java - 来自用字段值初始化的数组的 ArrayIndexOutOfBoundsException

Question

当我尝试将一个元素放入 Java 数组中时，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​​小时有效。

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;
    }
}

以下抛出异常:

new Stack(10).push(123);

Answer 1

当 this.size 的值是“正确的”时，您需要初始化数组。 this.size 的初始值是 0(零)(参见 Initial Values of Variables )所以这不是初始化数组的时间；您必须“等待”才能知道数组的大小。哪里提供了那个尺寸？在类构造函数中。

所以它在构造函数中，您必须在其中初始化数组(使用提供的大小)。

例如，参见下面的注释代码(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" any value in it)
    public int top = -1;

    //Methods
    public void push(int value)
    {
        top++;             // after this line `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
    }
}

因此，为了修复这个问题，您需要像这样更改代码:

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;
    }
}