An Array of ArrayLists (Null Pointer Exception)

Issue

i'm currently trying to add items to an generic "array" of arraylists but for some reason i keep getting a null pointer exception. The Structure is initialised and both my array index reference and the reference to the object i'm passing in are both visible within the body of code right before the exception occurs. I'm almost sure its down to the way i either declared the data structure or my way im trying to add it in. Any advice would be appreciated. Thanks in advance

ArrayList<Site>[] group = (ArrayList<Site>[])new ArrayList[entranceSites.size()];

group[i].add(sIndex(path));

sIndex is a function I'm using to convert integers to graph sites and the object is not null when I'm passing it in so i'm sure its not the problem. I is initialised and also visible to the program.

Solution

You have allocated an array of ArrayLists, but you have not allocated any actual ArrayLists inside that array. The array initially contains all null references. So your invocation to add is on a null reference and thus causes the exception. If you say:

group[i] = new ArrayList<Site>();

Before you call add it will work.

Note that it is generally a bad idea to mix primitive arrays and Java collections, and if you are new to Java, then you should probably stick to collections since they are going to be easier to work with.

You should also be aware that the cast you are making (ArrayList<Site>[]) is unchecked and will almost certainly generate a warning assuming you have warnings enabled, which you should be enabling warnings as a beginner. This is another reason why it is not a good idea to mix generics with primitive arrays.

Answered By - b4hand