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Iterators: Decoupling Algorithms from Containers

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This chapter has not had any significant translation yet.

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.

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, any 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.

The use of iterators also extends your code into the realm of functional programming, whose objective is to describe what a program is doing at every step rather than how it is doing it. That is, you say "sort" rather than describing the sort. The objective of the C++ STL was to provide this generic programming approach for C++ (how successful this approach will actually be remains to be seen).

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 Enumeration in Java 1.0/1.1 and the Iterator in Java 2. So you should already be familiar with their general use. If not, see Chapter 9, Holding Your Objects, under Iterators in Thinking in Java, 3rd edition (freely downloadable from www.BruceEckel.com).

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.

Type-Safe Iterators

In Thinking in Java, 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 java.util containers, but impose the constraint that the type of objects that it iterates over be of a particular type.

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 Objects, or to require a bit of hand-coding for every type that you want to iterate through. I will take the former approach.

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.

Lastly, how do we build the iterator? We cannot rewrite the existing Java library classes that already produce Enumerations and Iterators. However, we can use the Decorator design pattern, and create a class that simply wraps the Enumeration or Iterator that is produced, generating a new object that has the iteration behavior that we want (which is, in this case, to throw a RuntimeException if an incorrect type is encountered) but with the same interface as the original Enumeration or Iterator, so that it can be used in the same places (you may argue that this is actually a Proxy pattern, but it's more likely Decorator because of its intent). Here is the code:

# util/TypedIterator.py

class TypedIterator(Iterator):
    def __init__(self, it, type):
        self.imp = it
        self.type = type

    def hasNext(self):
        return imp.hasNext()

    def remove(self): imp.remove()
    def next(self):
        obj = imp.next()
        if(!type.isInstance(obj))
            throw ClassCastException(
              "TypedIterator for type " + type +
              " encountered type: " + obj.getClass())
        return obj