java-stream

问题 Experimenting with streams I ran into the following behavior which I don't quite understand. I created a parallel stream from an iterator and I noticed that it did not seem to be exhibiting parallelism. In the below example I've printed a counter to the console for two parallel streams, one created from an iterator and the other from a list. The stream created from the list exhibited the behavior I expected which was to print the counter in non-sequential order but the stream created from the

问题 list.stream().filter( a-> a < 20 && a > 7).forEach(a -> System.out.println(a)); fiter is lazily executed. forEach is eagerly executed. What does that mean? 回答1: Say you had the following operation. list.stream() .map(a -> a * a) .filter(a -> a > 0 && a < 100) .map(a -> -a) .forEach(a -> System.out.println(a)); The intermediate operations are the maps and filters, the terminal operation is the forEach . If intermediate operations were eagerly executed, then .map(a -> a * a) would immediately

问题 list.stream().filter( a-> a < 20 && a > 7).forEach(a -> System.out.println(a)); fiter is lazily executed. forEach is eagerly executed. What does that mean? 回答1: Say you had the following operation. list.stream() .map(a -> a * a) .filter(a -> a > 0 && a < 100) .map(a -> -a) .forEach(a -> System.out.println(a)); The intermediate operations are the maps and filters, the terminal operation is the forEach . If intermediate operations were eagerly executed, then .map(a -> a * a) would immediately

问题 Have two classes and two corresponding lists: class Click { long campaignId; Date date; } class Campaign { long campaignId; Date start; Date end; String type; } List<Click> clicks = ..; List<Campaign> campaigns = ..; And want to find all Click s in clicks that: Have a corresponding Campaign in campaigns list, i.e., Campaign with the same campaignId AND This Campaign has type = "prospective" AND This Campaigns.start < click.date < Campaigns.end So far I have the following implementation (which

问题 I want to use java streams to iterate a list and find the BigDecimal minimum price. The following illustrates, but does not work (because min() cannot accept BigDecimal . class Product { public BigDecimal price; } List<Product> products; products.stream().min((Product) p -> p.price); 回答1: Since BigDecimal already is Comparable , it is as simple as : BigDecimal min = products .stream() .map(Product::getPrice) .min(Comparator.naturalOrder()) .orElse(BigDecimal.ZERO); 回答2: You don't need a

问题 I have a class A Company { String name; Logo logo; } Logo { int color; //can have values=1 (green),2 (red),3 (blue) ... String name; String address; } Output needed : for each type 1,2,3 Group all instances of Logo by color. For each such group what were A.id Give me companies by their color logos. E.g. which companies have logo red? I tried following Input List<Company> company = {//initialization} company.stream().map(e -> e.getLogo()) .collect(Collectors.groupingBy(e -> {Logo b = new Logo(

问题 I have a class A Company { String name; Logo logo; } Logo { int color; //can have values=1 (green),2 (red),3 (blue) ... String name; String address; } Output needed : for each type 1,2,3 Group all instances of Logo by color. For each such group what were A.id Give me companies by their color logos. E.g. which companies have logo red? I tried following Input List<Company> company = {//initialization} company.stream().map(e -> e.getLogo()) .collect(Collectors.groupingBy(e -> {Logo b = new Logo(

问题 I am trying to read a YAML file in Java using the following code: public class LoadFile { public static void main(String[] args) throws IOException { Yaml yaml = new Yaml(); InputStream inputStream = LoadFile.class .getClassLoader() .getResourceAsStream("ABSOLUTE PATH TO YAML"); Object obj = yaml.load(inputStream); System.out.println(obj); } } But I get this exception which I don't understand why it occurs. Exception in thread "main" org.yaml.snakeyaml.error.YAMLException: java.io.IOException

问题 Consider this (completely contrived) Java code: final List<Integer> s = Arrays.asList(1, 2, 3); final int[] a = new int[1]; a[0] = 100; s.parallelStream().forEach(i -> { synchronized (a) { a[0] += i; } }); System.out.println(a[0]); Is this code guaranteed to output "106"? It seems like it is not unless there is a happens-before relationship established by parallelStream() , by which we can know for sure that the first accesses to a[0] in the lambda will see 100 and not zero (according to my

问题 Consider this (completely contrived) Java code: final List<Integer> s = Arrays.asList(1, 2, 3); final int[] a = new int[1]; a[0] = 100; s.parallelStream().forEach(i -> { synchronized (a) { a[0] += i; } }); System.out.println(a[0]); Is this code guaranteed to output "106"? It seems like it is not unless there is a happens-before relationship established by parallelStream() , by which we can know for sure that the first accesses to a[0] in the lambda will see 100 and not zero (according to my