How do I resolve a NullPointerException on a JUNG graph?

Question

So, basically this is what I have so far.

public List<String> cycleSearch(Graph<String,String> g) throws Exception{
    List<String> list = null;
    Graph<String,String> auxG = g;
    for(String aux : g.getVertices()){
        String aux2 = aux;
        if(g.degree(aux)>1){
            if(auxG.removeVertex(aux2)){
            for(String d : g.getSuccessors(aux2)){
                for(String a : g.getSuccessors(aux)){
                    if(a!=d){
                        list = findPath(auxG,d,a);
                        if(list!=null){
                            list.add(0,aux);
                            list.add(aux);
                            return list;
                            }
                        }
                    }
                }
            }
        }
        auxG = g;
    }
    return null;
}

What the method does is basically to search for a cycle in a hypergraph based on JUNG.

The idea it's to recieve a graph on the parameters and then create a variable (as the same type) to later remove a vertex from it without making any changes on the original graph, just in case a cycle isn't found. That way I can use a method called findPath(,,), once the vertex is removed. The method will 'create' another path without walking through the removed vertex.

My compiler says there is problem right here:

            for(String d : g.getSuccessors(aux2))

I've been programming graphs in Java(JUNG) for only 1 month. HELP

*** Read Graph simpleGraph.xml: #edges: 16 #nodes: 11 Finding diameter from simpleGraph.xml Exception in thread "main" java.lang.NullPointerException at GraphDFSWalker.cycleSearch(GraphDFSWalker.java:108) at GraphDFSWalker.main(GraphDFSWalker.java:153) The line #108 says that there is a error in for(String d : g.getSuccessors(aux2)) The line #153 says there is a problem in this line: List<String> path = walker.cycleSearch(g); — artavia23
What implementation of Graph are you using? None of the JUNG-provided implementations should be returning null for a getSuccessors() call (and in fact it's arguably a violation of the contract for that method to do so). — Joshua O'Madadhain

Mureinik Mureinik · Accepted Answer · 2015-05-06T07:45:09

Instead of using null to represnt no successors, you could just use an empty list, and thus avoid the NullPointerException. Replace return null; with return new LinkedList<>();.

How do I resolve a NullPointerException on a JUNG graph?

2 Answers