collectors

问题 I have a Java Map that I'd like to transform and filter. As a trivial example, suppose I want to convert all values to Integers then remove the odd entries. Map<String, String> input = new HashMap<>(); input.put("a", "1234"); input.put("b", "2345"); input.put("c", "3456"); input.put("d", "4567"); Map<String, Integer> output = input.entrySet().stream() .collect(Collectors.toMap( Map.Entry::getKey, e -> Integer.parseInt(e.getValue()) )) .entrySet().stream() .filter(e -> e.getValue() % 2 == 0)

问题 This question already has answers here : Word frequency count Java 8 (8 answers) Closed 4 years ago . 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 ? 回答1: 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

问题 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

问题 I'm trying to understand the implementation of downstream reduction in JDK. Here is it: public static <T, K, D, A, M extends Map<K, D>> Collector<T, ?, M> groupingBy(Function<? super T, ? extends K> classifier, Supplier<M> mapFactory, Collector<? super T, A, D> downstream) { Supplier<A> downstreamSupplier = downstream.supplier(); BiConsumer<A, ? super T> downstreamAccumulator = downstream.accumulator(); BiConsumer<Map<K, A>, T> accumulator = (m, t) -> { K key = Objects.requireNonNull

问题 I want to count the different elements of a stream and am wondering why Stream<String> stream = Stream.of("a", "b", "a", "c", "c", "a", "a", "d"); Map<String, Integer> counter1 = stream.collect(Collectors.toMap(s -> s, 1, Integer::sum)); doesn't work. Eclipse tells me The method toMap(Function, Function, BinaryOperator) in the type Collectors is not applicable for the arguments (( s) -> {}, int, Integer::sum) By the way, I know about that solution: Map<String, Long> counter2 = stream.collect

问题 In Java 8, this works: Stream<Class> stream = Stream.of(ArrayList.class); HashMap<Class, List<Class>> map = (HashMap)stream.collect(Collectors.groupingBy(Class::getSuperclass)); But this doesn't: Stream<Class> stream = Stream.of(List.class); HashMap<Class, List<Class>> map = (HashMap)stream.collect(Collectors.groupingBy(Class::getSuperclass)); Maps allows a null key, and List.class.getSuperclass() returns null. But Collectors.groupingBy emits a NPE, at Collectors.java, line 907: K key =

问题 I have a object with contains a list of other objects and I want to return a flatmap of the contained objects mapped by some property of the container. Any one if is it possible using stream and lambdas only? public class Selling{ String clientName; double total; List<Product> products; } public class Product{ String name; String value; } Lets supose a list of operations: List<Selling> operations = new ArrayList<>(); operations.stream() .filter(s -> s.getTotal > 10) .collect(groupingBy

问题 I have a collection that looks like below, and I want to filter out the everything except the dates that aren't the end of the months. 2010-01-01=2100.00, 2010-01-31=2108.74, 2010-02-01=2208.74, 2010-02-28=2217.92, 2010-03-01=2317.92, 2010-03-31=2327.57, 2010-04-01=2427.57, 2010-04-30=2437.67, 2010-05-01=2537.67, 2010-05-31=2548.22, 2010-06-01=2648.22, 2010-06-30=2659.24, 2010-07-01=2759.24, 2010-07-31=2770.72, 2010-08-01=2870.72, 2010-08-31=2882.66, 2010-09-01=2982.66, 2010-09-30=2995.07,

问题 I'm implementing own collector that uses merge function . Unfortunately, for some of my cases, I can't reuse the following JDK merger function that thrown IllegalStateException . java.util.stream.Collectors#throwingMerger It happens due to the fact that it has private access modifier and access from other(not inner) classes is restricted. However, javadoc says the following: This can be used to enforce the assumption that the elements being collected are distinct But, as I see, java doc is

问题 When a duplicate key entry is found during Collectors.toMap() , the merge function (o1, o2) is called. Question: how can I get the key that caused the duplication? String keyvalp = "test=one\ntest2=two\ntest2=three"; Pattern.compile("\n") .splitAsStream(keyval) .map(entry -> entry.split("=")) .collect(Collectors.toMap( split -> split[0], split -> split[1], (o1, o2) -> { //TODO how to access the key that caused the duplicate? o1 and o2 are the values only //split[0]; //which is the key, cannot