Is it possible to partition a List in pure Jdk8 into equal chunks (sublists).
I know it is possible using Guava Lists class, but can we do it with pure Jdk? I don\'t wan
That can be done easily using the subList()
method:
List<String> collection = new ArrayList<>(21);
// fill collection
int chunkSize = 10;
List<List<String>> lists = new ArrayList<>();
for (int i = 0; i < collection.size(); i += chunkSize) {
int end = Math.min(collection.size(), i + chunkSize);
lists.add(collection.subList(i, end));
}
private final String dataSheet = "103343262,6478342944, 103426540,84528784843, 103278808,263716791426, 103426733,27736529279,
103426000,27718159078, 103218982,19855201547, 103427376,27717278645,
103243034,81667273413";
final int chunk = 2;
AtomicInteger counter = new AtomicInteger();
Collection<List<String>> chuncks= Arrays.stream(dataSheet.split(","))
.map(String::trim)
.collect(Collectors.groupingBy(i->counter.getAndIncrement()/chunk))
.values();
result:
pairs =
"103218982" -> "19855201547"
"103278808" -> "263716791426"
"103243034" -> "81667273413"
"103426733" -> "27736529279"
"103426540" -> "84528784843"
"103427376" -> "27717278645"
"103426000" -> "27718159078"
"103343262" -> "6478342944"
We need to group each 2 elements into key, value pairs, so will partion the list into chunks of 2, (counter.getAndIncrement() / 2) will result same number each 2 hits ex:
IntStream.range(0,6).forEach((i)->System.out.println(counter.getAndIncrement()/2));
prints:
0
0
1
1
2
2
You may ajust chunk sizee to partition lists sizes.
I have tried my own solution with a custom made Collector. I hope someone will find it useful, or help me improve it.
class PartitioningCollector<T> implements Collector<T, List<List<T>>, List<List<T>>> {
private final int batchSize;
private final List<T> batch;
public PartitioningCollector(int batchSize) {
this.batchSize = batchSize;
this.batch = new ArrayList<>(batchSize);
}
@Override
public Supplier<List<List<T>>> supplier() {
return LinkedList::new;
}
@Override
public BiConsumer<List<List<T>>, T> accumulator() {
return (total, element) -> {
batch.add(element);
if (batch.size() >= batchSize) {
total.add(new ArrayList<>(batch));
batch.clear();
}
};
}
@Override
public BinaryOperator<List<List<T>>> combiner() {
return (left, right) -> {
List<List<T>> result = new ArrayList<>();
result.addAll(left);
result.addAll(left);
return result;
};
}
@Override
public Function<List<List<T>>, List<List<T>>> finisher() {
return result -> {
if (!batch.isEmpty()) {
result.add(new ArrayList<>(batch));
batch.clear();
}
return result;
};
}
@Override
public Set<Characteristics> characteristics() {
return emptySet();
}
}
Guava Lists class has a partition() method that does exactly this. See https://guava.dev/releases/21.0/api/docs/com/google/common/collect/Lists.html#partition-java.util.List-int-
Try using this code, it uses Java 8:
public static Collection<List<Integer>> splitListBySize(List<Integer> intList, int size) {
if (!intList.isEmpty() && size > 0) {
final AtomicInteger counter = new AtomicInteger(0);
return intList.stream().collect(Collectors.groupingBy(it -> counter.getAndIncrement() / size)).values();
}
return null;
}