Wait for library method to finish before continuing

Question

I have a problem. I have to wait for a method I call from a library to finish before I can continue with my code. How can I do that? My code:

Random random = new Random();
int node1 = random.nextInt(graph.getNumberOfVertices() + 1);
int node2 = random.nextInt(graph.getNumberOfVertices() + 1);

MatrixWrappedPath path = graph.getShortestPath(node1, node2);

int pathLength = 0;
if (path != null) {
    pathLength = path.getLength();
}

The exception I'm getting from the library ( http://grph.inria.fr/javadoc/index.html ) is:

Exception in thread "main" java.lang.IllegalStateException: cannot compute a distance because the two vertices are not connected

at grph.algo.distance.DistanceMatrix.getDistance(DistanceMatrix.java:56)

at grph.MatrixWrappedPath.getLength(MatrixWrappedPath.java:47)

The DistanceMatrix class runs a BFS (org.dipergrafs.algo.bfs.BFSAlgorithm) which is Multithreaded (org.dipergrafs.algo.SingleSourceSearchAlgorithm, method "compute":

public R[] compute(final Grph g, IntSet sources)
{
final R[] r = createArray(sources.getGreatest() + 1);

new MultiThreadProcessing(g.getVertices(), Grph.getNumberOfThreadsToCreate()) {

    @Override
    protected void run(int threadID, int source)
    {
    r[source] = compute(g, source);
    }

};

return r;
}

) and fills the DistanceMatrix. So if DistanceMatrix is not finished yet, the getDistance(node1, node2) method cannot get the values from the DistanceMatrix. I read about CountDownLatch and wait() , notify() , but I can't figure out how to do this. What would be a good way to solve this?

Answer 1

You're wrong when assuming there is a multithreading issue there.

The error message is very clear :

(...)cannot compute a distance because the two vertices are not connected 

You're taking 2 random nodes in a graph, but these nodes are not connected. that's why the library cannot compute its distance.

Are you sure you didn't forget to add the edges in your graph ? ;)