数组中的空指针异常初始化为null [英] Null pointer exception in array initialized to null

问题描述

这是下面的代码.我必须出于某种目的使数组为null，然后将数组组件初始化为1时，它显示了null指针异常.该如何处理?

This is the following code. I have to make the array null for a purpose, then when I initialize the array components to 1, it shows null pointer exception. How to handle this?

public static void main(String[] args) {
    double[] a;
    a=null;
    if(a==null)
        for(int i=0;i<12;i++)
            a[i]=1;        
}

推荐答案

您需要先创建一个数组对象并将其分配给该数组变量，然后再尝试使用该变量.否则，您将创建NullPointerException/NPE的定义:尝试使用(取消引用)引用null的引用变量.

You need to create an array object and assign it to the array variable before trying to use the variable. Otherwise you are creating the very definition of a NullPointerException/NPE: trying to use (dereference) a reference variable that refers to null.

//  constant to avoid "magic" numbers
private static final int MAX_A = 12; 

    // elsewhere in code
    if (a == null) {
       a = new double[MAX_A]; // you need this!
       for (int i = 0; i < a.length; i++) {
          a[i] = 1.0;
       }
    }

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