ArrayList of ArrayList in Java

We have discussed that an array of ArrayList is not possible without warning. A better idea is to use ArrayList of ArrayList.

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// Java code to demonstrate the concept of
// array of ArrayList
  
import java.util.*;
public class Arraylist {
    public static void main(String[] args)
    {
        int n = 3;
  
        // Here aList is an ArrayList of ArrayLists
        ArrayList<ArrayList<Integer> > aList = 
                  new ArrayList<ArrayList<Integer> >(n);
  
        // Create n lists one by one and append to the 
        // master list (ArrayList of ArrayList)
        ArrayList<Integer> a1 = new ArrayList<Integer>();
        a1.add(1);
        a1.add(2);
        aList.add(a1);
  
        ArrayList<Integer> a2 = new ArrayList<Integer>();
        a2.add(5);
        aList.add(a2);
  
        ArrayList<Integer> a3 = new ArrayList<Integer>();
        a3.add(10);
        a3.add(20);
        a3.add(30);
        aList.add(a3);
  
        for (int i = 0; i < aList.size(); i++) {
            for (int j = 0; j < aList.get(i).size(); j++) {
                System.out.print(aList.get(i).get(j) + " ");
            }
            System.out.println();
        }
    }
}

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Output:

1 2 
5 
10 20 30


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