08-02-2012, 01:41 PM
Data Types
[attachment=17287]
Some authors describe object-oriented programming as programming abstract data types and their relationships. Within this section we introduce abstract data types as a basic concept for object-orientation and we explore concepts used in the list example of the last section in more detail.
3.1 Handling Problems
The first thing with which one is confronted when writing programs is the problem. Typically you are confronted with ``real-life'' problems and you want to make life easier by providing a program for the problem. However, real-life problems are nebulous and the first thing you have to do is to 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 and is illustrated in
The model defines an abstract view to the problem. This implies that the model focusses only on problem related stuff and that you try to define properties of the problem. These properties include
• the data which are affected and
• the operations which are identified
by the problem.
As an example consider the administration of employees in an institution. The head of the administration comes to you and ask you to create a program which allows to administer the employees. Well, this is not very specific. For example, what employee information is needed by the administration? What tasks should be allowed? Employees are real persons who can be characterized with many properties; very few are:
• name,
• size,
• date of birth,
• shape,
• social number,
• room number,
• hair colour,
• hobbies.
Certainly not all of these properties are necessary to solve the administration problem. Only some of them are problem specific. Consequently you create a model of an employee for the problem. This model only implies properties which are needed to fulfill the requirements of the administration, for instance name, date of birth and social number. These properties are called the data of the (employee) model. Now you have described real persons with help of an abstract employee.
Of course, the pure description is not enough. There must be some operations defined with which the administration is able to handle the abstract employees. For example, there must be an operation which allows you to create a new employee once a new person enters the institution. Consequently, you have to identify the operations which should be able to be performed on an abstract employee. You also decide to allow access to the employees' data only with associated operations. This allows you to ensure that data elements are always in a proper state. For example you are able to check if a provided date is valid.
To sum up, abstraction is the structuring of a nebulous problem into well-defined entities by defining their data and operations. Consequently, these entities combine data and operations. They are not decoupled from each other.
3.2 Properties of Abstract Data Types
The example of the previous section shows, that 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.
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).
Figure 3.2 shows an ADT which consists of an abstract data structure and operations. Only the operations are viewable from the outside and define the interface.
[attachment=17287]
Some authors describe object-oriented programming as programming abstract data types and their relationships. Within this section we introduce abstract data types as a basic concept for object-orientation and we explore concepts used in the list example of the last section in more detail.
3.1 Handling Problems
The first thing with which one is confronted when writing programs is the problem. Typically you are confronted with ``real-life'' problems and you want to make life easier by providing a program for the problem. However, real-life problems are nebulous and the first thing you have to do is to 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 and is illustrated in
The model defines an abstract view to the problem. This implies that the model focusses only on problem related stuff and that you try to define properties of the problem. These properties include
• the data which are affected and
• the operations which are identified
by the problem.
As an example consider the administration of employees in an institution. The head of the administration comes to you and ask you to create a program which allows to administer the employees. Well, this is not very specific. For example, what employee information is needed by the administration? What tasks should be allowed? Employees are real persons who can be characterized with many properties; very few are:
• name,
• size,
• date of birth,
• shape,
• social number,
• room number,
• hair colour,
• hobbies.
Certainly not all of these properties are necessary to solve the administration problem. Only some of them are problem specific. Consequently you create a model of an employee for the problem. This model only implies properties which are needed to fulfill the requirements of the administration, for instance name, date of birth and social number. These properties are called the data of the (employee) model. Now you have described real persons with help of an abstract employee.
Of course, the pure description is not enough. There must be some operations defined with which the administration is able to handle the abstract employees. For example, there must be an operation which allows you to create a new employee once a new person enters the institution. Consequently, you have to identify the operations which should be able to be performed on an abstract employee. You also decide to allow access to the employees' data only with associated operations. This allows you to ensure that data elements are always in a proper state. For example you are able to check if a provided date is valid.
To sum up, abstraction is the structuring of a nebulous problem into well-defined entities by defining their data and operations. Consequently, these entities combine data and operations. They are not decoupled from each other.
3.2 Properties of Abstract Data Types
The example of the previous section shows, that 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.
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).
Figure 3.2 shows an ADT which consists of an abstract data structure and operations. Only the operations are viewable from the outside and define the interface.
