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[]

• JB Nizet: Read stackoverflow.com/questions/2770321/…
• cpp beginner: In addition to avoiding raw types you will have to change toArray(arr[0]) to toArray(new Integer[0])) to avoid a NullPointerException.

Answer 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];

• raj: But it is giving a warning unchecked conversion required List<Integer>[] found: ArrayList[]
• rkosegi: yes, new ArrayList<Integer>[2] will not compile. You should reconsider you data structure. Currently it seems pretty horrible to work with.