FIX: consistency of stream functions

src/mhtree/InternalNode.java

@@ -70,7 +70,8 @@ public class InternalNode extends Node implements Serializable {
         if (getLevel() == level) return Collections.singletonList(this);
 
         return children.stream()
-                .flatMap(child -> child.getNodesOnLevel(level).stream())
+                .map(child -> child.getNodesOnLevel(level))
+                .flatMap(Collection::stream)
                 .collect(Collectors.toList());
     }
 }

src/mhtree/Node.java

@@ -7,6 +7,7 @@ import messif.objects.LocalAbstractObject;
 
 import java.io.Serializable;
 import java.util.ArrayList;
+import java.util.Collection;
 import java.util.List;
 import java.util.stream.Collectors;
 
@@ -28,7 +29,8 @@ public abstract class Node implements Serializable {
 
     public static InternalNode createParent(List<Node> nodes, PrecomputedDistances distances, InsertType insertType) {
         List<LocalAbstractObject> objects = nodes.stream()
-                .flatMap(node -> node.getObjects().stream())
+                .map(Node::getObjects)
+                .flatMap(Collection::stream)
                 .collect(Collectors.toList());
 
         return new InternalNode(distances.getSubset(objects), insertType);