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?
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?)).
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>>.
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);
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]);
List<ArrayList<Integer>> myList = new ArrayList<>(n);rather thanArrayList<Integer>[] array = new ArrayList[n];.