Create Java 8 Stream from ArrayNode

问题

Is it possible to create stream from com.fasterxml.jackson.databind.node.ArrayNode?
I tried:

ArrayNode files = (ArrayNode) json.get("files");
Stream<JsonNode> stream = Stream.of(files);

But it will actually give stream of one element, the initial ArrayNode object.
Correct result should be Stream<JsonNode>, can I achieve it?

回答1:

ArrayNode implements Iterable. Iterable has a spliterator() method. You can create a sequential Stream from a Spliterator using

StreamSupport.stream(spliterator, false)

回答2:

An ArrayNode class provides random access: you can get size() and an element by index (using get(index)). This is all you need to create a good stream:

Stream<JsonNode> nodes = IntStream.range(0, files.size()).mapToObj(files::get);

Note that this solution is better than using default spliterator (as suggested by other answerers) as it can split well and reports the size properly. Even if you don't care about parallel processing, some operations like toArray() will work more effectively as knowing the size in advance will help to allocate an array of proper size.

回答3:

ArrayNode#elements returns an Iterator over it's elements you can use that to create a Stream (by leveraging StreamSupport). StreamSupport requires a Spliterator and to create a Spliterator from an Iterator you can use the Spliterators class.

  ArrayNode files = (ArrayNode) json.get("files");
  Stream<JsonNode>  elementStream = StreamSupport.stream(Spliterators
                  .spliteratorUnknownSize(files.elements(),
                        Spliterator.ORDERED),false);

cyclops-streams has a StreamUtils class has a static method that makes this a bit cleaner (I am the author).

 ArrayNode files = (ArrayNode) json.get("files");
 Stream<JsonNode>  elementStream = StreamUtils.stream(files.elements());

Taking into account @JB Nizet's answer that ArrayNode is an iterable with StreamUtils you can pass in the ArrayNode and get the Stream back directly.

Stream<JsonNode>  elementStream = StreamUtils.stream((ArrayNode) json.get("files"));

来源：https://stackoverflow.com/questions/32683785/create-java-8-stream-from-arraynode