How can I create an Array of ArrayLists?

Question

As per Oracle Documentation:

"You cannot create arrays of parameterized types"

Instead, you could do:

ArrayList<ArrayList<Individual>> group = new ArrayList<ArrayList<Individual>>(4);

As suggested by Tom Hawting - tackline, it is even better to do:

List<List<Individual>> group = new ArrayList<List<Individual>>(4);

As the others have mentioned it's probably better to use another List to store the ArrayList in but if you have to use an array:

ArrayList<Individual>[] group = (ArrayList<Individual>[]) new ArrayList[4];

You will need to suppress the warning but it's safe in this case.

This works:

ArrayList<String>[] group = new ArrayList[4];

Though it will produce a warning that you may want to suppress.

You can create a class extending ArrayList

class IndividualList extends ArrayList<Individual> {

}

and then create the array

IndividualList[] group = new IndividualList[10];

You can create Array of ArrayList

List<Integer>[] outer = new List[number];
for (int i = 0; i < number; i++) {
    outer[i] = new ArrayList<>();
}

This will be helpful in scenarios like this. You know the size of the outer one. But the size of inner ones varies. Here you can create an array of fixed length which contains size-varying Array lists. Hope this will be helpful for you.

In Java 8 and above you can do it in a much better way.

List<Integer>[] outer = new List[number];
Arrays.setAll(outer, element -> new ArrayList<>());

This works, array of ArrayList. Give it a try to understand how it works.

import java.util.*;

public class ArrayOfArrayList {
    public static void main(String[] args) {

        // Put the length of the array you need
        ArrayList<String>[] group = new ArrayList[15];
        for (int x = 0; x < group.length; x++) {
            group[x] = new ArrayList<>();
        }

        //Add some thing to first array
        group[0].add("Some");
        group[0].add("Code");

        //Add some thing to Secondarray
        group[1].add("In here");

        //Try to output 'em
        System.out.println(group[0]);
        System.out.println(group[1]);
    }
}

Credits to Kelvincer for some of codes.