java-stream

问题 Is there a better way to fill an ArrayList like this (I have done it like this in Java 7): List<ScheduleIntervalContainer> scheduleIntervalContainers = new ArrayList<>(); scheduleIntervalContainers.add(scheduleIntervalContainer); 回答1: To fill a List , it is possible to generate an infinite Stream using Stream.generate(s) and then limit the number of results with limit(maxSize). For example, to fill a List of 10 new ScheduleIntervalContainer objects: List<ScheduleIntervalContainer>

问题 Im having a double array , I need to convert the array into a JSONArray using java streams. I tried using forEach (shared mutability) which leads to loss of data. public static JSONArray arrayToJson(double[] array) throws JSONException{ JSONArray jsonArray = new JSONArray(); Arrays.stream(array) .forEach(jsonArray::put); return jsonArray; } Is there any way I could to create JSONArray using streams? 回答1: Your code works, but you can write something like this: return Arrays.stream(array)

问题 If the input size is too small the library automatically serializes the execution of the maps in the stream, but this automation doesn't and can't take in account how heavy is the map operation. Is there a way to force parallelStream() to actually parallelize CPU heavy maps? 回答1: There seems to be a fundamental misunderstanding. The linked Q&A discusses that the stream apparently doesn’t work in parallel, due to the OP not seeing the expected speedup. The conclusion is that there is no

问题 If the input size is too small the library automatically serializes the execution of the maps in the stream, but this automation doesn't and can't take in account how heavy is the map operation. Is there a way to force parallelStream() to actually parallelize CPU heavy maps? 回答1: There seems to be a fundamental misunderstanding. The linked Q&A discusses that the stream apparently doesn’t work in parallel, due to the OP not seeing the expected speedup. The conclusion is that there is no

问题 To sort it int ascending order I can use: myMap.entrySet().stream() .sorted(Map.Entry.comparingByValue()) .collect(Collectors.toMap(Entry::getKey, Entry::getValue)); How can I do it in decreasing order? 回答1: To sort in reverse order, pass Comparator.reverseOrder() as parameter to comparingByValue . To get a LinkedHashMap , you must specifically request one with the 4-argument toMap() . If you don't specify what kind of a map you want, you will get whatever the default is, which currently

问题 I've created method whih numerating each character of alphabet. I'm learning streams(functional programming) and try to use them as often as possible, but I don't know how to do it in this case: private Map<Character, Integer> numerateAlphabet(List<Character> alphabet) { Map<Character, Integer> m = new HashMap<>(); for (int i = 0; i < alphabet.size(); i++) m.put(alphabet.get(i), i); return m; } So, how to rewrite it using streams of Java 8? 回答1: Avoid stateful index counters like the

问题 I am posting my query after having searched in this forum & google, but was unable to resolve the same. eg: Link1 Link2 Link3 I am trying to filter List 2 (multi column) based on the values in List 1. List1: - [Datsun] - [Volvo] - [BMW] - [Mercedes] List2: - [1-Jun-1995, Audi, 25.3, 500.4, 300] - [7-Apr-1996, BMW, 35.3, 250.2, 500] - [3-May-1996, Porsche, 45.3, 750.8, 200] - [2-Nov-1998, Volvo, 75.3, 150.2, 100] - [7-Dec-1999, BMW, 95.3, 850.2, 900] expected o/p: - [7-Apr-1996, BMW, 35.3, 250

问题 How could I do the following with Java Streams? Let's say I have the following classes: class Foo { Bar b; } class Bar { String id; String date; } I have a List<Foo> and I want to convert it to a Map <Foo.b.id, Map<Foo.b.date, Foo> . I.e: group first by the Foo.b.id and then by Foo.b.date . I'm struggling with the following 2-step approach, but the second one doesn't even compile: Map<String, List<Foo>> groupById = myList .stream() .collect( Collectors.groupingBy( foo -> foo.getBar().getId()

问题 Suppose I have a list like this : [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] Is it possible to use a Java 8 stream to take every second element from this list to obtain the following? [1, 3, 5, 7, 9] Or maybe even every third element? [1, 4, 7, 10] Basically, I'm looking for a function to take every nth element of a stream: List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10); List<Integer> list2 = list.stream().takenth(3).collect(Collectors.toList()); System.out.println(list2); // => [1, 4

问题 byte[] a = {1,2,3}; System.out.println(Stream.of(a).count()); Byte[] b = {1,2,3}; System.out.println(Stream.of(b).count()); the result is 1 and 3, why? 回答1: Stream.of only accepts Objects as its arguments. A byte isn't an Object, but a byte array is. If a is an array of byte , then Stream.of(a) can only mean "stream of this one object, which is an array". If you have a Byte[] array, then each element of the array is an object, so the compiler can guess that's what you mean. There is