java-8

问题 Is there a predefined Function in Java 8 that does something like this: static <T, R> Function<T, R> constant(R val) { return (T t) -> { return val; }; } To answer people's query on why I need this function here is the real usage when I am trying to parse an integer to an roman numerals: // returns the stream of roman numeral symbol based // on the digit (n) and the exponent (of 10) private static Stream<Symbol> parseDigit(int n, int exp) { if (n < 1) return Stream.empty(); Symbol base =

问题 Based on the JSSE examples, I'm trying to get the value of the TLS parameter "Server Name Indication" (SNI) on the server side - without success. I'm sure that the value is sent by the client since I used a network sniffer (Wireshark) that shows the value. But when I use the following code fragment, the list of server name parameters is empty (while the "protocols" are shown): public void connectionAccepted(Socket socket) { System.out.println("Connection accepted!"); try { /* get SNI

问题 A question about spliterators that at first glance is not straightforward. In streams, .parallel() changes the behaviour that the stream is processed. However I was expecting the spliterators created from sequential and parallel streams to be the same. For example, in sequential streams typically, the .trySplit() is never invoked , while in parallel streams it is, in order to hand over the split spliterator to another thread. Differences between stream.spliterator() vs stream.parallel()

问题 HijrahDate hd=HijrahChronology.INSTANCE.date(LocalDate.of(2014,11, 25)); If we have HijrahDate Instance , it is expected to have a method in UmalquraCalendar API that shows the name of month : i inspect properties of this instance using groovy API : ['era':AH, 'class':class java.time.chrono.HijrahDate, 'prolepticMonth':17233, 'eraValue':1, 'dayOfWeek':2, 'leapYear':false, 'chronology':Hijrah-umalqura, 'dayOfYear':32] However we don't find the month name which must be one of the following list

问题 Is there a way to covert the below to using collectors yet? List<String[]> lines = getLines(); Multimap<String,String> multimap = ArrayListMultimap.create(); lines.forEach(line -> multimap.put(line[0],line[1]); ); 回答1: You can use Multimaps.toMultimap collector: ListMultimap<String, String> multimap = lines.stream() .collect(Multimaps.toMultimap( l -> l[0], l -> l[1], ArrayListMultimap::create )); Or if you don't need mutability, use ImmutableListMultimap.toImmutableListMultimap collector:

问题 Given: public class Test { public static void main(String[] args) { int nThreads = 1; Executor e = Executors.newFixedThreadPool(nThreads); CompletableFuture.runAsync(() -> { System.out.println("Task 1. Thread: " + Thread.currentThread().getId()); }, e).thenComposeAsync((Void unused) -> { return CompletableFuture.runAsync(() -> { System.out.println("Task 2. Thread: " + Thread.currentThread().getId()); }, e); }, e).join(); System.out.println("finished"); } } I am expecting a single executor

问题 I have a an object which looks like the following class MyObject { String type; List<String> subTypes; } Is it possible, given a list of MyObject's to use Java 8 streams to filter on both the type and then the subtype? So far I have myObjects.stream() .filter(t -> t.getType().equals(someotherType) .collect(Collections.toList()); but within this I also want another filter on each of the subTypes filtering those on a particular subtype too. I can't figure out how to do this. An example would be

问题 I've read here why Optional.of() should be used over Optional.ofNullable() , but the answer didn't satisfy me at all, so I ask slightly different: If you are SURE that your method does not return null , why should you use Optional at all? As far as I know, the more or less only purpose of it is to remind the "user of a method", that he might have to deal with null -values. If he does not have to deal with null -values, why should he be bothered with an Optional ? I ask, because I recently

问题 Closed . This question needs details or clarity. It is not currently accepting answers. Want to improve this question? Add details and clarify the problem by editing this post. Closed 4 years ago . How to return a String value from an Optional<String> using ifPresent and avoiding NullPointerException ? Example: public String longestName() { Optional<String> longName = someList.stream().reduce((name1, name2) -> name1.length() > name2.length() ? name1 : name2); // If I do not want to use

问题 Using stream.collect(Collectors.joining(", ")) I can easily join all the strings of my stream delimited by a comma. A possible result would be "a, b, c" . But what if I want the last delimiter to be different. For example to be " and " such that I get "a, b and c" as result. Is there an easy solution? 回答1: If they are already in a list, no stream is needed; simply join a sublist of all but the last element and concat the other delimiter and the final element: int last = list.size() - 1;