java-stream

问题 Closed . This question is opinion-based. It is not currently accepting answers. Want to improve this question? Update the question so it can be answered with facts and citations by editing this post. Closed 3 years ago . I am wondering how I could design methods that could either be run concurrently or single threaded. For example I have a method like this: /** * Produces the norm of the two vector {@code v1}. * * @param v1 * The first vector. * * @param v2 * The second vector * * @throws

问题 Question How to execute an action after the last item of an ordered stream is processed but before it's closed ? This Action should be able to inject zero or more items in the stream pipe. Context I've got a very large file of the form : MASTER_REF1 SUBREF1 SUBREF2 SUBREF3 MASTER_REF2 MASTER_REF3 SUBREF1 ... Where SUBREF (if any) is applicable to MASTER_REF and both are complex objects (you can imagine it somewhat like JSON). On first look I tried something like : public void process(Path

问题 How can I turn a finite Stream of things Stream<Thing> into an infinite repeating stream of things? 回答1: Boris the Spider is right: a Stream can only be traversed once, so you need a Supplier<Stream<Thing>> or you need a Collection. <T> Stream<T> repeat(Supplier<Stream<T>> stream) { return Stream.generate(stream).flatMap(s -> s); } <T> Stream<T> repeat(Collection<T> collection) { return Stream.generate(() -> collection.stream()).flatMap(s -> s); } Example invocations: Supplier<Stream<Thing>>

问题 Does Java 8 Stream.iterator() auto-close the stream when it's done? I suppose not... I have something like this: class Provider implements Serializable { Iterator<String> iterator() { Stream<String> stream = new BufferedReader(...).lines(); return stream.iterator(); } } This iterator is used by some other class that doesn't know the iterator is based on a file-reading resource. That is class Consumer { void f() { Iterator<String> iterator = provider.iterator(); // code that calls iterator

问题 Am trying to sort the filed in filter. Input Document / Sample Record: DocumentList: [ Document{ { _id=5975ff00a213745b5e1a8ed9, u_id=, mailboxcontent_id=5975ff00a213745b5e1a8ed8, idmapping=Document{ {ptype=PDF, cid=00988, normalizedcid=00988, systeminstanceid=, sourceschemaname=, pid=0244810006} }, batchid=null, pdate=Tue Jul 11 17:52:25 IST 2017, locale=en_US } }, Document{ { _id=597608aba213742554f537a6, u_id=, mailboxcontent_id=597608aba213742554f537a3, idmapping=Document{ {platformtype

问题 Inversion sum is not correct using Stream.reduce, what is going wrong here ? double[] array = {1.0, 2.0}; double iunversionSum = Arrays.stream(array).reduce(0.0, (a, b) -> Double.sum(1.0 / a, 1.0 / b)); output is .5 but expected is 1.5 (1/1 + 1/2) 回答1: I think using map() it could be simplier. double inversionSum = Arrays.stream(arr).map(val -> 1 / val).sum(); 回答2: The error in your reduce is: Double.sum(1.0 / a, 1.0 / b), starting the series with 0.0. Now check why your outcome is .5. Use

问题 I am trying to do a map operation on a List<Integer> : list.stream().map(i -> i - 2).collect(Collectors.toList()); Instead of performing the operation on the last element of the list, I would like it to just be passed through. ...Collectors.toList()).add(i) doesn't work, of course, because i is out of scope. For example, the input list [1, 2, 3, 4] should output [-1, 0, 1, 4] 回答1: You could stream the original list and limit the stream to exclude the last element, do the mapping and collect

问题 If I have 2 collections, List<String> domainArr; List<Person> personArr; I would like to make a minor transformation on each of the elements in the String and then iterate over the personArr to List<String> urlArr = strArr.stream() .map(str -> "https://" + strArr) .collect(Collectors.toList()); I have a method like List<Person> getPersons(String url){ /*makes a restful call to the url and gets a List of objects for each URL.*/ } I would like to iterate over each of the urls from urlArr and

问题 I want to construct a Map<Item, List<String>> , i.e. using the Item reference as key, with some arbitrary List<String> as value. I've attempted the following, but it will show type inference error in IntelliJ (however, it will still compile ); List<Item> items = getItems(...); Map<Item, List<String>> itemMap = items.stream().collect(Collectors.toMap(i->i, new ArrayList<>())); I then created a workaround through a helper method in Item , get() , which just returns its own instance; public Item

问题 IntStream may be a the easiest way but I can only pick up smallest M numbers as below: public class Test { private static final int[] arr = {5, 3, 4, 2, 9, 1, 7, 8, 6}; public static void main(String[] args) throws Exception { System.out.println(Arrays.asList(IntStream.of(arr).sorted().limit(5).boxed().toArray())); } } btw, considering algorithm complexity and assuming N >> M, a "sorted + limit" approach just have a complexity of O(N log(N)). I think the best complexity may reach to O(N log(M