TINKERPOP-3225 conjoin has incorrect null handling

CHANGELOG.asciidoc

@@ -22,6 +22,11 @@ under the License.
 
 image::https://raw.githubusercontent.com/apache/tinkerpop/master/docs/static/images/gremlin-zamfir.png[width=185]
 
+[[release-3-7-7]]
+=== TinkerPop 3.7.7 (NOT OFFICIALLY RELEASED YET)
+
+* Fixed conjoin has incorrect null handling.
+
 [[release-3-7-6]]
 === TinkerPop 3.7.6 (Release Date: April 1, 2026)
 

docs/src/upgrade/release-3.7.x.asciidoc

@@ -23,6 +23,42 @@ image::gremlin-zamfir.png[width=185]
 
 *Gremfir Master of the Pan Flute*
 
+== TinkerPop 3.7.7
+
+*Release Date: NOT OFFICIALLY RELEASED YET*
+
+Please see the link:https://github.com/apache/tinkerpop/blob/3.7.7/CHANGELOG.asciidoc#release-3-7-7[changelog] for a
+complete list of all the modifications that are part of this release.
+
+=== Upgrading for Users
+
+==== conjoin() Step Null Handling
+
+The `conjoin()` step previously returned `null` when elements in the incoming list are `null`. This behavior has
+been changed so that `conjoin()` now returns an empty string (`""`) in that case.
+
+[source,groovy]
+----
+// 3.7.6
+gremlin> g.inject([null]).conjoin("-")
+==>null
+gremlin> g.inject([null, null]).conjoin("-")
+==>null
+
+// 3.7.7
+gremlin> g.inject([null]).conjoin("+")
+==>
+gremlin> g.inject([null, null]).conjoin("+")
+==>
+----
+
+Code that checks the result of `conjoin()` for lists that include `null` elements should be updated to check for
+an empty string instead.
+
+See: link:https://issues.apache.org/jira/browse/TINKERPOP-3225[TINKERPOP-3225]
+
+
+
 == TinkerPop 3.7.6
 
 *Release Date: April 1, 2026*

gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/ConjoinStep.java

@@ -62,7 +62,7 @@ protected String map(Traverser.Admin<S> traverser) {
             joinResult.delete(joinResult.length() - delimiter.length(), joinResult.length());
             return joinResult.toString();
         } else {
-            return null;
+            return "";
         }
     }
 

gremlin-core/src/test/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/ConjoinStepTest.java

@@ -51,7 +51,8 @@ public void testReturnTypes() {
         assertEquals("5AA8AA10", __.__(new long[] {5L, 8L, 10L}).conjoin("AA").next());
         assertEquals("715", __.__(1).constant(new Long[] {7L, 15L}).conjoin("").next());
         assertEquals("5.5,8.0,10.1", __.__(new double[] {5.5, 8.0, 10.1}).conjoin(",").next());
-        assertNull(__.__(Arrays.asList(null, null)).conjoin(",").next());
+        assertEquals("", __.__(Arrays.asList(null, null)).conjoin(",").next());
+        assertEquals("1", __.__(Arrays.asList(null, 1, null)).conjoin(",").next());
 
         final Set<Integer> set = new LinkedHashSet<>();
         set.add(10); set.add(11); set.add(12);

gremlin-dotnet/test/Gremlin.Net.IntegrationTest/Gherkin/Gremlin.cs

@@ -633,6 +633,8 @@ private static IDictionary<string, List<Func<GraphTraversalSource, IDictionary<s
                {"g_V_out_out_path_byXnameX_conjoinXX", new List<Func<GraphTraversalSource, IDictionary<string, object>, ITraversal>> {(g,p) =>g.V().Out().Out().Path().By("name").Conjoin("")}}, 
                {"g_injectXa_null_bX_conjoinXxyzX", new List<Func<GraphTraversalSource, IDictionary<string, object>, ITraversal>> {(g,p) =>g.Inject(p["xx1"]).Conjoin("xyz")}}, 
                {"g_injectX3_threeX_conjoinX_X", new List<Func<GraphTraversalSource, IDictionary<string, object>, ITraversal>> {(g,p) =>g.Inject(p["xx1"]).Conjoin(";")}}, 
+               {"g_injectXnull_a_null_bX_fold_conjoinXplusX", new List<Func<GraphTraversalSource, IDictionary<string, object>, ITraversal>> {(g,p) =>g.Inject(p["xx1"]).Conjoin("+")}}, 
+               {"g_injectXnull_nullX_conjoinXplusX", new List<Func<GraphTraversalSource, IDictionary<string, object>, ITraversal>> {(g,p) =>g.Inject(p["xx1"]).Conjoin("+")}}, 
                {"g_V_connectedComponent_hasXcomponentX", new List<Func<GraphTraversalSource, IDictionary<string, object>, ITraversal>> {(g,p) =>g.V().ConnectedComponent().Has("gremlin.connectedComponentVertexProgram.component")}}, 
                {"g_V_dedup_connectedComponent_hasXcomponentX", new List<Func<GraphTraversalSource, IDictionary<string, object>, ITraversal>> {(g,p) =>g.V().Dedup().ConnectedComponent().Has("gremlin.connectedComponentVertexProgram.component")}}, 
                {"g_V_hasLabelXsoftwareX_connectedComponent_project_byXnameX_byXcomponentX", new List<Func<GraphTraversalSource, IDictionary<string, object>, ITraversal>> {(g,p) =>g.V().HasLabel("software").ConnectedComponent().Project<object>("name","component").By("name").By("gremlin.connectedComponentVertexProgram.component")}}, 

gremlin-go/driver/cucumber/gremlin.go

@@ -604,6 +604,8 @@ var translationMap = map[string][]func(g *gremlingo.GraphTraversalSource, p map[
     "g_V_out_out_path_byXnameX_conjoinXX": {func(g *gremlingo.GraphTraversalSource, p map[string]interface{}) *gremlingo.GraphTraversal {return g.V().Out().Out().Path().By("name").Conjoin("")}}, 
     "g_injectXa_null_bX_conjoinXxyzX": {func(g *gremlingo.GraphTraversalSource, p map[string]interface{}) *gremlingo.GraphTraversal {return g.Inject(p["xx1"]).Conjoin("xyz")}}, 
     "g_injectX3_threeX_conjoinX_X": {func(g *gremlingo.GraphTraversalSource, p map[string]interface{}) *gremlingo.GraphTraversal {return g.Inject(p["xx1"]).Conjoin(";")}}, 
+    "g_injectXnull_a_null_bX_fold_conjoinXplusX": {func(g *gremlingo.GraphTraversalSource, p map[string]interface{}) *gremlingo.GraphTraversal {return g.Inject(p["xx1"]).Conjoin("+")}}, 
+    "g_injectXnull_nullX_conjoinXplusX": {func(g *gremlingo.GraphTraversalSource, p map[string]interface{}) *gremlingo.GraphTraversal {return g.Inject(p["xx1"]).Conjoin("+")}}, 
     "g_V_connectedComponent_hasXcomponentX": {func(g *gremlingo.GraphTraversalSource, p map[string]interface{}) *gremlingo.GraphTraversal {return g.V().ConnectedComponent().Has("gremlin.connectedComponentVertexProgram.component")}}, 
     "g_V_dedup_connectedComponent_hasXcomponentX": {func(g *gremlingo.GraphTraversalSource, p map[string]interface{}) *gremlingo.GraphTraversal {return g.V().Dedup().ConnectedComponent().Has("gremlin.connectedComponentVertexProgram.component")}}, 
     "g_V_hasLabelXsoftwareX_connectedComponent_project_byXnameX_byXcomponentX": {func(g *gremlingo.GraphTraversalSource, p map[string]interface{}) *gremlingo.GraphTraversal {return g.V().HasLabel("software").ConnectedComponent().Project("name", "component").By("name").By("gremlin.connectedComponentVertexProgram.component")}}, 

gremlin-python/src/main/python/tests/feature/gremlin.py

@@ -606,6 +606,8 @@
     'g_V_out_out_path_byXnameX_conjoinXX': [(lambda g:g.V().out().out().path().by('name').conjoin(''))], 
     'g_injectXa_null_bX_conjoinXxyzX': [(lambda g, xx1=None:g.inject(xx1).conjoin('xyz'))], 
     'g_injectX3_threeX_conjoinX_X': [(lambda g, xx1=None:g.inject(xx1).conjoin(';'))], 
+    'g_injectXnull_a_null_bX_fold_conjoinXplusX': [(lambda g, xx1=None:g.inject(xx1).conjoin('+'))], 
+    'g_injectXnull_nullX_conjoinXplusX': [(lambda g, xx1=None:g.inject(xx1).conjoin('+'))], 
     'g_V_connectedComponent_hasXcomponentX': [(lambda g:g.V().connected_component().has('gremlin.connectedComponentVertexProgram.component'))], 
     'g_V_dedup_connectedComponent_hasXcomponentX': [(lambda g:g.V().dedup().connected_component().has('gremlin.connectedComponentVertexProgram.component'))], 
     'g_V_hasLabelXsoftwareX_connectedComponent_project_byXnameX_byXcomponentX': [(lambda g:g.V().has_label('software').connected_component().project('name','component').by('name').by('gremlin.connectedComponentVertexProgram.component'))], 

gremlin-test/src/main/resources/org/apache/tinkerpop/gremlin/test/features/map/Conjoin.feature

@@ -153,3 +153,29 @@ Feature: Step - conjoin()
     Then the result should be unordered
       | result |
       | 3;three |
+
+  @GraphComputerVerificationInjectionNotSupported
+  Scenario: g_injectXnull_a_null_bX_fold_conjoinXplusX
+    Given the empty graph
+    And using the parameter xx1 defined as "l[null,a,null,b]"
+    And the traversal of
+      """
+      g.inject(xx1).conjoin("+")
+      """
+    When iterated to list
+    Then the result should be unordered
+      | result |
+      | str[a+b] |
+
+  @GraphComputerVerificationInjectionNotSupported
+  Scenario: g_injectXnull_nullX_conjoinXplusX
+    Given the empty graph
+    And using the parameter xx1 defined as "l[null,null]"
+    And the traversal of
+      """
+      g.inject(xx1).conjoin("+")
+      """
+    When iterated to list
+    Then the result should be unordered
+      | result |
+      | str[] |