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<TITLE>Y: Iterators: decoupling algorithms from containers</TITLE>
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Thinking in Python<br>
<small>Revision 0.1.2 (12/31/01) -- Incomplete and Unfinished</small></FONT></H2>
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by Bruce Eckel ©2002 MindView, Inc.</FONT></H3>
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</P></DIV><A NAME="_Toc534420105"></A><A NAME="Heading56"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H1 ALIGN="LEFT">
Y: Iterators: decoupling algorithms from containers</H1></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia" SIZE=4 COLOR="Red">This chapter has not
had any significant translation yet.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia" SIZE=4>Alexander Stepanov thought for
years about the problem of generic programming techniques before creating the
STL (along with Dave Musser). He came to the conclusion that all algorithms are
defined on algebraic structures – what we would call containers.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_231">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">In the process, he realized that
iterators are central to the use of algorithms, because they decouple the
algorithms from the specific type of container that the algorithm might
currently be working with. This means that you can describe the algorithm
without worrying about the particular sequence it is operating on. More
generally, <I>any</I> code that you write using iterators is decoupled from the
data structure that the code is manipulating, and thus your code is more general
and reusable.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_232">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The use of iterators also extends your
code into the realm of <I>functional programming</I>, whose objective is to
describe <I>what</I> a program is doing at every step rather than <I>how</I> it
is doing it. That is, you say “sort” rather than describing the
sort. The objective of the C++ STL was to provide this <I>generic
programming</I> approach for C++ (how successful this approach will actually be
remains to be seen).
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_233">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">If you’ve used containers in Java
(and it’s hard to write code without using them), you’ve used
iterators – in the form of the <B>Enumeration</B> in Java 1.0/1.1 and the
<B>Iterator</B> in Java 2. So you should already be familiar with their general
use. If not, see Chapter 9, <I>Holding Your Objects</I>, under <I>Iterators</I>
in <I>Thinking in Java, 2<SUP>nd</SUP> edition</I> (freely downloadable from
<I>www.BruceEckel.com</I>).
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_234">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Because the Java 2 containers rely
heavily on iterators they become excellent candidates for generic/functional
programming techniques. This chapter will explore these techniques by converting
the STL algorithms to Java, for use with the Java 2 container library.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_235">Add Comment</A></FONT><A NAME="_Toc534420106"></A><BR></P></DIV>
<A NAME="Heading57"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H2 ALIGN="LEFT">
Type-safe iterators</H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">In <I>Thinking in Java, 2<SUP>nd</SUP>
edition</I>, I show the creation of a type-safe container that will only accept
a particular type of object. A reader, Linda Pazzaglia, asked for the other
obvious type-safe component, an iterator that would work with the basic
<B>java.util</B> containers, but impose the constraint that the type of objects
that it iterates over be of a particular type.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_236">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">If Java ever includes a template
mechanism, this kind of iterator will have the added advantage of being able to
return a specific type of object, but without templates you are forced to return
generic <B>Object</B>s, or to require a bit of hand-coding for every type that
you want to iterate through. I will take the former approach.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_237">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">A second design decision involves the
time that the type of object is determined. One approach is to take the type of
the first object that the iterator encounters, but this is problematic because
the containers may rearrange the objects according to an internal ordering
mechanism (such as a hash table) and thus you may get different results from one
iteration to the next. The safe approach is to require the user to establish the
type during construction of the iterator.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_238">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Lastly, how do we build the iterator? We
cannot rewrite the existing Java library classes that already produce
<B>Enumeration</B>s and <B>Iterator</B>s. However, we can use the
<I>Decorator</I> design pattern, and create a class that simply wraps the
<B>Enumeration</B> or <B>Iterator</B> that is produced, generating a new object
that has the iteration behavior that we want (which is, in this case, to throw a
<B>RuntimeException</B> if an incorrect type is encountered) but with the same
interface as the original <B>Enumeration</B> or <B>Iterator</B>, so that it can
be used in the same places (you may argue that this is actually a <I>Proxy</I>
pattern, but it’s more likely <I>Decorator</I> because of its intent).
Here is the code:
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_239">Add Comment</A></FONT><BR></P></DIV>
<BLOCKQUOTE><FONT SIZE = "+1"><PRE># util:TypedIterator.py
<font color=#0000ff>class</font> TypedIterator(Iterator):
private Iterator imp
private Class type
<font color=#0000ff>def</font> __init__(self, Iterator it, Class type):
imp = it
self.type = type
<font color=#0000ff>def</font> hasNext(self):
<font color=#0000ff>return</font> imp.hasNext()
<font color=#0000ff>def</font> remove(self): imp.remove()
<font color=#0000ff>def</font> next(self):
Object obj = imp.next()
<font color=#0000ff>if</font>(!type.isInstance(obj))
throw ClassCastException(
<font color=#004488>"TypedIterator for type "</font> + type +
<font color=#004488>" encountered type: "</font> + obj.getClass())
<font color=#0000ff>return</font> obj
# :~</PRE></FONT></BLOCKQUOTE>
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