使加权图在JGraphT中工作
16lz
2021-01-22
Why am I getting a NULL pointer error for this program, as far as I can see everything looks fine, I've just started using JGraphT and need a little help with it.
为什么我得到这个程序的NULL指针错误,只要我看到一切看起来很好,我刚开始使用JGraphT并需要一些帮助。
Context: I need to be able to add flights to the graph, each flight is bi-directional and the flights should be weighted with their prices.
背景:我需要能够向图表添加航班,每个航班都是双向的,航班应按其价格加权。
Error:
错误:
run:
Please enter the number of flights:
2
Please enter the flight destination for flight 1:
Edinburg
Please enter the flight destination for flight 2:
Heathrow
Enter the edges
Edinburg
Heathrow
Please enter a price for this edge:
25.99
Exception in thread "main" java.lang.NullPointerException
at org.jgrapht.graph.AbstractBaseGraph.setEdgeWeight(Unknown Source)
at graphapp.MyGraph.setEdgeWeight(MyGraph.java:25)
at graphapp.GraphApp.main(GraphApp.java:33)
Java Result: 1
BUILD SUCCESSFUL (total time: 14 seconds)
MyGraph.java (Class):
MyGraph.java(Class):
package graphapp;
import org.jgrapht.*;
import org.jgrapht.graph.*;
import org.jgrapht.alg.KruskalMinimumSpanningTree;
public class MyGraph {
private final SimpleDirectedWeightedGraph<String, DefaultWeightedEdge> g = new SimpleDirectedWeightedGraph<String, DefaultWeightedEdge>(DefaultWeightedEdge.class);
static final double DEFAULT_EDGE_WEIGHT=19;
//DefaultWeightedEdge > (DefaultWeightedEdge.class);
private DefaultWeightedEdge e1;
public void addVertex(String name) {
g.addVertex(name);
//graph.addVertex(name);
}
public void addEdge(String v1, String v2) {
g.addEdge(v1, v2);
e1 = g.addEdge(v1, v2);
}
public void setEdgeWeight(String EDGE_WEIGHT) {
g.setEdgeWeight(e1, Double.valueOf(EDGE_WEIGHT));
}
public SimpleDirectedWeightedGraph<String, DefaultWeightedEdge> getGraph() {
return g;
}
/*public SimpleWeightedGraph<String,DefaultWeightedEdge> getGraph() {
return graph;
}*/
public void getSpanningTree() {
KruskalMinimumSpanningTree k = new KruskalMinimumSpanningTree(g);
System.out.println(k.getEdgeSet().toString());
//KruskalMinimumSpanningTree k1=new KruskalMinimumSpanningTree(graph);
//System.out.println(k1.getEdgeSet().toString());
}
public void getSpanningTreeCost() {
KruskalMinimumSpanningTree k = new KruskalMinimumSpanningTree(g);
System.out.println(k.getSpanningTreeCost());
}
}
GraphApp (Main):
GraphApp(主要):
package graphapp;
import org.jgrapht.demo.*;
import java.util.Scanner;
import graphapp.*;
public class GraphApp{
public static void main(String args[]) {
int x;
Scanner sc = new Scanner(System.in);
MyGraph my = new MyGraph();
System.out.println("Please enter the number of flights: ");
int no_of_ver = sc.nextInt();
for(int i=1;i <= no_of_ver;i++) {
System.out.println("Please enter the flight destination for flight "+i+ ":");
my.addVertex(sc.next());
}
do {
System.out.println("Enter the edges");
String e1 = sc.next();
String e2 = sc.next();
my.addEdge(e1, e2);
System.out.println("Please enter a price for this edge:");
my.setEdgeWeight(sc.next());
System.out.println("Continue... Yes:1 ********** No:0");
x=sc.nextInt();
} while(x==1);
System.out.println("Graph\n" + my.getGraph().toString());
System.out.println("\n\n**********Spanning Tree*********");
my.getSpanningTree();
System.out.println("\nSpanning Tree Cost");
my.getSpanningTreeCost();
}
}
1 个解决方案
#1
5
My error was with this method:
我的错误是这个方法:
public void addEdge(String v1, String v2) {
g.addEdge(v1, v2);
e1 =g.addEdge(v1, v2);
System.out.println("Edge added: " + e1.toString());
}
I needed to change it to this:
我需要将其更改为:
public void addEdge(String v1, String v2) {
e1 =g.addEdge(v1, v2);
System.out.println("Edge added: " + e1.toString());
}
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