Data Structures and Algorithms in Java[1].pdf - Fulvio Frisone

Modularity
In addition to abstraction and encapsulation, a fundamental principle of object
oriented design is modularity. Modern software systems typically consist of
several different components that must interact correctly in order for the entire
system to work properly. Keeping these interactions straight requires that these
different components be well organized. In object-oriented design, this code
structuring approach centers around the concept of modularity. Modularity refers
to an organizing principle for code in which different components of a software
system are divided into separate functional units.
Hierarchical Organization
The structure imposed by modularity helps to enable software reusability. If
software modules are written in an abstract way to solve general problems, then
modules can be reused when instances of these same general problems arise in
other contexts.
For example, the structural definition of a wall is the same from house to house,
typically being defined in terms of 2- by 4-inch studs, spaced a certain distance
apart, etc. Thus, an organized architect can reuse his or her wall definitions from
one house to another. In reusing such a definition, some parts may require
redefinition, for example, a wall in a commercial building may be similar to that
of a house, but the electrical system and stud material might be different.
A natural way to organize various structural components of a software package is
in a hierarchical fashion, which groups similar abstract definitions together in a
level-by-level manner that goes from specific to more general as one traverses up
the hierarchy. A common use of such hierarchies is in an organizational chart,
where each link going up can be read as "is a," as in "a ranch is a house is a
building." This kind of hierarchy is useful in software design, for it groups
together common functionality at the most general level, and views specialized
behavior as an extension of the general one.
Figure 2.3: An example of an "is a" hierarchy
involving architectural buildings.
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