, public Members private, public, protected  Extending class can change access Override access to members in parent Valuable for ‘narrowing’ access polymorphism problem List StackQueue Remove some access

C++  Inheritance – partial – no single hierarchy  Efficient but complex

Java  Closer to pure object language than C++ No functions Dynamic by default One hierarchy of classes BUT  Imperative statements  Primitive data types are not objects

Smalltalk - pure OOP  every statement is a message with an object recipient and method invocation

Another ‘object’ model  Property lists / attribute-value pairs E.g., Lisp symbols: property list E.g., Javascript objects: hash table Properties are typeless – data and methods Set of properties is dynamically varying Ideally suited to tabulation – visual programming Are they objects? ADTs? Variant records?