# Array of ArrayList in Java

• Difficulty Level : Medium
• Last Updated : 11 Dec, 2018

We often come across 2D arrays where most of the part in the array is empty. Since space is a huge problem, we try different things to reduce the space. One such solution is to use jagged array when we know the length of each row in the array, but the problem arises when we do not specifically know the length of each of the rows. Here we use ArrayList since the length is unknown.
Following is a Java program to demonstrate the above concept.

 `// Java code to demonstrate the concept of ``// array of ArrayList`` ` `import` `java.util.*;``public` `class` `Arraylist {``    ``public` `static` `void` `main(String[] args)``    ``{``        ``int` `n = ``5``;`` ` `        ``// Here al is an array of arraylist having``        ``// n number of rows.The number of columns on``        ``// each row depends on the user.``        ``// al[i].size() will give the size of the``        ``// i'th row``        ``ArrayList[] al = ``new` `ArrayList[n];`` ` `        ``// initializing``        ``for` `(``int` `i = ``0``; i < n; i++) {``            ``al[i] = ``new` `ArrayList();``        ``}`` ` `        ``// We can add any number of columns to each``        ``// rows as per our wish``        ``al[``0``].add(``1``);``        ``al[``0``].add(``2``);``        ``al[``1``].add(``5``);``        ``al[``2``].add(``10``);``        ``al[``2``].add(``20``);``        ``al[``2``].add(``30``);``        ``al[``3``].add(``56``);``        ``al[``4``].add(``34``);``        ``al[``4``].add(``67``);``        ``al[``4``].add(``89``);``        ``al[``4``].add(``12``);`` ` `        ``for` `(``int` `i = ``0``; i < n; i++) {``            ``for` `(``int` `j = ``0``; j < al[i].size(); j++) {``                ``System.out.print(al[i].get(j) + ``" "``);``            ``}``            ``System.out.println();``        ``}``    ``}``}`

Output :

```1 2
5
10 20 30
56
34 67 89 12
```

The above code works fine, but shows below warning.

```prog.java:15: warning: [unchecked] unchecked conversion
ArrayList[] al = new ArrayList[n];
^
required: ArrayList[]
found:    ArrayList[]
1 warning
```

The warning comes basically due to below line.

 `ArrayList[] al = ``new` `ArrayList[n];`

How to fix above warning?
We cannot use array of ArrayList without warning. We basically need to use ArrayList of ArrayList.

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