Java: Add elements ArrayList

Question

I have this code that tells me a NullPointerException precisely at the method inizializza() at line:

valori[y].add(new Record(matriceBinaria[i][j], j));

How can I add items to the valori without that kind of exception?

Code java

Answer 1

You must also initialize each element in you array as follows :

if (valori[y] == null)
    valori[y] = new ArrayList();

valori[y].add(new Record(matriceBinaria[i][j], j));

ArrayList[size] is actually an array where each element is the ArrayList object. Since ArrayList is not a primitive you must alloacte memory for it by using new. So when you do valori[y] in your code you are accessing an uninitialized (null) object