Abstraction CMPS 2143

2 Abstraction Abstraction mechanisms are techniques to deal with creating, understanding and managing complex systems Abstraction is the purposeful suppression or hiding of some details of a process or artifact in order to bring out more clearly other aspects, details or structures Through abstraction one builds a model of the actual system

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Abstraction in Structured Programming void main () { ifstream infile; ofstream outfile; Container container; openFiles (infile, outfile); while moreData (infile) getData (infile, container); organizeData (container); displayData (container); closeFiles (infile, outfile); } 4

Another way to think of abstraction Factoring out redundancy ▫ Use loops ▫ Use functions (in structured programming) ▫ Use Abstract Data Types (ADTs) ▫ Use classes (in OOP) 5

6 Abstraction layers in OOP At the highest level a program is viewed as a community of cooperating objects. Each object in this community provides services At this highest level the important features are the services provided by each object and the lines of cooperation and communication between them.

7 Abstraction layers in OOP The next level of abstraction allows a group of objects working together to be grouped in a unit (packages, name spaces, units) allowing some names to be exposed to the world outside the unit while others remain hidden inside the unit library.dll

8 Abstraction layers in OOP The next two levels of abstraction deal with the interaction between 2 individual objects where one (the client) uses services from the other (the server) ▫ Server code advertises the services it can provide for the client as an interface (Java) or.h file (C++) and provides a concrete implementation for its interface ▫ Client code programs to this interface import java.util.*; Stack intStack = new Stack(); intStack.push (“data”);

9 Abstraction layers in OOP The last level of abstraction considers a single task in isolation, that is which steps implement a single method public class Stack implements IStack..{ : public void push(E e) { if top == MAX) throw new FullStackException(); items[++top] = e; } : }

10 General Forms of Abstraction abstraction division into parts “has a” multiple views Specialisation “is a” catalogs repetition service view class hierachies ADT OO programs patterns composition recursive data structures recursive algorithms dictionaries cross-references

11 Abstraction mechanisms in programming languages Procedures and Functions (function centered view) + information hiding for the detail of the behavior - no information hiding for the detail of the data - no encapsulation Modules and Packages (data centered view) + information hiding + encapsulation - instantiation not always supported Abstract Data Types + separates interface and implementation

12 Abstraction mechanisms in OO programming languages Classes are as Abstract Data Types in a service-centered view Message Passing and Method Binding bring polymorphism leading to more readable code Class Hierarchies and Inheritance bring code sharing which results in increased functionality and reduction of code size Inheritance and polymorphism together allow for tailoring shared code

Object Oriented Design Object oriented software development is NOT about the syntax of a programming language Object oriented software development is about a design technique driven by the determination and delegation of responsibilities Responsibility implies non-interference, it cuts or reduces the links between objects that depend on implementation details Goes beyond information hiding and encapsulation when dealing with programming in the large 13

References Budd Book 14