FordFulkersonAlgorithm - negative flows

[<monix_cora15 AT yahoo.com>]

Hello,

I'm a Computer Science student and I use GraphStream for a personal project.

One question regarding Flow algorithms:

I used FordFulkersonAlgorithm to compute the maximum flow in a bipartite 
graph,
but I get negative flows on some of the edges, and therefore it returns a 
wrong
maximum flow.

Is there any way to constraint the flow on any edge to be positive?  
My code example is in the end of the email.

Thank you very much for your help!

Kind regards,
Monica Dragan

============================================
Code:

import org.graphstream.algorithm.flow.FordFulkersonAlgorithm;
import org.graphstream.graph.Edge;
import org.graphstream.graph.Graph;
import org.graphstream.graph.implementations.SingleGraph;

public class MaxFlow {
        
        //example from Coreman, 3rd edition, page 733
        public static SingleGraph bipartite_graph(){
                                
                SingleGraph bipartite_graph = new SingleGraph("bipartite");
                bipartite_graph.addNode("1").setAttribute("ui.label", "1");   
 
        
                bipartite_graph.addNode("2").setAttribute("ui.label", "2");
                bipartite_graph.addNode("3").setAttribute("ui.label", "3");
                bipartite_graph.addNode("4").setAttribute("ui.label", "4");
                bipartite_graph.addNode("5").setAttribute("ui.label", "5");
                bipartite_graph.addNode("6").setAttribute("ui.label", "6");
                bipartite_graph.addNode("7").setAttribute("ui.label", "7");
                bipartite_graph.addNode("8").setAttribute("ui.label", "8");
                bipartite_graph.addNode("9").setAttribute("ui.label", "9");

                bipartite_graph.addNode("source").setAttribute("ui.label",
"s");
                bipartite_graph.addEdge("s-1", "source", "1", false);
                bipartite_graph.addEdge("s-2", "source", "2", false);
                bipartite_graph.addEdge("s-3", "source", "3", false);
                bipartite_graph.addEdge("s-4", "source", "4", false);
                bipartite_graph.addEdge("s-5", "source", "5", false);

                bipartite_graph.addNode("sink").setAttribute("ui.label", "t");
                bipartite_graph.addEdge("6-t", "6", "sink", false);
                bipartite_graph.addEdge("7-t", "7", "sink", false);
                bipartite_graph.addEdge("8-t", "8", "sink", false);
                bipartite_graph.addEdge("9-t", "9", "sink", false);

                bipartite_graph.addEdge("1-6", "1", "6", false);
                bipartite_graph.addEdge("2-6", "2", "6", false);
                bipartite_graph.addEdge("2-8", "2", "8", false);
                bipartite_graph.addEdge("3-7", "3", "7", false);
                bipartite_graph.addEdge("3-8", "3", "8", false);
                bipartite_graph.addEdge("3-9", "3", "9", false);
                bipartite_graph.addEdge("4-8", "4", "8", false);
                bipartite_graph.addEdge("5-8", "5", "8", false);
                
                return bipartite_graph;
        }
        
        public static double getMaxFlow(Graph bipartite_graph){
                
                FordFulkersonAlgorithm flow = new FordFulkersonAlgorithm();
                flow.init(bipartite_graph, "source", "sink");
                flow.setAllCapacities(1.0);
                flow.compute();
                
                System.out.println(flow.getMaximumFlow());      
                for(Edge e: bipartite_graph.getEachEdge()) {
                        System.out.println(e.getNode0().getId() + " " +
e.getNode1().getId() + " - flow=" + flow.getFlow(e.getNode0(), e.getNode1()));
                }
                System.out.println("Max flow = " + flow.getMaximumFlow());    
 
        
                return flow.getMaximumFlow();
        }
        
        public static void main(String[] s){
                
                SingleGraph bipartite_graph = bipartite_graph();              
 

                getMaxFlow(bipartite_graph);
                        
        }
}