The Iterator design pattern introduces an external object that allows to go through the items of collections without knowing the internal structure of the collections. This is one of the most popular pattern, and you can find it implemented in standard libraries of almost all programming languages. Sometimes, it's called Enumerator instead of Iterator.
Today's programming languages often provide rich variety of collections,
i.e., arrays, lists, dictionaries (hashmap), but also e.g. trees. Sometimes,
it's useful to implement our own collection, especially if it's supposed to
provide some special operations over its elements. At some point, we need to
iterate through every collection, i.e. to go through all its elements from the
first to the last one and print them, for example. If each collection
implemented its own interface to iterate over its elements, first, we'd have to
examine them, and collections are sometimes really complicated. And second, the
language itself won't be able to support syntactic sugar such as the
foreach loop. If you think of introducing some general interface
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