listOfData.stream() .flatMap(e -> e.entrySet().stream()) .collect(Collectors.groupingBy(Map.Entry::getKey, Collectors.mapping(Map.Entry::getValue, Collectors.toList()))); update: Slightly different variant to user1692342‘s answer for completeness. list.stream() .map(e -> Arrays.asList(e.get(“Role”), e.get(“Name”))) .collect(Collectors.groupingBy(e -> e.get(0), Collectors.mapping(e -> e.get(1), Collectors.toList())));
I’m guessing a bit at what you’re expecting, but I think I have a good hunch. The stream of a queue, like iterating over a queue, represents the current contents of the queue. When the iterator or the stream reaches the tail of the queue, it doesn’t block awaiting further elements to be added. The … Read more
If you want primitive int values, do not call IntStream::boxed as that produces Integer objects by boxing. Simply use Random::ints which returns an IntStream: int[] array = new Random().ints(size, lowBound, highBound).toArray();
One can implement a parallel sort in scala e.g. http://blog.yunglinho.com/blog/2013/03/19/parallel-external-merge-sort/ . I don’t know why ParSeq doesn’t offer sorting in its API. Note that there are a number of alternatives to .par, so don’t necessarily assume that your results from ParSeq are representative of Scala parallelism in general. (But benchmarks are valuable and I wish … Read more
When an exception is thrown in one of the stages, it does not wait for other operations to finish, the exception is re-thrown to the caller. That is how ForkJoinPool handles that. In contrast findFirst for example when run in parallel, will present the result to the caller only after ALL operations have finished processing … Read more
We did in fact prototype some Collector.OfXxx specializations. What we found — in addition to the obvious annoyance of more specialized types — was that this was not really very useful without having a full complement of primitive-specialized collections (like Trove does, or GS-Collections, but which the JDK does not have). Without an IntArrayList, for … Read more
My free StreamEx library allows you to process the pairs of the stream elements using additional pairMap intermediate operation. Like this: StreamEx.of(input).pairMap((current, next) -> doSomethingWith(current, next)); Where input is a Collection, array or Stream. For example, this way you can easily check whether input is sorted: boolean isSorted = StreamEx.of(input) .pairMap((current, next) -> next.compareTo(current)) .allMatch(cmp … Read more
Well, in Java 8, there is Pattern.splitAsStream which will provide a stream of items split by a delimiter pattern but unfortunately no support method for getting a stream of matches. If you are going to implement such a Stream, I recommend implementing Spliterator directly rather than implementing and wrapping an Iterator. You may be more familiar … Read more
I was going to point you to my earlier answer on this topic but it turns out that you’ve already linked to that question. The other answer also provides useful information. If you want char values, you can use the IntStream returned by String.chars() and cast the int values to char without loss of information. … Read more
You can make use of the Collectors.filtering API introduced since Java-9 for this: Map<String, List<Employee>> output = list.stream() .collect(Collectors.groupingBy(Employee::getDepartment, Collectors.filtering(e -> e.getSalary() > 2000, Collectors.toList()))); Important from the API note : The filtering() collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy or partitioningBy. A filtering collector differs … Read more