Java实现邻接表无向图，总是报空指针异常

import java.util.ArrayList;

public class ListUDG {

//节点类
class vNode {
    char data;
    //存储和data形成边的另一顶点索引的链表
    ArrayList<Integer> firstNode;
}

//创建存储节点数组
static vNode[] vertex = new vNode[200];

public ListUDG(char[] vex, char[][] edg) {

    //对节点的数组初始化
    for (int i = 0; i < vex.length; i++) {
        vertex[i] = new vNode();
        vertex[i].data = vex[i];
        vertex[i].firstNode = null;
    }

    //遍历边的集合，并将边的顶点加到vNode类的链表里
    for (int i = 0; i < edg.length; i++) {
        char c1 = edg[i][0];
        char c2 = edg[i][1];
        int p1 = getPosition(c1);
        int p2 = getPosition(c2);


        if (vertex[p1].firstNode == null) {

            vertex[p1].firstNode.add(0,p2);
        } else
            vertex[p1].firstNode.add(p2);

        if (vertex[p2].firstNode == null) {
            vertex[p2].firstNode.add(0,p1);
        } else
            vertex[p2].firstNode.add(p1);
    }
}

private int getPosition(char c) {
    for (int i = 0; i < vertex.length; i++)
        if (vertex[i].data == c)
            return i;
    return -1;
}

public static void main(String[] args) {
    char[] vexs = {'A', 'B', 'C', 'D', 'E', 'F', 'G','H','I','J','K'};
    char[][] edges = new char[][]{
            {'A', 'C'},
            {'A', 'D'},
            {'A', 'F'},
            {'B', 'C'},
            {'C', 'D'},
            {'E', 'G'},
            {'D', 'G'},
            {'I','J'},
            {'J','G'},};
    ListUDG lu = new ListUDG(vexs,edges);

    StringBuffer s = new StringBuffer();

    ///输出图
    for (int i = 0; i < vexs.length; i++) {
        s = null;
        s.append(vertex[i].data + " ");
        for (Integer num : vertex[i].firstNode)
            s.append(num.toString() + " ");
        System.out.println(s);
    }
}

}

https://blog.csdn.net/XU906722/article/details/80687154

main函数最后的那个for循环里，将StringBuffer类型变量s设成了null

  ///输出图
     for (int i = 0; i < vexs.length; i++) {
          s = null; //错误代码
          s.append(vertex[i].data + " ");
         for (Integer num : vertex[i].firstNode)
           s.append(num.toString() + " ");
        System.out.println(s);
 }