Java8StreamsBeyondBasics.md

This tutorial teaches some useful techniques involving streams that let you employ them in a much wider set of uses.

Using flatMap to merge sequences

Using Optional effectively

Creating streams from iterators

When you work with Java collections, the stream method of Collection objects helps you easily create a stream based on the contents of the collection. But not all iterable objects have in-built methods to get a stream. For example, I use the Jackson library for JSON quite heavily and often miss being able to process sequences of JSON elements as a stream. But, given an interator to the underlying sequence, it is not that hard to create a stream on it.

import java.util.stream.StreamSupport;
 
Iterator<ObjectType> iter = container.iterator();
Iterable<ObjectType> iterable = () -> iter;    // java.lang.Iterable's only method iterator is assigned the lambda
Boolean parallel = true;
Stream<ObjectType> stream = StreamSupport.stream(iterable.spliterator, !parallel);

You can easily fold this into a generic function:

public static <T, U> Stream<U> getStream(T container, Function<T, Iterator<U>> iterGetter, 
                                         Boolean parallel) {
    Iterator<U> iter = iterGetter.apply(container);
    Iterable<U> iterable = () -> iter;
    return StreamSupport.stream(iterable.spliterator(), parallel);
}

If you now want to apply it on say the JsonNode class from Jackson, you could do so easily:

JsonNode node = ...;
 
if (node.isArray()) {
    Stream<JsonNode> stream = getStream(node, JsonNode::elements, false);
} else if (node.isObject()) {
    Stream<Map.Entry<String, JsonNode>> stream = getStream(node, JsonNode::fields, false);
}