Java Reference
In-Depth Information
The only additions that cannot be directly devised from the static object
structure are the each-other containment relationships between
Heading
and
Link
; these relationships have been added based on knowledge of the
HTML language.
All containment relationships in this class diagram are ordered because
the order in which contained elements appear in the container element is
important (e.g. the heading of a section should appear before the text that
constitutes its content).
Class
Page
does not contain any specific information. Class
Head
contains
only the title of the HTML page (i.e.
Test
in the example above). Class
Body
contains the background colour, which is encoded in the
bgcolor
parameter
inside the
<body>
tag. Class
Text
contains the fragment of text it represents.
Class
Heading
contains the text that constitutes the heading. Finally class
Link
contains the destination URL of the hyperlink. In addition, all classes
except
Head
and
Text
can contain other elements that are linked to them.
8.4.2
Design
We try now to refine the analysis model described so far. First we identify
commonalities among different classes and factor them into a common base
class connected to the specific classes by inheritance. This is an application
of the class generalization idiom presented in Chapter 6. Three main
characteristics can be identified as common to the elements of the analysis
model.
1 Most elements have a string associated with them, which represents the
content of the element or some other parameter. The information asso-
ciated with the various elements may be very different; the common
characteristic is that it can be stored in a string. Examples of such
information are the background colour of the tag body, or the text that
constitutes a document fragment.
2 All elements must provide an opening tag and a closing one.
3 Most elements must provide mechanisms to iterate on the contained
elements for exploring the page elements and for serializing the page
contents.
The main differences among these elements are related to the mechanisms
used to iterate on the document structure. Each class must provide specific
methods for accessing its content; for instance class
Page
should provide a
method for accessing the
Head
component and another method for accessing
its
Body
component.
Decision point
Document elements must implement iteration mechanisms.
