classfile/visitor/package.html

<body>
This package contains interfaces and classes for processing class files from
the <code>{@link proguard.classfile proguard.classfile}</code> package using
the <i>visitor pattern</i>. Cfr., for instance, "Design Patterns, Elements of
Reusable OO Software", by Gamma, Helm, Johnson, and Vlissider.
<p>
Why the visitor pattern? Class files frequently contain lists of elements of
various mixed types: class items, constant pool entries, attributes,...
These lists and types are largely fixed; they won't change much in future
releases of the Java class file specifications. On the other hand, the kinds
of operations that we may wish to perform on the class files may change and
expand. We want to separate the objects and the operations performed upon them.
This is a good place to use the visitor pattern.
<p>
Visitor interfaces avoid having to do series of <code>instanceof</code> tests
on the elements of a list, followed by type casts and the proper operations.
Every list element is a visitor accepter. When its <code>accept</code> method
is called by a visitor, it calls its corresponding <code>visitX</code> method
in the visitor, passing itself as an argument. This technique is called
double-dispatch.
<p>
As already mentioned, the main advantage is avoiding lots of
<code>instanceof</code> tests and type casts. Also, implementing a visitor
interface ensures you're handling all possible visitor accepter types. Each
type has its own method, which you simply have to implement.
<p>
A disadvantage is that the visitor methods always get the same names, specified
by the visitor interface. These names aren't descriptive at all, making code
harder to read. It's the visitor classes that describe the operations now.
<p>
Also, the visitor methods always have the same parameters and return values, as
specified by the visitor interfaces. Passing additional parameters is done by
means of extra fields in the visitor, which is somewhat of a kludge.
<p>
Because objects (the visitor accepters) and the operations performed upon them
(the visitors) are now separated, it becomes harder to associate some state
with the objects. For convenience, we always provide an extra <i>visitor
info</i> field in visitor accepters, in which visitors can put any temporary
information they want.
</body>