Question

我想将边缘添加到Vertex类的私有链表边缘中。请帮助我解决此功能。我收到此错误：

线程“ main”中的异常java.lang.NullPointerException在 sample.Vertex.setEdges（Vertex.java:19）在 sample.Main.main（Main.java:24）

Vertex.java

package sample;

import java.util.LinkedList;
import java.util.List;


    public class Vertex {
        private String label;
       private LinkedList<Integer> edges;

        public void setLabel(String label) {
            this.label = label;

        }}

//        public void setEdges(Edge e) {
//
//            edges.add(e);
//        }}

Edge.java

public class Edge {

        private Vertex destination;
        private double weight;

    public void setWeight(double weight) {
        this.weight = weight;
    }

    public void setDestination(Vertex destination) {
        this.destination = destination;
    }

    public double getWeight() {
        return weight;
    }

    public Vertex getDestination() {
        return destination;
    }


}

Main.java

public class Main {

      //  Parent root = FXMLLoader.load(getClass().getResource("sample.fxml"));

            public static void main(String[] args) {
                // creating the graph A --1.0--> B
                Vertex n = new Vertex();
                n.setLabel("A");
                Vertex b = new Vertex();
                b.setLabel("B");
                Edge e = new Edge();
                e.setDestination(b);
                e.setWeight(1.0);
//                n.setEdges(e);

                // returns the destination Node of the first Edge
double weight = e.getWeight();
System.out.print(e);
                System.out.print(n);
                // returns the weight of
                // the first Edge
            }


    public static void main2(String[] args) {
        launch(args);
    }
}

Answer 1

您收到此错误的原因是，当您创建Vertex的新实例时，尚未初始化该Vertex的{{1}}成员变量。让我们来看一下您的Main class：

edges

因此，在创建新的public class Main { public static void main(String[] args) { Vertex n = new Vertex(); // At this point, n.edges is null because it was never initialized n.setLabel("A"); Vertex b = new Vertex(); b.setLabel("B"); Edge e = new Edge(); e.setDestination(b); e.setWeight(1.0); n.setEdges(e); // Now you are attempting to add an edge to a null LinkedList, hence the NullPointerException ... } public static void main2(String[] args) { launch(args); } }时需要初始化edges成员变量。我们可以在构造函数中做到这一点：

Vertex

如何使我的图表的增加功能。我需要将边缘添加到节点

1 个答案: