Index: trunk/src/org/openstreetmap/josm/data/osm/ChangesetDataSet.java
===================================================================
--- trunk/src/org/openstreetmap/josm/data/osm/ChangesetDataSet.java	(revision 18605)
+++ trunk/src/org/openstreetmap/josm/data/osm/ChangesetDataSet.java	(revision 18606)
@@ -62,14 +62,13 @@
      * @throws IllegalArgumentException if the same primitive was already stored with a higher or equal version
      */
-    public void put(HistoryOsmPrimitive primitive, ChangesetModificationType cmt) {
+    public void put(final HistoryOsmPrimitive primitive, ChangesetModificationType cmt) {
         CheckParameterUtil.ensureParameterNotNull(primitive, "primitive");
         CheckParameterUtil.ensureParameterNotNull(cmt, "cmt");
         DefaultChangesetDataSetEntry csEntry = new DefaultChangesetDataSetEntry(cmt, primitive);
-        Object val = entryMap.get(primitive.getPrimitiveId());
+        final PrimitiveId pid = primitive.getPrimitiveId();
         ChangesetDataSetEntry[] entries;
-        if (val == null) {
-            entryMap.put(primitive.getPrimitiveId(), csEntry);
+        Object val = entryMap.computeIfAbsent(pid, k -> csEntry);
+        if (val == csEntry)
             return;
-        }
         if (val instanceof ChangesetDataSetEntry) {
             entries = new ChangesetDataSetEntry[2];
@@ -78,5 +77,5 @@
                 throw new IllegalArgumentException(
                         tr("Changeset {0}: Unexpected order of versions for {1}: v{2} is not higher than v{3}",
-                                String.valueOf(primitive.getChangesetId()), primitive.getPrimitiveId(),
+                                String.valueOf(primitive.getChangesetId()), pid,
                                 primitive.getVersion(), entries[0].getPrimitive().getVersion()));
             }
@@ -86,9 +85,9 @@
         if (entries[1] != null) {
             Logging.info("Changeset {0}: Change of {1} v{2} is replaced by version v{3}",
-                    String.valueOf(primitive.getChangesetId()), primitive.getPrimitiveId(),
+                    String.valueOf(primitive.getChangesetId()), pid,
                     entries[1].getPrimitive().getVersion(), primitive.getVersion());
         }
         entries[1] = csEntry;
-        entryMap.put(primitive.getPrimitiveId(), entries);
+        entryMap.put(pid, entries);
     }