在java中初始化自定义类的二维数组

Question

当我尝试实例化一个包含二维接口数组的类时，我收到了 NullPointerException。 在另一个类中，我有一个 CompetitionGround 类型的对象，我尝试做这样的事情来初始化它：

CompetitionGround groud;
ground=new CompetitionGround(5);

我的 CompetitionGround 类的构造函数如下所示：

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

整个班级是：

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);
    }
}

实现 IEgg 的 Egg 类有两种类型的构造函数。我尝试了两者并遇到了同样的问题。我究竟做错了什么？我想不通。

Answer 1

数组本身从未被初始化，所以你还不能给它的元素分配任何东西。在 2 个嵌套的 for 循环中初始化之前，先创建 2D 数组本身。

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

您从未初始化过eggs，这导致了您的问题。现在您正在初始化eggs 的每个元素，但如果您不先初始化eggs 本身，就会给您带来问题。

我建议在构造函数中这样做：

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();
        }
    }
}

此外，这是不相关的，但边界拼写错误。以后可能会出现问题。