Limitations of collections

                          Java collections have served the Java language extremely well. However, they are based upon the idea that all the elements of a collection exist and are represented somewhere in memory. This means that they are not capable of representing more-general data, such as infinite sets.

Consider, for example, the set of all prime numbers.

                         This cannot be modeled as Set<Integer> because we don’t know what all the prime numbers are, and we certainly don’t have enough heap space to represent them all. In Java 7 and earlier versions, the only way to model the set would be to work with an Iterator representing the set instead. It is possible to construct a view of data that works primarily with iterators and relegates the underlying collections to a supporting role. However, this requires discipline and is not an immediately obvious approach to working with the Java collections.With Java 7 and earlier versions, if you wanted to use this type of approach, you would typically depend upon an external library that provided better support for this functionality. The upcoming release of Java 8 addresses this use case by introducing the Stream interface as an abstraction that is better suited to dealing with more-general data structures than basic, finite collections. This means that a Stream can be thought of as more general than an Iterator or a Collection. However, a Stream is not really a data structure; instead, it’s an abstraction for handling data. What does this distinction mean? It means a Stream does not manage the storage for elements nor does it provide a way to access individual elements directly from the Stream.