2D static array of dynamic array in Java

Question

I have a 2D array as an input of NxM size, where N is known and static, and M actually grows dynamically and will be different for each index of the array[0...N-1].

I was thinking I could initialize my 2D array like so:

ArrayList<Integer>[] array = new ArrayList[n];

but this leaves all sub-arrays initialized to null instead of an ArrayList instance. For example, calling

array[0].add(1);

crashes with a NullPointerException

How do I properly initialize the ArrayLists?

as an aside, consider using `List> myList = new ArrayList<>(n);` rather than `ArrayList[] array = new ArrayList[n];`. — Ousmane D., Mar 24 '18 at 22:31

score 3 · Accepted Answer · answered Mar 24 '18 at 22:31

3

You have initialized the array itself, not the list at the 1st index (and so on...).

List<Integer>[] array = new ArrayList[n];
array[0] = new ArrayList<>();
array[0].add(1);

Anyway, I recommend you to avoid the array structure and pick List<List<Integer>> instead. Or create tuples class (more info at A Java collection of value pairs? (tuples?)).

answered Mar 24 '18 at 22:31

Nikolas Charalambidis

40,893
16
117
183

My N is 100K-200K. Would looping through and initializing for each ArrayList be unavoidable? – adhdj Mar 24 '18 at 22:36
The initialization is unavoidable regardless the number. Read the recommendation on the second part of my updated answer. – Nikolas Charalambidis Mar 24 '18 at 22:39

score 1 · Answer 2 · answered Mar 24 '18 at 22:38

1

As you'll see at the Oracle documentation

You cannot create arrays of parameterized types.

You could use an ArrayList<ArrayList<T>> or a List<List<T>>.

answered Mar 24 '18 at 22:38

ryanm

451
2
7

NSchorr · Answer 3 · 2018-04-22T02:57:18.237

0

Here's how I would do this:

        List<ArrayList<Integer>> complex = new ArrayList <ArrayList<Integer>>();
        ArrayList<Integer> simple = new  ArrayList<Integer>();
        simple.add((Integer)5555);
        complex.add(simple);

edited Apr 22 '18 at 02:57

answered Mar 24 '18 at 23:01

NSchorr

875
10
13

score 0 · Answer 4 · edited May 07 '18 at 09:50

0

ArrayList<Integer>[] array = new ArrayList[n];

Instead of doing it. You can do like below:

List<List<Integer>> list1 = new ArrayList<>();
List<Integer> list2 = new ArrayList<>();
list2.add(2);
list1.add(list2);
ArrayList[] array = list1.toArray(new ArrayList[10]);
System.out.println(array[0]);

edited May 07 '18 at 09:50

MSDN.WhiteKnight

664
7
30

answered Apr 22 '18 at 04:20

Varun Srivastawa

72
4

2D static array of dynamic array in Java

4 Answers4