diff --git a/java/server/src/main/java/org/softwareheritage/graph/Endpoint.java b/java/server/src/main/java/org/softwareheritage/graph/Endpoint.java
index 17b7541..35939b4 100644
--- a/java/server/src/main/java/org/softwareheritage/graph/Endpoint.java
+++ b/java/server/src/main/java/org/softwareheritage/graph/Endpoint.java
@@ -1,230 +1,230 @@
 package org.softwareheritage.graph;
 
 import java.util.ArrayList;
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import org.softwareheritage.graph.Graph;
 import org.softwareheritage.graph.SwhId;
 import org.softwareheritage.graph.SwhPath;
 import org.softwareheritage.graph.algo.Traversal;
 
 /**
  * REST API endpoints wrapper functions.
  *
  * @author Thibault Allançon
  * @version 1.0
  * @since 1.0
  */
 
 public class Endpoint {
   /** Graph where traversal endpoint is performed */
   Graph graph;
   /** Internal traversal API */
   Traversal traversal;
 
   /** Timings logger */
   private static final Logger logger = LoggerFactory.getLogger(Endpoint.class);
 
   /**
    * Constructor.
    *
    * @param graph the graph used for traversal endpoint
    * @param direction a string (either "forward" or "backward") specifying edge orientation
    * @param edgesFmt a formatted string describing allowed edges (TODO: link API doc)
    */
   public Endpoint(Graph graph, String direction, String edgesFmt) {
     this.graph = graph;
     this.traversal = new Traversal(graph, direction, edgesFmt);
   }
 
   /**
    * Returns measurement starting timestamp for logging purposes.
    *
    * @return timestamp used for time measurement
    */
   private long startTiming() {
     return System.nanoTime();
   }
 
   /**
    * Ends timing measurement and returns total duration in seconds for logging purposes.
    *
    * @param startTime measurement starting timestamp
    * @return time in seconds elapsed since starting point
    */
   private float stopTiming(long startTime) {
     long endTime = System.nanoTime();
     float duration = (float) (endTime - startTime) / 1_000_000_000;
     return duration;
   }
 
   /**
    * Converts a list of (internal) long node ids to a list of corresponding (external) SWH PIDs.
    *
    * @param nodeIds the list of long node ids
    * @return a list of corresponding SWH PIDs
    */
   private ArrayList<SwhId> convertNodesToSwhIds(ArrayList<Long> nodeIds) {
     long startTime = this.startTiming();
     ArrayList<SwhId> swhIds = new ArrayList<>();
     for (long nodeId : nodeIds) {
       swhIds.add(graph.getSwhId(nodeId));
     }
     float duration = this.stopTiming(startTime);
     logger.debug("convertNodesToSwhIds() took {} s.", duration);
     return swhIds;
   }
 
   /**
    * Converts a list of (internal) long node ids to the corresponding {@link SwhPath}.
    *
    * @param nodeIds the list of long node ids
    * @return the corresponding {@link SwhPath}
    * @see org.softwareheritage.graph.SwhPath
    */
   private SwhPath convertNodesToSwhPath(ArrayList<Long> nodeIds) {
     long startTime = this.startTiming();
     SwhPath path = new SwhPath();
     for (long nodeId : nodeIds) {
       path.add(graph.getSwhId(nodeId));
     }
     float duration = this.stopTiming(startTime);
     logger.debug("convertNodesToSwhPath() took {} s.", duration);
     return path;
   }
 
   /**
    * Converts a list of paths made of (internal) long node ids to one made of {@link SwhPath}-s.
    *
    * @param pathsNodeId the list of paths with long node ids
    * @return a list of corresponding {@link SwhPath}
    * @see org.softwareheritage.graph.SwhPath
    */
   private ArrayList<SwhPath> convertPathsToSwhIds(ArrayList<ArrayList<Long>> pathsNodeId) {
     long startTime = this.startTiming();
     ArrayList<SwhPath> paths = new ArrayList<>();
     for (ArrayList<Long> path : pathsNodeId) {
       paths.add(convertNodesToSwhPath(path));
     }
     float duration = this.stopTiming(startTime);
     logger.debug("convertPathsToSwhIds() took {} s.", duration);
     return paths;
   }
 
   /**
    * Leaves endpoint wrapper (performs input/output node ids conversions).
    *
    * @param src source node of endpoint call specified as a SwhId
    * @return the resulting list of SwhId from endpoint call
    * @see org.softwareheritage.graph.SwhId
    * @see org.softwareheritage.graph.algo.Traversal#leaves(long)
    */
   public ArrayList<SwhId> leaves(SwhId src) {
     long srcNodeId = graph.getNodeId(src);
 
     long startTime = this.startTiming();
     ArrayList<Long> nodeIds = traversal.leaves(srcNodeId);
     float duration = this.stopTiming(startTime);
     logger.debug("leaves({}) took {} s.", src, duration);
 
     return convertNodesToSwhIds(nodeIds);
   }
 
   /**
    * Neighbors endpoint wrapper (performs input/output node ids conversions).
    *
    * @param src source node of endpoint call specified as a SwhId
    * @return the resulting list of SwhId from endpoint call
    * @see org.softwareheritage.graph.SwhId
    * @see org.softwareheritage.graph.algo.Traversal#neighbors(long)
    */
   public ArrayList<SwhId> neighbors(SwhId src) {
     long srcNodeId = graph.getNodeId(src);
 
     long startTime = this.startTiming();
     ArrayList<Long> nodeIds = traversal.neighbors(srcNodeId);
     float duration = this.stopTiming(startTime);
     logger.debug("neighbors({}) took {} s.", src, duration);
 
     return convertNodesToSwhIds(nodeIds);
   }
 
   /**
    * Walk endpoint wrapper (performs input/output node ids conversions).
    *
    * @param src source node of endpoint call specified as a SwhId
    * @param dstFmt destination used in endpoint call (TODO: link to API doc)
    * @param algorithm traversal algorithm used in endpoint call (either "dfs" or "bfs")
    * @return the resulting {@link SwhPath} from endpoint call
    * @see org.softwareheritage.graph.SwhId
    * @see org.softwareheritage.graph.SwhPath
    * @see org.softwareheritage.graph.algo.Traversal#walk
    */
   public SwhPath walk(SwhId src, String dstFmt, String algorithm) {
     long srcNodeId = graph.getNodeId(src);
-    ArrayList<Long> nodeIds = null;
+    ArrayList<Long> nodeIds = new ArrayList<Long>();
 
     // Destination is either a SWH ID or a node type
     try {
       SwhId dstSwhId = new SwhId(dstFmt);
       long dstNodeId = graph.getNodeId(dstSwhId);
 
       long startTime = this.startTiming();
       nodeIds = traversal.walk(srcNodeId, dstNodeId, algorithm);
       float duration = this.stopTiming(startTime);
       logger.debug("walk({}) took {} s.", src, duration);
     } catch (IllegalArgumentException ignored1) {
       try {
         Node.Type dstType = Node.Type.fromStr(dstFmt);
 
         long startTime = this.startTiming();
         nodeIds = traversal.walk(srcNodeId, dstType, algorithm);
         float duration = this.stopTiming(startTime);
         logger.debug("walk({}) took {} s.", src, duration);
       } catch (IllegalArgumentException ignored2) { }
     }
 
     return convertNodesToSwhPath(nodeIds);
   }
 
   /**
    * VisitNodes endpoint wrapper (performs input/output node ids conversions).
    *
    * @param src source node of endpoint call specified as a SwhId
    * @return the resulting list of SwhId from endpoint call
    * @see org.softwareheritage.graph.SwhId
    * @see org.softwareheritage.graph.algo.Traversal#visitNodes(long)
    */
   public ArrayList<SwhId> visitNodes(SwhId src) {
     long srcNodeId = graph.getNodeId(src);
 
     long startTime = this.startTiming();
     ArrayList<Long> nodeIds = traversal.visitNodes(srcNodeId);
     float duration = this.stopTiming(startTime);
     logger.debug("visitNodes({}) took {} s.", src, duration);
 
     return convertNodesToSwhIds(nodeIds);
   }
 
   /**
    * VisitPaths endpoint wrapper (performs input/output node ids conversions).
    *
    * @param src source node of endpoint call specified as a SwhId
    * @return the resulting list of {@link SwhPath} from endpoint call
    * @see org.softwareheritage.graph.SwhId
    * @see org.softwareheritage.graph.SwhPath
    * @see org.softwareheritage.graph.algo.Traversal#visitPaths(long)
    */
   public ArrayList<SwhPath> visitPaths(SwhId src) {
     long srcNodeId = graph.getNodeId(src);
 
     long startTime = this.startTiming();
     ArrayList<ArrayList<Long>> paths = traversal.visitPaths(srcNodeId);
     float duration = this.stopTiming(startTime);
     logger.debug("visitPaths({}) took {} s.", src, duration);
 
     return convertPathsToSwhIds(paths);
   }
 }