Java Iterators and Streams

Question

I am trying to convert a loop that I have made into Java streams, though the code uses iterators and I am finding it hard to convert it into readable code.

private void printKeys() throws IOException {
    ClassLoader classLoader = getClass().getClassLoader();

    // read a json file
    ObjectMapper objectMapper = new ObjectMapper();
    JsonNode root = objectMapper.readTree(classLoader.getResource("AllSets.json"));
    Set<String> names = new HashSet<>();

    // loop through each sub node and store the keys
    for (JsonNode node : root) {
        for (JsonNode cards : node.get("cards")) {
            Iterator<String> i = cards.fieldNames();
            while(i.hasNext()){
                String name = i.next();
                names.add(name);
            }
        }
    }

    // print each value
    for (String name : names) {
        System.out.println(name);
    }

   }

I have tried the following though I feel like its not going the right way.

List<JsonNode> nodes = new ArrayList<>();
root.iterator().forEachRemaining(nodes::add);

Set<JsonNode> cards = new HashSet<>();
nodes.stream().map(node -> node.get("cards")).forEach(cards::add);

Stream s = StreamSupport.stream(cards.spliterator(), false);
//.. unfinished and unhappy

You can find the Json file I used here: https://mtgjson.com/json/AllSets.json.zip Be warned its quite large.

Could you attach AllSets.json to test code, please?

Andrii Abramov
– Andrii Abramov

2016-12-23 14:37:00 +00:00
Commented Dec 23, 2016 at 14:37 — Andrii Abramov
– Andrii Abramov, Commented Dec 23, 2016 at 14:37
@AndriiAbramov I added a link to the file in the question.

I am not smart
– I am not smart

2016-12-23 16:14:43 +00:00
Commented Dec 23, 2016 at 16:14 — I am not smart
– I am not smart, Commented Dec 23, 2016 at 16:14

Jorn Vernee · Accepted Answer · 2016-12-23 15:21:56Z

3

You can do most of the things in one swoop, but it's a shame this json api does not support streams better.

List<JsonNode> nodes = new ArrayList<>();
root.iterator().forEachRemaining(nodes::add);

Set<String> names = nodes.stream()
    .flatMap(node -> StreamSupport.stream(
        node.get("cards").spliterator(), false))
    .flatMap(node -> StreamSupport.stream(
        ((Iterable<String>) () -> node.fieldNames()).spliterator(), false))
    .collect(Collectors.toSet());

Or with Patrick's helper method (from the comments):

Set<String> names = stream(root)
    .flatMap(node -> stream(node.get("cards")))
    .flatMap(node -> stream(() -> node.fieldNames()))
    .collect(Collectors.toSet());

...

public static <T> Stream<T> stream(Iterable<T> itor) {
    return StreamSupport.stream(itor.spliterator(), false);
}

And printing:

names.stream().forEach(System.out::println);

edited Dec 23, 2016 at 15:21

answered Dec 23, 2016 at 15:11

Jorn Vernee

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1 Comment

Patrick Parker Over a year ago

This could look a lot better with a helper method public static <T> Stream<T> stream(Iterable<T> itor) { return StreamSupport.stream(itor.spliterator(), false); }

Andrii Abramov · Accepted Answer · 2016-12-23 14:52:29Z

0

If you provide 'json' file to us, it will be very useful.

At least now, I can make some suggestions to you:

Set<JsonNode> cards = new HashSet<>();
nodes.stream().map(node -> node.get("cards")).forEach(cards::add);

Replace with:

Set<JsonNode> cards = nodes.stream().map(node -> node.get("cards")).collect(Collectors.toSet());

for (String name : names) {
    System.out.println(name);
}

Replace with:

names.forEach(System.out::println);

answered Dec 23, 2016 at 14:52

Andrii Abramov

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1 Comment

I am not smart Over a year ago

Added a link to the json file in the original question

Balkrishna Rawool · Accepted Answer · 2016-12-23 15:17:37Z

0

Replace

Set<JsonNode> cards = new HashSet<>();

with

List<JsonNode> cards = new ArrayList<>();

Remove

Stream s = StreamSupport.stream(cards.spliterator(), false);

Then add below lines

cards.stream().forEach( card -> {
    Iterable<String> iterable = () -> card.fieldNames();
    Stream<String> targetStream = StreamSupport.stream(iterable.spliterator(), false);
    targetStream.forEach(names::add);
});
names.forEach(System.out::println);

edited Dec 23, 2016 at 15:17

answered Dec 23, 2016 at 15:04

Balkrishna Rawool

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