How to create and use a two dimensional array of classes

Question

I was just experimenting with a two dimensional array of classes in java. I wrote the following (wrong) code:

public class GameEvent
{
    static int LAUNCH_ALIEN = 0;
    static int END = 1;

    long time;
    long cum_time;
    int alien_type;
    int event_code;
}

then

GameEvent[][] event_array = new GameEvent[MAX_LEVELS][MAX_EVENTS];

and accessed it with code like below (read comment in code):

event_array[lev][ctr].event_code = code; // nullpointer exception at runtime

So my question now is, what is the least painful way to change my code so that it works.

how does it not work?

Woot4Moo
– Woot4Moo

2013-05-14 16:12:19 +00:00
Commented May 14, 2013 at 16:12 — Woot4Moo
– Woot4Moo, Commented May 14, 2013 at 16:12
Just initialize the array and it will work.

Zutty
– Zutty

2013-05-14 16:12:48 +00:00
Commented May 14, 2013 at 16:12 — Zutty
– Zutty, Commented May 14, 2013 at 16:12
because that position is pointing to null :)

Triode
– Triode

2013-05-14 16:14:50 +00:00
Commented May 14, 2013 at 16:14 — Triode
– Triode, Commented May 14, 2013 at 16:14

Jack · Accepted Answer · 2013-05-14 16:14:18Z

3

GameEvent[][] event_array = new GameEvent[MAX_LEVELS][MAX_EVENTS];

This allocates the array of references to GameEvent objects but not the objects themselves.

To actually allocate the obects it is necessary to do it for every cell of the array:

for (int i = 0; i < MAX_LEVELS; ++i)
  for (int j = 0; j < MAX_EVENTS; ++j)
    event_array[i][j] = new GameEvent();

answered May 14, 2013 at 16:14

Jack

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