java-stream

I have a list of objects. At first, I need to sort it by type. Than by faceValue. In the end, summarize all quantities: class Coin{ String type; BigInteger faceValue; BigInteger quantity; ... } List<Coin> coins = new ArrayList<>(); coins.add(new Coin("USD", 1, 150)); coins.add(new Coin("USD", 1, 6)); coins.add(new Coin("USD", 1, 60)); coins.add(new Coin("USD", 2, 100)); coins.add(new Coin("USD", 2, 100)); coins.add(new Coin("CAD", 1, 111)); coins.add(new Coin("CAD", 1, 222)); Result list must contains only 3 new coin objects: Coin("USD", 1 , 216) Coin("USD", 2 , 200) Coin("CAD", 1 , 333) How

This is a follow-up of How to get the count of keys for values in a hash map using lambda . I have a HashMap and I want to find the number of keys for each value Map<Integer, List<Integer>> map = new HashMap<Integer, List<Integer>>() {{ put(0, Arrays.asList(1, 2)); put(1, Arrays.asList(2, 0, 3)); put(2, Arrays.asList(4,0,1)); put(3, Arrays.asList(4,1, 5)); put(4, Arrays.asList(5,2,3)); put(5, Arrays.asList(4,3)); }}; According to the above post, I tried flat mapping: Map<Object, Long> ex = map.values() .stream() .flatMap(Collection::stream) .collect(Collectors.groupingBy(v -> v, Collectors

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,

I'm new to Java8 and I can't use streams to map one array into another 2 dimensional array. I have one 2-dimensional array which is a pattern: boolean[][] pattern = { {true, true, false}, {true, false, true}, {false, true, true} }; And second array which contains keys. 0 means: take 0-element from pattern 1 means: take 1-element from pattern and so on int[] keys = {2, 1, 0}; From these 2 arrays I'd like to produce another 2-dimensional array. In this case the result will look like this: boolean[][] result = { {false, true, true}, {true, false, true}, {true, true, false} }; This is the code in

A question about spliterators that at first glance is not straightforward. In streams, .parallel() changes the behaviour that the stream is processed. However I was expecting the spliterators created from sequential and parallel streams to be the same. For example, in sequential streams typically, the .trySplit() is never invoked , while in parallel streams it is, in order to hand over the split spliterator to another thread. Differences between stream.spliterator() vs stream.parallel().spliterator() : They may have different characteristics: Stream.of(1L, 2L, 3L).limit(2); // ORDERED Stream

Suppose, however, that the result container used in this reduction was a concurrently modifiable collection -- such as a ConcurrentHashMap. In that case, the parallel invocations of the accumulator could actually deposit their results concurrently into the same shared result container, eliminating the need for the combiner to merge distinct result containers. This potentially provides a boost to the parallel execution performance. We call this a concurrent reduction. also A Collector that supports concurrent reduction is marked with the Collector.Characteristics.CONCURRENT characteristic.

I'm trying to make a List from a primitive array int[] values={4,5,2,3,42,60,20}; List<Integer> greaterThan4 = Arrays.stream(values) .filter(value -> value > 4) .collect(Collectors.toList()); But the last function collect gives me an error because it wants other arguments. It wants 3 arguments Supplier, ObjIntConsumer and BiConsumer. I don't understand why it wants 3 arguments when I have seen different examples that just use collect(Collectors.toList()); and get the list. What I'm doing wrong? Yes this is because Arrays.stream returns an IntStream . You can call boxed() to get a Stream