public class ListNode {
    public int val;
    public ListNode next = null;

    public ListNode(){
    }

    public ListNode(int val) {
        this.val = val;
    }

    public void addListNode(ListNode nextNode){
        this.next = nextNode;
    }

    @Override
    public String toString(){
        System.out.println(val);
        System.out.println("das");
//        while(next != null){
//            System.out.println(next.val);
//            next = next.next;
//        }
        return "" + val;
    }
    
}

public class No52FirstCommonNodesInLists {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int m = scanner.nextInt();
        int n = scanner.nextInt();

        if (m<1 || n<1){
            return ;
        }
        int num1 = scanner.nextInt();
        ListNode list1 = new ListNode(num1);
        System.out.println(list1);
        ListNode head1 = list1;
        while (--m > 0){
            int num2 = scanner.nextInt();
            list1.next = new ListNode(num2);
            list1 = list1.next;
        }

        ListNode list2 = new ListNode(scanner.nextInt());
        ListNode head2 = list2;
        while (--n > 0){
            list2.next = new ListNode(scanner.nextInt());
            list2 = list2.next;
        }
        scanner.close();

    }
}

debug过程中，查看变量内容的时候，显示内容会调用toString对象，当tostring内部如上代码所示，循环到了链表尾节点，next为空，造成后续添加节点报空异常。在IDEA中可以设置，其他的未知。