java-stream

问题 I have 3 interfaces public interface IGhOrg { int getId(); String getLogin(); String getName(); String getLocation(); Stream<IGhRepo> getRepos(); } public interface IGhRepo { int getId(); int getSize(); int getWatchersCount(); String getLanguage(); Stream<IGhUser> getContributors(); } public interface IGhUser { int getId(); String getLogin(); String getName(); String getCompany(); Stream<IGhOrg> getOrgs(); } and I need to implement Optional<IGhRepo> highestContributors(Stream<IGhOrg>

问题 For example: I have a list of user objects with e-mails. I would like to collect the list of users with distinct e-mails (because two users with the same e-mail would be probably the same person, so I do not care about the differences). In other words, for each distinct key (e-mail), I want to grab the first value (user) found with that key and ignore the rest. This is a code I have came up with: public List<User> getUsersToInvite(List<User> users) { return users .stream() // group users by

问题 I want to create a function that builds an array of incremental numbers. For example, I want to obtain something like: int[] array = new int[]{1, 2, 3, 4, 5, 6, 7, 8, ..., 1000000}; The function will receive two parameters: start number (inclusive) and the final length of the array: public int[] buildIncrementalArray(int start, int length) { ... } I know how to do it using a for loop: public int[] buildIncrementalArray(int start, int length) { int[] result = new int[length]; for(int i = 0 ; i

问题 It's easy to convert List<V> into Map<K, List<V>> . For example: public Map<Integer, List<String>> getMap(List<String> strings) { return strings.stream() .collect(Collectors.groupingBy(String::length)); } But I want to do it with my own List and Map suppliers . I have come up with this: public Map<Integer, List<String>> getMap(List<String> strings) { return strings.stream() .collect(Collectors.toMap( String::length, item -> {List<String> list = new ArrayList<>(); list.add(item); return list;}

问题 It's easy to convert List<V> into Map<K, List<V>> . For example: public Map<Integer, List<String>> getMap(List<String> strings) { return strings.stream() .collect(Collectors.groupingBy(String::length)); } But I want to do it with my own List and Map suppliers . I have come up with this: public Map<Integer, List<String>> getMap(List<String> strings) { return strings.stream() .collect(Collectors.toMap( String::length, item -> {List<String> list = new ArrayList<>(); list.add(item); return list;}

问题 What I have: A text-file which is read line by line. Each String contains a line. What I want: Group ALL words by first character using Java Streams. What I have so far: public static Map<Character, List<String>> groupByFirstChar(String fileName) throws IOException { return Files.lines(Paths.get(PATH)). flatMap(s -> Stream.of(s.split("[^a-zA-Z]"))). map(s -> s.toLowerCase()). sorted((s1, s2) -> s1.compareTo(s2)). collect(Collectors.groupingBy(s -> s.charAt(0))); } Problem: I get an Exception

问题 I want to convert int array to Map<Integer,Integer> using Java 8 stream api int[] nums={2, 7, 11, 15, 2, 11, 2}; Map<Integer,Integer> map=Arrays .stream(nums) .collect(Collectors.toMap(e->e,1)); I want to get a map like below, key will be integer value, value will be total count of each key map={2->3, 7->1, 11->2, 15->1} compiler complains " no instance(s) of type variable(s) T, U exist so that Integer confirms to Function " appreciate any pointers to resolve this 回答1: You need to box the

问题 This question already has answers here : Is there a concise way to iterate over a stream with indices in Java 8? (22 answers) Closed 2 years ago . List<Rate> rateList = guestList.stream() .map(guest -> buildRate(ageRate, guestRate, guest)) .collect(Collectors.toList()); class Rate { protected int index; protected AgeRate ageRate; protected GuestRate guestRate; protected int age; } In the above code, is it possible to pass index of guestList inside buildRate method. I need to pass index also

问题 Why summaryStatistics() method on an empty IntStream returns max and min value of integer as the maximum and minimum int value present in the stream? IntStream intStream = IntStream.of(); IntSummaryStatistics stat = intStream.summaryStatistics(); System.out.println(stat); Output : IntSummaryStatistics{count=0, sum=0, min=2147483647, average=0.000000, max=-2147483648} What is the point of returning these values? Should not it considered the wrong result? 回答1: See IntSummaryStatistics.getMax()

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