访问 ArrayList 中的引用时出现问题

Question

无法添加新元素（节点）到ArrayList

Node N = new Node(5,"Sandeep");

Node N1 = new Node(5,"qwert");

在下面的行中我收到空指针异常

N.children.add(N1)

代码：

class Node {

    public int val;

    public String data;

    public ArrayList<Node> children;

    public Node(int val, String data) {

        this.val = val;

        this.data = data;

        ArrayList<Node> children = new ArrayList<Node>();

    }

}

public class Nary {

    public static void main(String[] args) {            

       // ArrayList<Node> children = new ArrayList<Node>();

        Node N = new Node(5,"Sandeep");

        Node N1 = new Node(5,"qwert");

        N.children.add(N1);

    }

}

Answer 1

在您的代码中更改此设置class Node {&nbsp; &nbsp; public int val;&nbsp; &nbsp; public String data;&nbsp; &nbsp; public ArrayList<Node> children;&nbsp; &nbsp; public Node(int val, String data) {&nbsp; &nbsp; &nbsp; &nbsp; this.val = val;&nbsp; &nbsp; &nbsp; &nbsp; this.data = data;&nbsp; &nbsp; &nbsp; &nbsp; this.children = new ArrayList<Node>();&nbsp; &nbsp; }}

Answer 2

在 Node 构造函数中，您正在创建 Children 的新本地属性如果您在构造函数中进行以下更改，它将正常工作 this.children = new ArrayList();