java - 使用用户输入和 for 循环的数组获取 NullPointerException

Question

为什么我会收到此 NulPointerException:

Please Give the length of the Array: 
5
Please give the entries for the array for 0th index
2
Exception in thread "main" java.lang.NullPointerException
    at Array.main(Array.java:18)

In this code?

public class Array{

public static void main(String[] args) 
{
    Scanner scan = new Scanner(System.in);
    System.out.println("Please Give the length of the Array: ");
    int num= scan.nextInt();
    int arr[] = null;
    int i;
    for (i=0; i<num;i++)
    {
        if(i<num-1)
        {
            System.out.println("Please give the entries for the array for " +i+"th index");
            arr[i]=scan.nextInt();

        }
        else
        {   
            System.out.println("Please give the entries for the array for last index");
            arr[i]=scan.nextInt();

        }
    }
    scan.close();   
}
}

Answer 1

您需要初始化数组:

int arr[] = new int[num];

否则你不能在里面放任何东西。