OOPs

1. OOPs Object oriented programming

2. 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

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

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

5. 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

6. 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

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

8. Scope & objects in java looking for a variable while executing a method: block, local, instance, class, superclass 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; } } } superclass w class z instance x method r,q block t w in superclass, either protected or public, may be static

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

10. 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)

11. Need for multiple inheritance • ‘orthogonal’ properties – the ‘diamond’ problem Vehicle GMVehicle KiaVehicle Car, SUV properties repeated Manufacturer properties repeated GMCar GMSUV KiaCar KiaSUV Vehicle Car SUV GMCar KiaCar GMSUV KiaSUV

12. Using multiple inheritance Vehicle GMVehicle KiaVehicle Car SUV GMCar GMSUV KiaCar KiaSUV

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

14. 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

15. 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

16. Polymorphism in method calls 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(); class Super method x() class Sub1 method x() class Sub2 class Sub3 method x()

17. 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

18. 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

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

20. 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 X X X X

21. 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 Remove some access Stack Queue

22. C++ • Inheritance – partial – no single hierarchy • Efficient but complex

23. 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

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

25. 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?