如何创建和使用二维类数组

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.

Answer 1

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();