Creating a Stream using Java StreamSupport api and iterator, spliterator and then partition the stream using Collections.partitionBy() and Collections.groupBy() method.

Stream:

Introduced in Java 8, the Stream API is used to process collections of objects. A stream is a sequence of objects that supports various methods which can be pipelined to produce the desired result. The features of Java stream are –

  • A stream is not a data structure instead it takes input from the Collections, Arrays or I/O channels.
  • Streams don’t change the original data structure, they only provide the result as per the pipelined methods.
  • Each intermediate operation is lazily executed and returns a stream as a result, hence various intermediate operations can be pipelined. Terminal operations mark the end of the stream and return the result.

To make a Stream from Iterator We first need to make a Spliterator first. So lets first understand what is Iterator and Spliterator is.

Iterator:

A Java Cursor is an Iterator, which is used to iterate or traverse or retrieve a Collection or Stream object’s elements one by one. There are three cursors in Java. Spliterator is also a special type of Iterator so its also a cursor.

  1. Iterator
  2. Enumeration
  3. ListIterator
  4. Spliterator

Iterators in Java are used in the Collection framework to retrieve elements one by one lazily. It is a universal iterator as we can apply it to any Collection object. By using Iterator, we can perform both read/update and remove operations. It is an improved version of Enumeration with the additional functionality of removing an element.

Iterator must be used whenever we want to enumerate elements in all Collection framework implemented interfaces like Set, List, Queue, Deque, and all implemented classes of Map interface.

Spliterator:

Besides traversing sequences of data, like an Iterator<T> a Spliterator<T> can also partition it:

Iterator + Splitting => Spliterator

The trySplit() method allows it to partition off some elements of the sequence as another Spliterator<T>.

This particular advantage over Iterator makes it the core component of the Stream API. By splitting up data into apt sub-sequences, it allows parallel processing.

Iterator to Spliterator:

To convert an Iterator to Spliterator there is two api’s. One is for unknownsized collection and another is for known sized collections. In the known sized Spliterators.spliterator() requires the size whereas the unknownsized doesnt required the size.

There is another parameter that is needed to be provided to the method, and that is characterstics, which is an integer that defines the characterstics of the spliterator.

Type of spliterator’s

  1. Empty Spliterator [Spliterators.emptySpliterator();]
  2. Sized Spliterator [Spliterators.spliterator();]
  3. Unknownsized Spliterator [Spliterators.spliteratorUnknownSize();]

Now creating a unknownsized spliterator from and Iterator

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 Iterator<Node> nodeIterator = Iterators.nodes(result);
Spliterator<Node> nodeSpliterator = Spliterators.spliteratorUnknownSize(nodeIterator,Spliterator.CONCURRENT);

Now StreamSupport.stream() method can take that Spliterator Object and a boolean option to enable parallel processing of stream elements. This method Creates a new sequential or parallel Stream from a Spliterator.

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Stream<Node> nodeStream = StreamSupport.stream(nodeSpliterator,false);

The spliterator is only traversed, split, or queried for estimated size after the terminal operation of the stream pipeline commences.

If parallel option is enabled then the returned stream will be parallel and if false then it will return a sequential stream.

This is a great stackoverflow article to understand if we should use parallel everywhere possible?