Add shortcut in BasicGraph#getEdge

src/de/tudarmstadt/maki/simonstrator/api/common/graph/BasicGraph.java

@@ -381,7 +381,9 @@ public class BasicGraph implements Graph {
 		if (!this.containsNode(to))
 			return null;
 
-		// TODO@rkluge: May be a performance issue.
+		if (!this.containsEdge(from, to))
+			return null;
+
 		final Set<IEdge> outgoingEdgesOfFrom = this.outgoingAdjacencyList.get(from);
 		for (final IEdge outgoingEdgeOfFrom : outgoingEdgesOfFrom) {
 			if (outgoingEdgeOfFrom.toId().equals(to)) {
@@ -402,6 +404,9 @@ public class BasicGraph implements Graph {
 		if (!this.containsNode(to))
 			return resultSet;
 
+		if (!this.containsEdge(from, to))
+			return resultSet;
+
 		final Set<IEdge> outgoingEdgesOfFrom = this.outgoingAdjacencyList.get(from);
 		for (final IEdge outgoingEdgeOfFrom : outgoingEdgesOfFrom) {
 			if (outgoingEdgeOfFrom.toId().equals(to)) {