在Java中初始化自定义类的二维数组

Question

I'm getting a NullPointerException when I try to instantiate a class that contains a 2d array of interface. In a other class i have an Object of type CompetitionGround and i try to do something like this to initialize it:

CompetitionGround groud;
ground=new CompetitionGround(5);

My constructor for the class CompetitionGround looks like this:

public CompetitionGround(int boundries) {
    for (int i = 0; i <boundries; i++)
        for (int j = 0; j <boundries; j++)
            eggs[i][j]=new Egg();
}

and the whole class is:

public class CompetitionGround {

    private IEgg eggs[][];

    public void goIn(Rabbit rabbit) {
        IPozition temp = rabbit.getPozition();
        rabbit.Collect(eggs[temp.getPozitionX()][temp.getPozitionY()]);
    }

    public CompetitionGround(int boundries) {
        for (int i = 0; i < boundries; i++)
            for (int j = 0; j < boundries; j++)
                eggs[i][j] = new Egg();
    }

    public void AddEgg(int x, int y, int points) {
        eggs[x][y] = new Egg(points);
    }
}

Class Egg that implements IEgg has two types of constructors. I tried with both and get the same problem. What am I doing wrong? I can't figure it out.

Answer 1

The array itself was never initialized, so you can't assign anything to its elements yet. Before initializing in the 2 nested for loop, create the 2D array itself first.

public CompetitionGround(int boundries  /* [sic] */) {
    // Init array here.
    eggs = new IEgg[boundries][boundries];

    // You should use proper indenting.
    for (int i = 0; i < boundries; i++)
       for (int j = 0; j < boundries; j++)
           eggs[i][j] = new Egg();
}

Answer 2

You aren't ever initializing eggs , which is causing your issues. Right now you are initializing each element of eggs , but that will give you problems if you don't initialize eggs itself first.

I recommend doing this in the constructor like this:

public CompetitionGround(int boundries)
{
    //Remember, you want IEgg, not Egg here
    //This is so that you can add elements that implement IEgg but aren't Eggs
    eggs = new IEgg[boundries][boundries];

    for (int i = 0; i <boundries; i++)
    {
        for (int j = 0; j <boundries; j++)
        {
            eggs[i][j]=new Egg();
        }
    }
}

Also, this is unrelated, but boundaries is spelled wrong. Could cause problems later.