Update conjoin() and asString() null handling.

The handling of null values in some recently implemented Steps were
inconsistent. This addresses that issue by making conjoin() behave like
concat() and asString() like the other String functions.

Author: kenhuuu

docs/src/dev/provider/gremlin-semantics.asciidoc

@@ -632,8 +632,9 @@ incoming list traverser as string.
 *Arguments:*
 
 * `scope` - Determines the type of traverser it operates on. Both scopes will operate on the level of individual traversers.
-The `global` scope will operate on individual traverser, casting all to string. The `local` scope will behave like `global`
-for everything except lists, where it will cast individual elements inside the list into string and return a list of string instead.
+The `global` scope will operate on individual traverser, casting all (except null) to string. The `local` scope will behave like
+`global` for everything except lists, where it will cast individual non-null elements inside the list into string and return a
+list of string instead.
 
 Null values from the incoming traverser are not processed and remain as null when returned.
 
@@ -1121,9 +1122,9 @@ None
 
 *Considerations:*
 
-Every element in the list (including nulls) is converted to a String. The delimiter is inserted between neighboring
-elements to form the final result. This step only applies to list types which means that non-iterable types (including
-null) will cause exceptions to be thrown.
+Every element in the list (except null) is converted to a String. Null values are ignored. The delimiter is inserted
+between neighboring elements to form the final result. This step only applies to list types which means that
+non-iterable types (including null) will cause exceptions to be thrown.
 
 *Exceptions*
 

docs/src/reference/the-traversal.asciidoc

@@ -782,7 +782,7 @@ link:++https://tinkerpop.apache.org/javadocs/x.y.z/core/org/apache/tinkerpop/gre
 [[asString-step]]
 === AsString Step
 
-The `asString()`-step (*map*) returns the value of incoming traverser as strings. Null values are returned as a string value "null".
+The `asString()`-step (*map*) returns the value of incoming traverser as strings. Null values are returned unchanged.
 
 [gremlin-groovy,modern]
 ----
@@ -1215,7 +1215,7 @@ link:++https://tinkerpop.apache.org/docs/x.y.z/dev/provider/#concat-step++[`Sema
 The `conjoin()`-step (*map*) joins together the elements in the incoming list traverser together with the provided argument
 as a delimiter. The resulting `String` is added to the Traversal Stream. This step only expects list data (array or
 Iterable) in the incoming traverser and will throw an `IllegalArgumentException` if any other type is encountered
-(including null). However, null is allowed as an element within the list and is converted to "null".
+(including null). Null values are skipped and not included in the result.
 
 [gremlin-groovy,modern]
 ----

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

@@ -27,7 +27,7 @@
 
 /**
  * Reference implementation for asString() step, a mid-traversal step which returns the incoming traverser value
- * as a string. Null values are returned as a string value "null".
+ * as a string. Null values are returned unchanged.
  *
  * @author David Bechberger (http://bechberger.com)
  * @author Yang Xia (http://github.com/xiazcy)
@@ -40,7 +40,8 @@ public AsStringGlobalStep(final Traversal.Admin traversal) {
 
     @Override
     protected E map(final Traverser.Admin<S> traverser) {
-        return (E) String.valueOf(traverser.get());
+        S traverserObject = traverser.get();
+        return (E) ((traverserObject == null) ? null : String.valueOf(traverserObject));
     }
 
     @Override

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

@@ -31,7 +31,7 @@
 
 /**
  * Reference implementation for asString() step, a mid-traversal step which returns the incoming traverser value
- * as a string. Null values are returned as a string value "null".
+ * as a string. Null values are returned unchanged.
  *
  * @author David Bechberger (http://bechberger.com)
  * @author Yang Xia (http://github.com/xiazcy)
@@ -54,7 +54,7 @@ protected E map(final Traverser.Admin<S> traverser) {
             final Iterator<E> iterator = IteratorUtils.asIterator(item);
             while (iterator.hasNext()) {
                 final E i = iterator.next();
-                resList.add(String.valueOf(i));
+                resList.add((i == null) ? null : String.valueOf(i));
             }
             return (E) resList;
         } else {

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

@@ -48,18 +48,22 @@ public ConjoinStep(final Traversal.Admin traversal, final String delimiter) {
     @Override
     protected String map(Traverser.Admin<S> traverser) {
         final Collection elements = convertTraverserToCollection(traverser);
+        if (elements.isEmpty()) { return ""; }
 
         final StringBuilder joinResult = new StringBuilder();
 
         for (Object elem : elements) {
-            joinResult.append(String.valueOf(elem)).append(delimiter);
+            if (elem != null) {
+                joinResult.append(String.valueOf(elem)).append(delimiter);
+            }
         }
 
         if (joinResult.length() != 0) {
             joinResult.delete(joinResult.length() - delimiter.length(), joinResult.length());
+            return joinResult.toString();
+        } else {
+            return null;
         }
-
-        return joinResult.toString();
     }
 
     @Override

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

@@ -49,7 +49,7 @@ public void testReturnTypes() {
         assertEquals("1", __.__(Arrays.asList(1, 2)).unfold().asString().next());
         assertArrayEquals(new String[]{"1", "2"}, __.inject(Arrays.asList(1, 2)).unfold().asString().toList().toArray());
 
-        assertEquals("null", __.__(null).asString().next());
+        assertEquals(null, __.__(null).asString().next());
 
         assertEquals("[1, 2]test", __.__(Arrays.asList(1, 2)).asString().concat("test").next());
         assertEquals("1test", __.__(Arrays.asList(1, 2)).unfold().asString().concat("test").next());

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

@@ -46,7 +46,7 @@ public void testReturnTypes() {
         assertEquals("1", __.__(1).asString(Scope.local).next());
         assertArrayEquals(new String[]{"1", "2"}, __.inject(1, 2).asString(Scope.local).toList().toArray());
         assertArrayEquals(new String[]{}, ((List<?>) __.__(Collections.emptyList()).asString(Scope.local).next()).toArray());
-        assertArrayEquals(new String[]{"1", "2", "null"}, ((List<?>) __.__(Arrays.asList(1, 2, null)).asString(Scope.local).next()).toArray());
+        assertArrayEquals(new String[]{"1", "2", null}, ((List<?>) __.__(Arrays.asList(1, 2, null)).asString(Scope.local).next()).toArray());
 
         assertArrayEquals(new String[]{"1test", "2test"},
                 __.__(Arrays.asList(1, 2)).asString(Scope.local).unfold().concat("test").toList().toArray());

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

@@ -23,12 +23,14 @@
 import org.apache.tinkerpop.gremlin.process.traversal.step.StepTest;
 import org.junit.Test;
 
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 
@@ -49,6 +51,7 @@ 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());
 
         final Set<Integer> set = new HashSet<>();
         set.add(10); set.add(11); set.add(12);

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

@@ -72,7 +72,7 @@ Feature: Step - asString()
     When iterated to list
     Then the result should be unordered
       | result |
-      | str[null] |
+      | null |
       | 1 |
 
   @GraphComputerVerificationInjectionNotSupported
@@ -86,7 +86,7 @@ Feature: Step - asString()
     When iterated to list
     Then the result should be unordered
       | result |
-      | l[1,str[null]] |
+      | l[1,null] |
 
   Scenario: g_V_valueMapXnameX_asString
     Given the modern graph

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

@@ -139,7 +139,7 @@ Feature: Step - conjoin()
     When iterated to list
     Then the result should be unordered
       | result |
-      | str[axyznullxyzb] |
+      | str[axyzb] |
 
   @GraphComputerVerificationInjectionNotSupported
   Scenario: g_injectX3_threeX_conjoinX_X