collectors

问题 Process list of string via method which returns ImmutableTable<R,C,V> . For instance ImmutableTable<Integer,String,Boolean> process(String item) { /*...*/} . Collect the result i.e, merge all results (individual table may contain duplicates) and return ImmutableTable . My current implementation works when there are no duplicates: final ImmutableTable<Integer, String, Boolean> result = itemsToProcess.parallelStream() .map(item -> ProcessorInstanceProvider.get() .buildTable(item)) .collect

问题 Process list of string via method which returns ImmutableTable<R,C,V> . For instance ImmutableTable<Integer,String,Boolean> process(String item) { /*...*/} . Collect the result i.e, merge all results (individual table may contain duplicates) and return ImmutableTable . My current implementation works when there are no duplicates: final ImmutableTable<Integer, String, Boolean> result = itemsToProcess.parallelStream() .map(item -> ProcessorInstanceProvider.get() .buildTable(item)) .collect

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 My problem essentially comes down to this simplified example. I have data coming back from a DB which has some duplicate information in the rows. In this example I have a list of TeamRow objects that come back from the DB. I can easily group these using Collectors.groupingBy : public class TeamRow { private int id; private String name; private String player; public TeamRow(int id, String name, String player) { this.id = id; this.name = name; this.player = player; } public int getId() {return

问题 As for example, there are two lists: List<Double> list1 = Arrays.asList(1.0, 2.0); List<String> list2 = Arrays.asList("one_point_zero", "two_point_zero"); Using Stream, I want to create a map composed of these lists, where list1 is for keys and list2 is for values. To do it, I need to create an auxiliary list: List<Integer> list0 = Arrays.asList(0, 1); Here is the map: Map<Double, String> map2 = list0.stream() .collect(Collectors.toMap(list1::get, list2::get)); list0 is used in order list1:

问题 I am currently creating a Map<String, Map<LocalDate, Integer>> like this, where the Integer represents seconds: Map<String, Map<LocalDate, Integer>> map = stream.collect(Collectors.groupingBy( x -> x.getProject(), Collectors.groupingBy( x -> x.getDate(), Collectors.summingInt(t -> t.getDuration().toSecondOfDay()) ) )); How could I instead create a Map<String, Map<LocalDate, Duration>> ? 回答1: To change that Integer from Collectors.summingInt to a Duration , you simply need to replace that

问题 I have two objects like following: public class A { private Integer id; private String name; private List<B> list; public A(Integer id, String name, List<B> list) { this.id = id; this.name = name; this.list = list; } //getters and setters } and public class B { private Integer id; private String name; public B(Integer id, String name) { this.id = id; this.name = name; } //getters and setters } So, A holds a list of B and there is a list of A populated as follows: List<A> list = new ArrayList<

问题 I have a Stream< Map< K, V > > and I'm trying to merge those maps together, but preserve duplicate values in a list, so the final type would be Map< K, List<V> > . Is there a way to do this? I know the toMap collector has a binary function to basically choose which value is returned, but can it keep track of the converted list? i.e. if a is a Stream< Map< String, Int > > a.flatMap(map -> map.entrySet().stream()).collect( Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (val1, val2) ->

问题 For the Collectors.groupingBy() that returns Map<K,List<T>> is it implied that the List<T> is in order that the stream is evaluated? I see no explicit description of the ordering of the list, whereas the concurrent version explicitly states no ordering. If it weren't ordered somehow, I'd expect it to be a Collection though, and I don't see what other ordering it could possibly be, other than order received. I'm hoping it's guaranteed that the last value in each list is the last value received

问题 Collectors.toSet() does not preserve order. I could use Lists instead, but I want to indicate that the resulting collection does not allow element duplication, which is exactly what Set interface is for. 回答1: You can use toCollection and provide the concrete instance of the set you want. For example if you want to keep insertion order: Set<MyClass> set = myStream.collect(Collectors.toCollection(LinkedHashSet::new)); For example: public class Test { public static final void main(String[] args)