Doh! Forgot to actually check for negative cycles in relaxEdge.

@@ -148,15 +148,22 @@ public abstract class AbstractGraphPathSearch<V extends Vertex, E extends Edge<V
          * If possible, relax the specified edge using the supplied base cost
          * and edge-weight function.
          *
-         * @param e    edge to be relaxed
-         * @param cost base cost to reach the edge destination vertex
-         * @param ew   optional edge weight function
+         * @param e               edge to be relaxed
+         * @param cost            base cost to reach the edge destination vertex
+         * @param ew              optional edge weight function
+         * @param forbidNegatives if true negative values will forbid the link
          * @return true if the edge was relaxed; false otherwise
          */
-        boolean relaxEdge(E e, double cost, EdgeWeight<V, E> ew) {
+        boolean relaxEdge(E e, double cost, EdgeWeight<V, E> ew,
+                          boolean... forbidNegatives) {
             V v = e.dst();
             double oldCost = cost(v);
-            double newCost = cost + (ew == null ? 1.0 : ew.weight(e));
+            double hopCost = ew == null ? 1.0 : ew.weight(e);
+            if (hopCost < 0 && forbidNegatives.length == 1 && forbidNegatives[0]) {
+                return false;
+            }
+
+            double newCost = cost + hopCost;
             boolean relaxed = newCost < oldCost;
             boolean same = Math.abs(newCost - oldCost) < samenessThreshold;
             if (same || relaxed) {

@@ -40,7 +40,7 @@ public class DijkstraGraphSearch<V extends Vertex, E extends Edge<V>>
             if (cost < Double.MAX_VALUE) {
                 // If the vertex is reachable, relax all its egress edges.
                 for (E e : graph.getEdgesFrom(nearest)) {
-                    result.relaxEdge(e, cost, weight);
+                    result.relaxEdge(e, cost, weight, true);
                 }
             }
 

@@ -91,4 +91,21 @@ public class DijkstraGraphSearchTest extends BreadthFirstSearchTest {
         executeSearch(graphSearch(), graph, A, E, weight, 3, 3.0);
     }
 
+    @Test
+    public void negativeWeights() {
+        graph = new AdjacencyListsGraph<>(of(A, B, C, D, E, F, G),
+                                          of(new TestEdge(A, B, 1),
+                                             new TestEdge(A, C, -1),
+                                             new TestEdge(B, D, 1),
+                                             new TestEdge(D, A, -2),
+                                             new TestEdge(C, D, 1),
+                                             new TestEdge(D, E, 1),
+                                             new TestEdge(D, F, 1),
+                                             new TestEdge(E, G, 1),
+                                             new TestEdge(F, G, 1),
+                                             new TestEdge(G, A, -5),
+                                             new TestEdge(A, G, 4)));
+        executeSearch(graphSearch(), graph, A, G, weight, 3, 4.0);
+    }
+
 }