Software Engineering for Students A Programming Approach

78 Chapter 6 ■ Modularity
3. Modifying data within another component
Allowing one component to alter another component’s data seems rather less harmful
than changing another component’s code. However, the objection is the same and the
coupling is strong because a fault that appears in one component may be caused by
another.
4. Shared or global data
Shared data is data that two or more components have access to. The data is in a distinct
component. Global data means a collection of data that is accessible to a large
number of, perhaps all, components. The facility to access data in this way is present in
nearly all widely used programming languages.
We have already seen why this is undesirable.
SELF-TEST QUESTION
6.1 Give one reason why global data is undesirable.
5. A method call with a parameter that is a switch
We have seen that both shared data and unusual transfers of control result in strong
coupling between components. The solution is, of course, to use method calls with
their attendant parameters. Even so, it is possible to worsen the coupling by passing as
a parameter not pure data but an element of control. An example is where a component
is passed an indicator telling the method which action to take from among a
number of available actions. (This indicator is sometimes called a switch.) Here is an
example of a method call on a general-purpose input-output method:
inputOutput(command, device, buffer, length);
The parameter command has values 0, 1, 2, etc. that specify whether the operation is
a read, write, open, etc. This is undesirable simply because it is unnecessarily complicated.
This method can be divided into several methods – each carrying out a single
action. As an alternative to calling a single method and passing it a switch, we can
instead call the individual appropriate method, like this:
read(device, buffer, length);
We have eliminated a parameter from the interaction and at the same time created
well-defined methods, each with a specific function. This contrasts with a single, multipurpose
method. Arguably this modularization is easier to understand and maintain.