Java: how can I split an ArrayList in multiple small ArrayLists?
Solution 1:
You can use subList(int fromIndex, int toIndex)
to get a view of a portion of the original list.
From the API:
Returns a view of the portion of this list between the specified
fromIndex
, inclusive, andtoIndex
, exclusive. (IffromIndex
andtoIndex
are equal, the returned list is empty.) The returned list is backed by this list, so non-structural changes in the returned list are reflected in this list, and vice-versa. The returned list supports all of the optional list operations supported by this list.
Example:
List<Integer> numbers = new ArrayList<Integer>(
Arrays.asList(5,3,1,2,9,5,0,7)
);
List<Integer> head = numbers.subList(0, 4);
List<Integer> tail = numbers.subList(4, 8);
System.out.println(head); // prints "[5, 3, 1, 2]"
System.out.println(tail); // prints "[9, 5, 0, 7]"
Collections.sort(head);
System.out.println(numbers); // prints "[1, 2, 3, 5, 9, 5, 0, 7]"
tail.add(-1);
System.out.println(numbers); // prints "[1, 2, 3, 5, 9, 5, 0, 7, -1]"
If you need these chopped lists to be NOT a view, then simply create a new List
from the subList
. Here's an example of putting a few of these things together:
// chops a list into non-view sublists of length L
static <T> List<List<T>> chopped(List<T> list, final int L) {
List<List<T>> parts = new ArrayList<List<T>>();
final int N = list.size();
for (int i = 0; i < N; i += L) {
parts.add(new ArrayList<T>(
list.subList(i, Math.min(N, i + L)))
);
}
return parts;
}
List<Integer> numbers = Collections.unmodifiableList(
Arrays.asList(5,3,1,2,9,5,0,7)
);
List<List<Integer>> parts = chopped(numbers, 3);
System.out.println(parts); // prints "[[5, 3, 1], [2, 9, 5], [0, 7]]"
parts.get(0).add(-1);
System.out.println(parts); // prints "[[5, 3, 1, -1], [2, 9, 5], [0, 7]]"
System.out.println(numbers); // prints "[5, 3, 1, 2, 9, 5, 0, 7]" (unmodified!)
Solution 2:
You can add the Guava library to your project and use the Lists.partition method, e.g.
List<Integer> bigList = ...
List<List<Integer>> smallerLists = Lists.partition(bigList, 10);
Solution 3:
Apache Commons Collections 4 has a partition method in the ListUtils
class. Here’s how it works:
import org.apache.commons.collections4.ListUtils;
...
int targetSize = 100;
List<Integer> largeList = ...
List<List<Integer>> output = ListUtils.partition(largeList, targetSize);