Lecture 2

IDENTIFYING OBJECTS AND CLASSES FROM THE REQUIREMENT SPECIFICATION

Mistakes done creating classes and objects

There are few signs which indicate a bas design of classes. There are no hundred percent negatives in class designs. One can create a design which is bad, but in that circumstance that design may be legitimate. It’s meant you should try as possible to avoid those pitfalls being occurred in the class design but it’s not meant that you can t design classes as follows.

Designing a class that isn’t

The main drawbacks that can occur are designing a class which is not a class. The main concept behind object oriented construction is to build modules around object types not around functions. Some beginners often fall into a big pitfall calling a routine as a class. The remedy is to be conscious about whether each class being corresponds to a meaningful data abstraction.

Class that performs one task

The next drawback is writing a class that performs one task. Such a class is not considered as a real class. For example people some times say this class prints this thing and that class parses the inputs so on. This is not a good habit of creating classes.

Using Imperative Names

This weakness is regarding the naming conventions of a class. Its not possible to use verbs for naming the classes. If you use a verb such as a ‘Print’ for a class that gives the viewer a illusion that the class is performing only one task which is a drawback. In the earlier drawback explained a class performing only one task is not considered as a good design of a class. There is a rule for naming the classes and it says that a class name should be a Noun or an adjective. So you can use nouns such as List, Tree as appropriately and nouns with adjectives such as Linked List.

Still you can use verbs also to name classes but I the form of nouns. For example instead of the word DELETE_ WORD you can use WORD_DELETION. English provides a better support for this from its grammar because almost all the verbs can be converted to nouns very easily.

Premature Classification

This is also a very common mistake done by the programmers. The mistake is worrying about the inheritance more early in the class creation process. What they do is before identifying all the classes they try to think about the inheritance of the classes. That will lead to bad selection of classes.

First you have to identify all the classes with their attributes and functions. And then should try to identify the common features in those classes and separate those in to a super class. Then you will be having a better design and a structure for your classes. For example of being over concentrating on the inheritance, you will be creating two separate classes for COLOMBO and KANDY which extends from the class CITY where you can you use two instances of the same class CITY at the run time.

No command classes

Some times you have come across classes that have no routine at all. They only have queries to access objects. This is bad design technique but there are special situations where this design is not considered as an improper design. One situation is where the class is representing a object obtained from the outside world which the object oriented software cannot change. Such can be data coming from a sensor of a process control system. Another situation is the class is only used for encapsulating data as constants such as in interfaces. There is another situation where classes instead of modifying the object they produce new objects of the same type. For example addition operation in a class doesn’t modify the object instead for given two values X and Y it produces the value X+Y where the result is in the same format.

Up to now we have gone through the possible mistakes that can happen when designing classes. Then what will be the ideal class look like. They are given below.

• There is a clearly associated abstraction with a class which can be considered as a data abstraction
• The class name is a Noun or an Adjective
• Several queries are available to find out properties of an instance.
• Several commands are available to change the state of the instance.
• There are some classes which don’t have commands to modify the objects but they produce objects of the same type as with the example of addition of two values.