How to use the GraphStream library for Scala

Question

I tried to use the GraphStream library to find the shortest path between 2 nodes in a graph. At the end I'm able to print the edges of the path (it.foreach(println)) but I can't access one element at the time. This is the code:

import org.graphstream.algorithm.Dijkstra;
import org.graphstream.graph.Edge;
import org.graphstream.graph.Graph;
import org.graphstream.graph.Node;
import org.graphstream.graph.Path;
import org.graphstream.graph.implementations.SingleGraph;
import scala.collection.JavaConverters._


object MainApp extends App{


        def exampleGraph():Graph={
                  val g:Graph = new SingleGraph("example");
                  g.addNode("N1_S1");
                  g.addNode("N1_J1");
                  g.addNode("N1_H1");
                  g.addNode("N1_J2");
                  g.addNode("N1_H2");
                  g.addNode("N1_W1");
                  var e:Edge=g.addEdge("N1_S1-N1_J1", "N1_S1", "N1_J1")
                  e.addAttribute("length",Int.box(6))
                  e=g.addEdge("N1_J1-N1_H1", "N1_J1", "N1_H1")
                  e.addAttribute("length",Int.box(8))
                  e=g.addEdge("N1_J1-N1_J2", "N1_J1", "N1_J2")
                  e.addAttribute("length",Int.box(8))
                  e=g.addEdge("N1_J2-N1_H2", "N1_J2", "N1_H2")
                  e.addAttribute("length",Int.box(4))
                  e=g.addEdge("N1_J2-N1_W1", "N1_J2", "N1_W1")
                  e.addAttribute("length",Int.box(10))

                  return g
        }

    val g:Graph = exampleGraph();
    g.display(false);

    val dijkstra:Dijkstra = new Dijkstra(Dijkstra.Element.EDGE, null, "length");

    dijkstra.init(g);

    dijkstra.setSource(g.getNode("N1_S1"));

    println(dijkstra.getPath(g.getNode("N1_W1")));
    val myPath:Path=dijkstra.getPath(g.getNode("N1_W1"))
    val it=(myPath.getEachEdge).asScala
    println("edges")
    it.foreach(println)
}

The problem is that the prototype of getEachEdge is getEachEdge[T <: Edge](): Iterable[_ <: T] and asScala returns a Iterable[_ <: Nothing]. So the final question is "How can I access each element of the shortest path?"

Giovanni Caporaletti Giovanni Caporaletti · Accepted Answer · 2016-11-14T14:17:55

I'm not sure why, but you have to explicitly annotate the type.

myPath.getEachEdge[Edge].asScala

You could get runtime cast exceptions if you specify the wrong subtype.

The following throws a ClassCastException:

@ trait OtherEdge extends Edge
defined trait OtherEdge
@ myPath.getEachEdge[OtherEdge].asScala.head
java.lang.ClassCastException: org.graphstream.graph.implementations.AbstractEdge cannot be cast to $sess.cmd17$OtherEdge
  $sess.cmd18$.<init>(cmd18.sc:1)
  $sess.cmd18$.<clinit>(cmd18.sc:-1)

How to use the GraphStream library for Scala

2 Answers