diff --git a/jsprit-core/src/main/java/com/graphhopper/jsprit/core/algorithm/ruin/RuinString.java b/jsprit-core/src/main/java/com/graphhopper/jsprit/core/algorithm/ruin/RuinString.java
index 8341ddd4..ff6155ee 100644
--- a/jsprit-core/src/main/java/com/graphhopper/jsprit/core/algorithm/ruin/RuinString.java
+++ b/jsprit-core/src/main/java/com/graphhopper/jsprit/core/algorithm/ruin/RuinString.java
@@ -44,11 +44,11 @@ import java.util.*;
  */
 public final class RuinString extends AbstractRuinStrategy {
 
-    private Logger logger = LoggerFactory.getLogger(RuinString.class);
+    private static Logger logger = LoggerFactory.getLogger(RuinString.class);
 
-    private VehicleRoutingProblem vrp;
+    private final VehicleRoutingProblem vrp;
 
-    private JobNeighborhoods jobNeighborhoods;
+    private final JobNeighborhoods jobNeighborhoods;
 
     private int kMin = 1;