Array of ArrayList

Question

import java.util.ArrayList;
import java.util.*;
import java.util.List;
class A
{
    public static void main(String args[])
    {
        ArrayList arrayOfArrayList[]=new ArrayList[2];
        int i;
        for(i=0;i<2;i++)
        {
            arrayOfArrayList[i] = new ArrayList<Integer>();
        }
        arrayOfArrayList[0].add(1);
        arrayOfArrayList[0].add(2);
        arrayOfArrayList[1].add(3);
        arrayOfArrayList[1].add(4);
        Integer arr[][] = new Integer[2][];
        arr[0] = arrayOfArrayList[0].toArray(arr[0]);
        arr[1] = arrayOfArrayList[1].toArray(arr[1]);
        for (Integer x : arr[0])
            System.out.print(x + " ");
    }
}

I am trying to create an array of arrayLists and later convert it to array. But the compile time error error: Incompatible type conversion object[] to Integer[]

In addition to avoiding raw types you will have to change toArray(arr[0]) to toArray(new Integer[0])) to avoid a NullPointerException. — cpp beginner
– cpp beginner, Commented Oct 15, 2017 at 8:14

rkosegi · Accepted Answer · 2017-10-15 08:02:36Z

1

Here you created raw type :

ArrayList arrayOfArrayList[]=new ArrayList[2];

If you need to store Integer there, you should use correct generics:

ArrayList<Integer> arrayOfArrayList[]=new ArrayList[2];

edited Oct 15, 2017 at 8:02

answered Oct 15, 2017 at 7:57

rkosegi

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2 Comments

raj Over a year ago

But it is giving a warning unchecked conversion required List<Integer>[] found: ArrayList[]

rkosegi Over a year ago

yes, new ArrayList<Integer>[2] will not compile. You should reconsider you data structure. Currently it seems pretty horrible to work with.

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