A Survey of Object-Oriented Concept

1. A Survey of Object-Oriented Concept Oscar Nierstrasz

2. 1.1 What is Object-Oriented Programming Language • Restrictive – The programming language that supports both object classes and class inheritance. • Loose - Any programming language that provides mechanisms that can be used to exploit encapsulation is (at least to some degree) object-oriented.

3. Object models for programming languages often encapsulate objects in terms of a set of operations as a visible interface, while hiding the object’s realization.

4. Our Discussion will center on: • 1.1.1 Reusability • 1.1.2 Object types • 1.1.3 Concurrency

5. 1.1.1 Reusability • Instantiation • Object Classes • Inheritance • Polymorphism • Overloading • Generic Classes

6. Instantiation • Instantiation is perhaps the most basic OO reusability mechanism. • There are static and dynamic instantiation. • Statically instantiated objects are allocated at compile time and exist for the duration that the program executes. • Dynamically instantiated objects require run-time support for allocation and for either explicit deallocation or some form of garbage collection.

7. Instantiation • One way for programmers to define and instantiate their own objects is providing the programmer with a facility to define object classes. • An alternative approach to instantiation is to use prototypical objects.

8. Object Classes • An object class specifies a set of visible operations, a set of hidden instance variables, and a set of hidden methods that implement the operations. • The difference between object classes and prototypical objects is brought out sharply when viewed in terms of applicable inheritance mechanisms.

9. Inheritance • In most cases, inheritance is strictly a reusability mechanism for sharing behavior between objects. • Inheritance can be statically or dynamically. • Class inheritance is often represented as the fundamental feature that distinguishes object-oriented from other programming languages. • Single inheritance • Multiple inheritance. • Dynamic inheritance refers to mechanisms that permit objects to alter their behaviour in the course of normal interactions between objects. • Part inheritance. • Scope inheritance.

10. Some interesting points concering class inheritance • Not all languages with class inheritance support multiple inheritance. • It is important to be able to override inherited methods • Subclass may or may not be permitted direct access to inherited instance variables. • The issue of name clashes in teh presence of multiple inheritance.

11. Polymorphism and overloading • A polymorphic function is one that can be applied uniformly to a variety of objects. • Class inheritance is closely related to polymorphism. • Polymorphism enhances software reusability by making it possible to implement generic software that will work not only for a range of existing objects, but also for objects to be added later.

12. Generic classes • Generic object classes achieves software reusability by partially describing a class and parameterizing the unknowns. • Two basic categories of generic object: • Container objects: arrays and lists. • Tool objects: sorters and editors. • In case of tool objects, the parameter must be constrained to indicate the required parent class of the parameter.

13. 1.1.2 Object types • An object type is superficially the same thing as an object class. The difference is that when we manipulate typed objects, we can make static type-checking. • The declared types of a varibles and of arguments to operations serve as specification for balid bindings and invocations. • In the presence of dynamic binding, it is generally impossible to determine statically the class of a variable, but with teh appropriate type rules we can still perform type-checking.

14. 1.1.3 Concurrency • Two ways in which programing languages have dealt with concurrency and communication. • Active entities (process)communicate indirectly through shared passive objects. • Active entities communicate directly with one another by message passing.

15. 1.2 OO systems • Two important kinds of OO system • Those that provide run-time support for OO application. • Those that form an environment for OO software development.

16. 1.2.1 Object Management • Object management refers to a mixed bag of run-time issues such as concurrency, distribution, version control and security. • Minimal object management support is provided for C++ objects. • Smalltalk and LISP additionally provide fro automatic garbage collection. • Object-Oriented Operating system may provide support for persistence, reliable communication, ro distributed object-naming at a low level. • OO database.

17. 1.2.2 OO programing Environment • This is about tools and environments for application development. • First problem is how should we decompose our application into objects in such a way as to best exploit the OO pradigms available to us? (consept model techniques) • The next problem is that of selecting objects from a software base (personal expertise, software document, and browsing tools). • Third problem is to cope with evolution of software base (making interface unchanged)

18. Summary • The term Object-oriented is best interpreted as referring to any approach that exploits encapsulation or packaging in the process of designing and building software. • We have surveyed OO techniques in programing languages and system support.