使用字段值初始化的数组中的ArrayIndexOutOfBoundsException

Question

当我尝试将一个元素推入Java数组时，我从构造函数参数中获取数组大小，它会抛出一个ArrayIndexOutOfBoundsException异常。但是，当我在声明添加元素的数组时设置大​​小。

这是我的代码：

public class Stack {
    public int size;

    public Stack(int size)
    {
        this.size = size;
    }

    public int[] arr = new int[size];

    public int top = -1;

    // Methods
    public void push(int value)
    {
        top++;
        arr[top] = value;
    }
}

以下引发异常：

new Stack(10).push(123);

Answer 1

当this.size的值是“正确”值时，您需要初始化数组。最初this.size的值是0（零）（参见Initial Values of Variables）所以这不是初始化数组的时间;你必须“等待”才能知道阵列的大小。提供那么大小的？在类构造函数中。

所以它在构造函数中必须初始化数组（使用提供的大小）。

例如，请参阅下面注释的代码（您的）：

public class Stack {
    public int size ;                   // size = 0 at this time
    public Stack(int size)
    {
        this.size = size;
    }
    public int[] arr = new int[size];  // still size = 0 at this time!
                                       // so you're creating an array of size zero (you won't be able to "put" eny value in it)
    public int top = -1;

    //Methods
    public void push(int value)
    {
        top++;             // at this time top is 0
        arr[top] = value;  // in an array of zero size you are trying to set in its "zero" position the value of `value`
                          // it's something like this:
                          // imagine this array (with no more room)[], can you put values?, of course not
    }
}

所以，为了解决这个问题，你需要改变你的代码：

public class Stack {

    public int size;
    public int[] arr;       // declare the array variable, but do not initialize it yet
    public int top = -1;

    public Stack(int size) {
        this.size = size;
        arr = new int[size];  // when we know in advance which will be the size of the array, then initialize it with that size
    }

    //Methods
    public void push(int value) {
        top++;
        arr[top] = value;
    }
}