collectors

The follow code : public class Test { public static void main(String[] args) { Stream.of(1,2,3).map(String::valueOf).collect(Collectors::toList) } } intellij tell me : Collector<String, A, R> is not a functional interface but when i modify the code as follows, everything is ok, i don't know why? public class Test { public static void main(String[] args) { Stream.of(1,2,3).map(String::valueOf).collect(Collectors.<String>toList) } } The reason that the first syntax is illegal is that the target type implied by the method signature— Stream.collect(Collector) —is a Collector . Collector has

I have recently started using collectingAndThen and found that it is taking a bit long time comparatively to the other coding procedures, which i used for performing the similar tasks. Here is my code: System.out.println("CollectingAndThen"); Long t = System.currentTimeMillis(); String personWithMaxAge = persons.stream() .collect(Collectors.collectingAndThen( Collectors.maxBy(Comparator.comparing(Person::getAge)), (Optional<Person> p) -> p.isPresent() ? p.get().getName() : "none" )); System.out.println("personWithMaxAge - "+personWithMaxAge + " time taken = "+(System.currentTimeMillis() - t));

Can any one explain why the below code will not compile but the second one does? Do not compile private void doNotCompile() { List<Integer> out; out = IntStream .range(1, 10) .filter(e -> e % 2 == 0) .map(e -> Integer.valueOf(2 * e)) .collect(Collectors.toList()); System.out.println(out); } Compilation errors on the collect line The method collect(Supplier, ObjIntConsumer, BiConsumer) in the type IntStream is not applicable for the arguments (Collector>) Type mismatch: cannot convert from Collector> to Supplier Compiles private void compiles() { List<Integer> in; in = Arrays.asList(1, 2, 3, 4,

This question already has an answer here: Word frequency count Java 8 8 answers I have an Object[] array I need to create map Map<Obejct, Integer> , where Integer value contains frequency of key Object in array. How can i do it in java 8 style, using Collectors ? You can do (I hope I don't have any typos) : Map<Object,Long> map = Stream.of(array) .collect(Collectors.groupingBy(o -> o, Collectors.counting())); This should group the elements of the array by equality and count the number of Objects in each group. 来源： https://stackoverflow.com/questions/29450169/java-8-frequency-object-in-array

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]); ); 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: ListMultimap<String, String> multimap = lines.stream() .collect(toImmutableListMultimap(l -> l[0], l -> l[1]))

I have below code snap with Java 8. List<Employee> employees = DataProvider.getEmployees(); Set<Employee> set = employees.stream().filter(emp -> { System.out.println(emp.getName()); return emp.getName().equals("Vishal"); }).collect(Collectors.toSet()); I just want to know which implementation of Set it is using by default when we use Collectors.toSet() (refer above example)? Also, is there any way to tell the Java API to use a particular implementation (for example, HashSet )? The toSet() collector does not specify which implementation it uses; you get a Set , that's all. If you want a

With the classes: public class Person { private String name; private Color favouriteColor; } public enum Color {GREEN, YELLOW, BLUE, RED, ORANGE, PURPLE} Having a List<Person> using the Java8 Stream API can I trasform it in a Map<Color, Long> having the count of each Color , also for the color that aren't included in the list. Example: List<Person> list = List.of( new Person("Karl", Color.RED), new Person("Greg", Color.BLUE), new Person("Andrew", Color.GREEN) ); Trasforming this list in a Map with all the colors of the enum with their count. Thanks SOLVED Solved using a custom collector: