Java中的多个链接列表

Question

I am trying to create multiple linked list in java, but at one of the lines i am getting NullPointerException and i am unable to find out the reason as to why such error is arising.

import java.util.*;

class node{
int data;
node link;

public node(){
    data = 0;
    link = null;
}
}

public class ll{
    static node add(node head[], int x){
    node temp = new node();
    System.out.println("Enter value");
    temp.data = new Scanner(System.in).nextInt();
    temp.link = head[x].link;
    head[x].link = temp;
    return head[x];
  }


public static void main(String []args){
    int m =0;
    int x = 0; int flag = 0;
    System.out.println("Enter the size of index");
    m = new Scanner(System.in).nextInt();
    node []head = new node[m];
    while(x<head.length){
        head[x].data = 0; //error arises here 
        head[x].link = null;
        x++;
    }
    System.out.println(head[0].data); //error arises here.


}
}

Answer 1

那是因为head[x] / head[0]包含null引用。

Answer 2

You have defined a node array but not populated it with anything, so

head[x]

refers to a null item, and using it to access the .data field results in an NPE.