Java program to group a sequential number from given numbers in a <list>

Your general logic is mostly correct. However, you have a couple of problems when doing this.

• You are getting an array index out of bounds exception because you are calling a[i+1]
  
  when i = a.length
  
  . Change the loop condition to a.length - 1
  
  .
• Every time you need to use a new internal one ArrayList
  
  , otherwise each array will be destroyed. Change temp.clear();
  
  to temp = new ArrayList<>();
  
  .
• You need to initialize a new list for the first element, not an empty list. Add temp.add(a[0]);
  
  at the beginning and temp.add(a[i+1])
  
  when you find you need a new sublist.
• You need to add the final list at the end because your condition will not be called at the end of the loop.

Here's the modified program:

import java.util.ArrayList;
import java.util.List;
import java.util.Arrays;

public class SubList {
    public static void main(String... args) {
        int[] a = { 11, 2, 7, 6, 13,17, 8, 9, 3, 5, 12 };

        Arrays.sort(a);
        List<List<Integer>> ListMain = new ArrayList<>();
        List<Integer> temp = new ArrayList<>();
        temp.add(a[0]);

        for (int i = 0; i < a.length - 1; i++) {
            if (a[i + 1] == a[i] + 1) {
                temp.add(a[i + 1]);
            } else {
                ListMain.add(temp);
                temp = new ArrayList<>();
                temp.add(a[i+1]);
            }
        }

        ListMain.add(temp);

        System.out.println(ListMain);
    }
}

      

        
        
        
      

    

Output:

[[2, 3], [5, 6, 7, 8, 9], [11, 12, 13], [17]]