java-stream

问题 I try to understand how does the reduce() method work in java-8. For example I have this code: public class App { public static void main(String[] args) { String[] arr = {"lorem", "ipsum", "sit", "amet"}; List<String> strs = Arrays.asList(arr); int ijk = strs.stream().reduce(0, (a, b) -> { System.out.println("Accumulator, a = " + a + ", b = " + b); return a + b.length(); }, (a, b) -> { System.out.println("Combiner"); return a * b; }); System.out.println(ijk); } } And the output is this:

问题 This question already has an answer here : How to ensure order of processing in java8 streams? (1 answer) Closed last year . TL;DR; I am looking for a place where I can lookup whether a certain intermediate or terminal operation. Where can I find such documentation? Edit This is not a duplicate of How to ensure order of processing in java8 streams?, since that question does not provide a comprehensive list of operations. Background The the package documentation says: Whether or not a stream

问题 Like just about everyone, I'm still learning the intricacies (and loving them) of the new Java 8 Streams API. I have a question concerning usage of streams. I'll provide a simplified example. Java Streams allows us to take a Collection , and use the stream() method on it to receive a stream of all of its elements. Within it, there are a number of useful methods, such as filter() , map() , and forEach() , which allow us to use lambda operations on the contents. I have code that looks something

问题 I was watching a presentation on Java, and at one point, the lecturer said: "Mutability is OK, sharing is nice, shared mutability is devil's work." What he was referring to is the following piece of code, which he considered an "extremely bad habit": //double the even values and put that into a list. List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 1, 2, 3, 4, 5); List<Integer> doubleOfEven = new ArrayList<>(); numbers.stream() .filter(e -> e % 2 == 0) .map(e -> e * 2) .forEach(e ->

问题 I am using Java 8 streams to iterate over a list with sublists. The outer list size varies between 100 to 1000 (different test runs) and the inner list size is always 5. There are 2 benchmark runs which show unexpected performance deviations. package benchmark; import org.openjdk.jmh.annotations.*; import org.openjdk.jmh.infra.Blackhole; import java.io.IOException; import java.util.concurrent.ThreadLocalRandom; import java.util.*; import java.util.function.*; import java.util.stream.*;

问题 Using a Java 8 lambda expression, I'm trying to do something like this. List<NewObject> objs = ...; for (OldObject oldObj : oldObjects) { NewObject obj = oldObj.toNewObject(); obj.setOrange(true); objs.add(obj); } I wrote this code. oldObjects.stream() .map(old -> old.toNewObject()) .forEach({new.setOrange("true")}) .collect(Collectors.toList()); This is invalid code because I'm then trying to do .collect() on what's returned by .forEach() , but forEach is void and does not return a list. How

问题 Using a Java 8 lambda expression, I'm trying to do something like this. List<NewObject> objs = ...; for (OldObject oldObj : oldObjects) { NewObject obj = oldObj.toNewObject(); obj.setOrange(true); objs.add(obj); } I wrote this code. oldObjects.stream() .map(old -> old.toNewObject()) .forEach({new.setOrange("true")}) .collect(Collectors.toList()); This is invalid code because I'm then trying to do .collect() on what's returned by .forEach() , but forEach is void and does not return a list. How

问题 In this case just odd lines have meaningful data and there is no character that uniquely identifies those lines. My intention is to get something equivalent to the following example: Stream<DomainObject> res = Files.lines(src) .filter(line -> isOddLine()) .map(line -> toDomainObject(line)) Is there any “clean” way to do it, without sharing global state? 回答1: A clean way is to go one level deeper and implement a Spliterator . On this level you can control the iteration over the stream elements

问题 Which is the best way to create a stream out of a collection: final Collection<String> entities = someService.getArrayList(); entities.stream(); Stream.of(entities); 回答1: The second one does not do what you think it does! It does not give you a stream with the elements of the collection; instead, it will give you a stream with a single element, which is the collection itself (not its elements). If you need to have a stream containing the elements of the collection, then you must use entities

问题 I am trying to sort an array of Strings by reverse order (ignoring case), without modifying it, and just printing it. So I am using Java8 stream. But I can't manage to do it. Here is my attempt : package experimentations.chapter02; import java.util.Arrays; import java.util.Comparator; import java.util.stream.Collectors; public class StringStream { public static void main(String[] args) { sortStrings(); } public static void sortStrings(){ String[] stringsArray = "The quick brown fox has a