I have an array of size 1000. I want to use the stream operations to performe like this :-
List list= new ArrayList();
//list is initialize to 1000 elements
List subList = list.subList(0, 100);
// perform some operaions over the subarray
List subList1 = list.subList(101, 200);
// perform some operaions over the subarray
.... so on
}
I want code using stream API. Thanks in advance
What about :
List<List<Integer>> result = IntStream.range(0, list.size() / 100)
.mapToObj(index -> list.subList(index * 100, index * 100 + 100))
.collect(Collectors.toList());
You could either use Collectors.partitioningBy:
Map<Boolean, List<Integer>> map = list.stream().collect(Collectors.partitioningBy(element -> list.indexOf(element) >= 100));
and then do:
List<List<Integer>> results = new ArrayList(map.values());
Update:
Collectors.partitioningBy takes a predicate and thus is not able to solve the desired use case.
Or if you want to split a list into equal parts (what I think is more youre use case), you could use Collectors.groupingBy():
Map<Integer, List<Integer>> groups =
list.stream().collect(Collectors.groupingBy(element -> (element - 1) / YOUR_NUMBER_OF_PIECES_PER_SUBLIST));
List<List<Integer>> subLists= new ArrayList<List<Integer>>(groups.values());
System.out.println("Number of sublists " + subLists.size());
This gives you:
Number of sublists: 5
when running with NUMBER_OF_PIECES_PER_SUBLIST = 200, which seems your use case.
To use the Stream API with an array, you want to use StreamSupport and a Spliterator.
Arrays provides utility methods to create a Spliterator.
For example:
int[] array = new int[1000];
StreamSupport.stream(Arrays.spliterator(array, 0, 100), false)
.forEach(e -> {});
StreamSupport.stream(Arrays.spliterator(array, 100, 200), false)
.forEach(e -> {});
Note - it's zero-based indexing, and the start index is inclusive and the end index is exclusive.
You can use IntStream.iterate() to achieve that:
int sublistItems = 100;
List<List<Integer>> result = IntStream.iterate(0, i -> i + sublistItems)
.limit((list.size() + sublistItems - 1) / sublistItems)
.mapToObj(startIndex -> list.subList(startIndex, Math.min(startIndex + sublistItems, list.size())))
.collect(Collectors.toList());
If you are using Java 9 or above you can simplify it like that:
int sublistItems = 100;
List<List<Integer>> result = IntStream.iterate(0, i -> i < list.size(), i -> i + sublistItems)
.mapToObj(startIndex -> list.subList(startIndex, Math.min(startIndex + sublistItems, list.size())))
.collect(Collectors.toList());
0, right?.subList(0, 100)has only 100 elements. Whereas.subList(101, 200)will have 99 elements.