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Type-Safe Iterators

Метки: python

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