Lecture 1. Abstraction

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1 Lecture 1 Abstraction 1

2 Programming Techniques Unstructured Programming Procedural Programming Modular & Structural Programming Abstract Data Type Object-Oriented Programming 2

3 Unstructured Programming Usually, people start learning programming by writing small and simple programs consisting only of one main program. Here ``main program'' stands for a sequence of commands or statements which modify data which is global throughout the whole program. Main Program Data 3

4 Procedural Programming With procedural programming, you are able to combine sequences of calling statements into one single place. A procedure call is used to invoke the procedure. After the sequence is processed, flow of control proceeds right after the position where the call was made. Main Program Procedure 4

5 Procedure Program view Now a program can be viewed as a sequence of procedure calls. The main program is responsible to pass data to the individual calls, the data is processed by the procedures and the resulting data is presented. Thus, the flow of data can be illustrated as a hierarchical graph, a tree. 5

6 Procedure Program view Main Program Data Procedure 1 Procedure 2 Procedure 3 6

7 Modular Programming With modular programming, procedures of a common functionality are grouped together into separate modules. A program therefore no longer consists of only one single part. It is now divided into several smaller parts which interact through procedure calls and which form the whole program. 7

8 Main Program(Also a module) Data Module 1 Module Data Data 1 Data Data 2 Procedure 1 Procedure 2 Procedure 3 The main program coordinates calls to procedures in separate modules and hands over appropriate data as parameters. 8

9 Structural Programming How many basic structures for programming? Three Types of Structures in a structured program Statement sequence(s1,s2,,sn) Branch(if-then-else) Loop(for,do, and while loops) Keep no goto!!! Keep modules!!! 9

10 Abstraction --Handling Problems Try to understand the problem to separate necessary from unnecessary details: You try to obtain your own abstract view, or model, of the problem. This process of modeling is called abstraction. 10

11 Abstraction A named collection of attributes and behavior relevant to modeling a given entity for some particular purpose 11

12 Abstraction Problem Model 12

13 Mapping Abstraction to Software real-world abstraction software attributes {data, data, } entity behavior {method, method, } 13

14 Abstraction 14

15 Abstraction of a Sales Person 15

16 Abstraction of a Patient in a Medical Database 16

17 Is-a a and Has-A A abstraction Two of the most important types of abstraction are the following: Division into parts -- Has-a abstraction Division into specialization -- Is-a abstraction 17

18 Is-A A Abstraction Is-a abstraction takes a complex system, and views it as an instance of a more general abstraction. Characterized by sentences that have the words ``is=a'' A car is a wheeled vehicle, which is-a means of transportation A bicycle is-a wheeled vehicle A pack horse is-a means of transportation Allows us to categorize artifacts and information and make it applicable to many different situations. 18

19 Has-A A Abstraction Division into parts takes a complex system, and divides into into component parts, which can then be considered in isolation. Characterized by sentences that have the words ``has-a'' A car has-a engine, and has-a transmission A bicycle has-a wheel A window has-a menu bar Allows us to drop down a level of complexity when we consider the component in isolation. 19

20 Abstract Data Types(ADTs) Abstraction Model Properties of Abstract Data Types Abstract Data Types and Object-Orientation 20

21 Properties of ADT With abstraction, you create a well-defined entity which can be properly handled. These entities define the data structure of a set of items. For example, each administered employee has a name, date of birth and social number... 21

22 Properties of ADT(Cont d) The data structure can only be accessed with defined operations. This set of operations is called interface and is exported by the entity. An entity with the properties just described is called an abstract data type (ADT). 22

23 ADT Abstract Data Type Abstract Data Structure Operations Interface 23

24 Definition (ADT) ADT is characterized by the following properties: 1. It exports a type. 2. It exports a set of operations. This set is called interface. 3. Operations of the interface are the only access mechanism to the type's data structure. 4. Axioms and preconditions define the application domain of the type. 24

25 Example: ADT List Type List. The interface to instances of type List is defined by the interface definition file. Operations: insert, get, append, delete,search, 25

26 List The application domain is defined by the semantical meaning of the provided operations. Axioms and preconditions include statements such as ``An empty list is a list.'' ``Let l=(d1, d2, d3,..., dn) be a list. Then l.append(dm) results in l=(d1, d2, d3,..., dn, dm).'' ``The first element of a list can only be deleted if the list is not empty.'' 26

27 Encapsulation Combine the data and the operations Enclosing of both variables and functions Keep details of the data and operations from the users of the ADT Once you have created an ADT for complex numbers, say Complex, you can use it in the same way like well-known data types such as integers. 27

28 Encapsulation cont d Allows Modularity Controlled access to data Separates implementation from interface It extends the built-in types 28

29 Object-Oriented Programming Object is derived from abstract data type Object-oriented programming has a web of interacting objects, each house-keeping its own state. Objects of a program interact by sending messages to each other. 29

30 Object 1 Data 1 +Procedures 1 Object 2 Data 2 + Procedures Data 12 Object3 Data 3 + Procedures 3 Object 4 Data 4 + Procedures 4 30

31 Object-Oriented Programming cont d In object-oriented programming, instead of calling a procedure which we must provide with the correct handle, we would directly send a message to the object in questions. Roughly speaking, each object implements its own module. 31

32 Object-Oriented Programming cont d Each object is responsible to initialize and destroy itself correctly. Consequently, there is no longer the need to explicitly call a creation or termination procedure. 32

33 Kay s Description of OOP Everything is an object Objects perform computation by making requests of each other through the passing of messages Every object has it's own memory, which consists of other objects. Every object is an instance of a class. A class groups similar objects. The class is the repository for behavior associated with an object Classes are organized into singly-rooted tree structure, called an inheritance hierarchy. 33

34 Object-Oriented Approach 34

35 35

36 ADT and Object-Orientation ADTs define functionality by putting main emphasis on the involved data, their structure, operations as well as axioms and preconditions. Object-oriented programming is ``programming with ADTs'': combining functionality of different ADTs to solve a problem. Therefore instances (objects) of ADTs (classes) are dynamically created, destroyed and used. 36

37 The Formula Object-Orientation = Object (Abstraction) + Class (ADT & Classification) + Inheritance (Reusing) + Dynamic Binding (Polymorphism, Flexibility) 37

38 Inheritance(Hierarchy) Expresses commonality among objects Allows code reusability Highlights Generalization/Specialization relationships We will learn more in the following lectures. 38

39 Polymorphism The ability of objects to respond differently to the same message or function call. We will discuss more in the following lectures. 39

40 Object-Orientation Evolution Modules Information hiding Data encapsulation Abstract data types Objects 40

41 Simply Remember Encapsulation(Data & Operations)--- A technique for Information Hiding-----the users of the objects could not see the details of the data and operations of the objects. Data Abstraction ---- the procedure to find a class from objects. Abstract Data Type---- Class. 41

Elementary Concepts of Object Class

Elementary Concepts of Object Class Modeling entities and their behaviour by objects. A class as a specification of objects and as an object factory, computation as message passing/function call between