chore: use EmptyVisitationContext.INSTANCE instead of null

core/src/main/java/io/substrait/expression/proto/ExpressionProtoConverter.java

@@ -47,7 +47,7 @@ public TypeProtoConverter getTypeProtoConverter() {
   }
 
   public io.substrait.proto.Expression toProto(io.substrait.expression.Expression expression) {
-    return expression.accept(this, null);
+    return expression.accept(this, EmptyVisitationContext.INSTANCE);
   }
 
   public List<io.substrait.proto.Expression> toProto(

core/src/main/java/io/substrait/extendedexpression/ExtendedExpressionProtoConverter.java

@@ -6,6 +6,7 @@
 import io.substrait.proto.ExtendedExpression;
 import io.substrait.relation.AggregateFunctionProtoConverter;
 import io.substrait.type.proto.TypeProtoConverter;
+import io.substrait.util.EmptyVisitationContext;
 
 /**
  * Converts from {@link io.substrait.extendedexpression.ExtendedExpression} to {@link
@@ -27,7 +28,7 @@ public ExtendedExpression toProto(
       if (expressionReference
           instanceof io.substrait.extendedexpression.ExtendedExpression.ExpressionReference et) {
         io.substrait.proto.Expression expressionProto =
-            et.getExpression().accept(expressionProtoConverter, null);
+            et.getExpression().accept(expressionProtoConverter, EmptyVisitationContext.INSTANCE);
         ExpressionReference.Builder expressionReferenceBuilder =
             ExpressionReference.newBuilder()
                 .setExpression(expressionProto)

core/src/main/java/io/substrait/relation/AggregateFunctionProtoConverter.java

@@ -5,6 +5,7 @@
 import io.substrait.extension.ExtensionCollector;
 import io.substrait.proto.AggregateFunction;
 import io.substrait.type.proto.TypeProtoConverter;
+import io.substrait.util.EmptyVisitationContext;
 import java.util.stream.IntStream;
 
 /**
@@ -34,7 +35,10 @@ public AggregateFunction toProto(Aggregate.Measure measure) {
         .setOutputType(measure.getFunction().getType().accept(typeProtoConverter))
         .addAllArguments(
             IntStream.range(0, args.size())
-                .mapToObj(i -> args.get(i).accept(aggFuncDef, i, argVisitor, null))
+                .mapToObj(
+                    i ->
+                        args.get(i)
+                            .accept(aggFuncDef, i, argVisitor, EmptyVisitationContext.INSTANCE))
                 .collect(java.util.stream.Collectors.toList()))
         .setFunctionReference(
             functionCollector.getFunctionReference(measure.getFunction().declaration()))

core/src/main/java/io/substrait/relation/RelProtoConverter.java

@@ -77,7 +77,7 @@ public io.substrait.proto.RelRoot toProto(Plan.Root relRoot) {
   }
 
   public io.substrait.proto.Rel toProto(io.substrait.relation.Rel rel) {
-    return rel.accept(this, null);
+    return rel.accept(this, EmptyVisitationContext.INSTANCE);
   }
 
   protected io.substrait.proto.Expression toProto(io.substrait.expression.Expression expression) {
@@ -136,7 +136,10 @@ private AggregateRel.Measure toProto(Aggregate.Measure measure) {
             .setOutputType(toProto(measure.getFunction().getType()))
             .addAllArguments(
                 IntStream.range(0, args.size())
-                    .mapToObj(i -> args.get(i).accept(aggFuncDef, i, argVisitor, null))
+                    .mapToObj(
+                        i ->
+                            args.get(i)
+                                .accept(aggFuncDef, i, argVisitor, EmptyVisitationContext.INSTANCE))
                     .collect(Collectors.toList()))
             .addAllSorts(toProtoS(measure.getFunction().sort()))
             .setFunctionReference(
@@ -463,7 +466,11 @@ private List<ConsistentPartitionWindowRel.WindowRelFunction> toProtoWindowRelFun
 
               var arguments =
                   IntStream.range(0, args.size())
-                      .mapToObj(i -> args.get(i).accept(aggFuncDef, i, argVisitor, null))
+                      .mapToObj(
+                          i ->
+                              args.get(i)
+                                  .accept(
+                                      aggFuncDef, i, argVisitor, EmptyVisitationContext.INSTANCE))
                       .collect(Collectors.toList());
               var options =
                   f.options().stream()

core/src/test/java/io/substrait/type/proto/IfThenRoundtripTest.java

@@ -10,6 +10,7 @@
 import io.substrait.expression.ExpressionCreator;
 import io.substrait.expression.proto.ExpressionProtoConverter;
 import io.substrait.expression.proto.ProtoExpressionConverter;
+import io.substrait.util.EmptyVisitationContext;
 import java.util.Arrays;
 import org.junit.jupiter.api.Test;
 
@@ -26,7 +27,7 @@ void ifThenNotNullable() {
 
     var to = new ExpressionProtoConverter(null, null);
     var from = new ProtoExpressionConverter(null, null, EMPTY_TYPE, protoRelConverter);
-    assertEquals(ifRel, from.from(ifRel.accept(to, null)));
+    assertEquals(ifRel, from.from(ifRel.accept(to, EmptyVisitationContext.INSTANCE)));
   }
 
   @Test
@@ -40,6 +41,6 @@ void ifThenNullable() {
 
     var to = new ExpressionProtoConverter(null, null);
     var from = new ProtoExpressionConverter(null, null, EMPTY_TYPE, protoRelConverter);
-    assertEquals(ifRel, from.from(ifRel.accept(to, null)));
+    assertEquals(ifRel, from.from(ifRel.accept(to, EmptyVisitationContext.INSTANCE)));
   }
 }

core/src/test/java/io/substrait/type/proto/LiteralRoundtripTest.java

@@ -7,6 +7,7 @@
 import io.substrait.expression.ExpressionCreator;
 import io.substrait.expression.proto.ExpressionProtoConverter;
 import io.substrait.expression.proto.ProtoExpressionConverter;
+import io.substrait.util.EmptyVisitationContext;
 import java.math.BigDecimal;
 import org.junit.jupiter.api.Test;
 
@@ -17,6 +18,6 @@ void decimal() {
     var val = ExpressionCreator.decimal(false, BigDecimal.TEN, 10, 2);
     var to = new ExpressionProtoConverter(null, null);
     var from = new ProtoExpressionConverter(null, null, EMPTY_TYPE, protoRelConverter);
-    assertEquals(val, from.from(val.accept(to, null)));
+    assertEquals(val, from.from(val.accept(to, EmptyVisitationContext.INSTANCE)));
   }
 }

isthmus/src/main/java/io/substrait/isthmus/SchemaCollector.java

@@ -79,7 +79,7 @@ private TableGatherer() {
      */
     public static Map<List<String>, NamedStruct> gatherTables(Rel rootRel) {
       var visitor = new TableGatherer();
-      rootRel.accept(visitor, null);
+      rootRel.accept(visitor, EmptyVisitationContext.INSTANCE);
       return visitor.tableMap;
     }
 

isthmus/src/main/java/io/substrait/isthmus/SubstraitToCalcite.java

@@ -178,7 +178,7 @@ private NamedStructGatherer() {
 
     public static Map<List<String>, NamedStruct> gatherTables(Rel rel) {
       var visitor = new NamedStructGatherer();
-      rel.accept(visitor, null);
+      rel.accept(visitor, EmptyVisitationContext.INSTANCE);
       return visitor.tableMap;
     }
 

isthmus/src/test/java/io/substrait/isthmus/ProtoPlanConverterTest.java

@@ -87,7 +87,10 @@ public Optional<Rel> visit(Cross cross, EmptyVisitationContext context)
               "orders" o
             """,
             new SqlToSubstrait(featureBoard));
-    plan1.getRoots().forEach(t -> t.getInput().accept(crossJoinCountingVisitor, null));
+    plan1
+        .getRoots()
+        .forEach(
+            t -> t.getInput().accept(crossJoinCountingVisitor, EmptyVisitationContext.INSTANCE));
     assertEquals(1, counter[0]);
 
     Plan plan2 =
@@ -101,7 +104,10 @@ public Optional<Rel> visit(Cross cross, EmptyVisitationContext context)
               "orders" o
             """,
             new SqlToSubstrait(featureBoard));
-    plan2.getRoots().forEach(t -> t.getInput().accept(crossJoinCountingVisitor, null));
+    plan2
+        .getRoots()
+        .forEach(
+            t -> t.getInput().accept(crossJoinCountingVisitor, EmptyVisitationContext.INSTANCE));
     assertEquals(2, counter[0]);
   }
 

isthmus/src/test/java/io/substrait/isthmus/RelCopyOnWriteVisitorTest.java

@@ -156,7 +156,8 @@ public void replaceCountDistinctsInUnion() throws IOException, SqlParseException
   private static class HasTableReference {
     public boolean hasTableReference(Plan plan, String name) {
       HasTableReferenceVisitor visitor = new HasTableReferenceVisitor(Arrays.asList(name));
-      plan.getRoots().stream().forEach(r -> r.getInput().accept(visitor, null));
+      plan.getRoots().stream()
+          .forEach(r -> r.getInput().accept(visitor, EmptyVisitationContext.INSTANCE));
       return (visitor.hasTableReference());
     }
 
@@ -190,7 +191,8 @@ private static class CountCountDistinct {
 
     public int getCountDistincts(Plan plan) {
       CountCountDistinctVisitor visitor = new CountCountDistinctVisitor();
-      plan.getRoots().stream().forEach(r -> r.getInput().accept(visitor, null));
+      plan.getRoots().stream()
+          .forEach(r -> r.getInput().accept(visitor, EmptyVisitationContext.INSTANCE));
       return visitor.getCountDistincts();
     }
 
@@ -221,7 +223,8 @@ private static class CountApproxCountDistinct {
 
     public int getApproxCountDistincts(Plan plan) {
       CountCountDistinctVisitor visitor = new CountCountDistinctVisitor();
-      plan.getRoots().stream().forEach(r -> r.getInput().accept(visitor, null));
+      plan.getRoots().stream()
+          .forEach(r -> r.getInput().accept(visitor, EmptyVisitationContext.INSTANCE));
       return visitor.getApproxCountDistincts();
     }
 

isthmus/src/test/java/io/substrait/isthmus/RelExtensionRoundtripTest.java

@@ -18,6 +18,7 @@
 import io.substrait.relation.Rel;
 import io.substrait.relation.RelProtoConverter;
 import io.substrait.type.Type;
+import io.substrait.util.EmptyVisitationContext;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
@@ -62,7 +63,8 @@ void extensionMultiRelDetailTest() {
   void roundtrip(Rel pojo1) {
     // Substrait POJO 1 -> Substrait Proto
     io.substrait.proto.Rel proto =
-        pojo1.accept(new RelProtoConverter(new ExtensionCollector()), null);
+        pojo1.accept(
+            new RelProtoConverter(new ExtensionCollector()), EmptyVisitationContext.INSTANCE);
 
     // Substrait Proto -> Substrait POJO 2
     var pojo2 = (new CustomProtoRelConverter(new ExtensionCollector())).from(proto);

spark/src/main/scala/io/substrait/debug/RelToVerboseString.scala

@@ -42,7 +42,7 @@ class RelToVerboseString(addSuffix: Boolean) extends DefaultRelVisitor[String] {
   }
 
   def apply(rel: Rel, maxFields: Int): String = {
-    rel.accept(this, null)
+    rel.accept(this, EmptyVisitationContext.INSTANCE)
   }
 
   override def visit(fetch: Fetch, context: EmptyVisitationContext): String = {

spark/src/main/scala/io/substrait/spark/logical/ToLogicalPlan.scala

@@ -65,7 +65,7 @@ class ToLogicalPlan(spark: SparkSession = SparkSession.builder().getOrCreate())
     val function = measure.getFunction
     var arguments = function.arguments().asScala.zipWithIndex.map {
       case (arg, i) =>
-        arg.accept(function.declaration(), i, expressionConverter, null)
+        arg.accept(function.declaration(), i, expressionConverter, EmptyVisitationContext.INSTANCE)
     }
     if (function.declaration.name == "count" && function.arguments.size == 0) {
       // HACK - count() needs to be rewritten as count(1)
@@ -92,7 +92,7 @@ class ToLogicalPlan(spark: SparkSession = SparkSession.builder().getOrCreate())
       })
 
     val filter = Option(measure.getPreMeasureFilter.orElse(null))
-      .map(_.accept(expressionConverter, null))
+      .map(_.accept(expressionConverter, EmptyVisitationContext.INSTANCE))
 
     AggregateExpression(
       aggregateFunction,
@@ -213,7 +213,7 @@ class ToLogicalPlan(spark: SparkSession = SparkSession.builder().getOrCreate())
   }
 
   private def toSortOrder(sortField: SExpression.SortField): SortOrder = {
-    val expression = sortField.expr().accept(expressionConverter, null)
+    val expression = sortField.expr().accept(expressionConverter, EmptyVisitationContext.INSTANCE)
     val (direction, nullOrdering) = sortField.direction() match {
       case SExpression.SortDirection.ASC_NULLS_FIRST => (Ascending, NullsFirst)
       case SExpression.SortDirection.DESC_NULLS_FIRST => (Descending, NullsFirst)
@@ -449,7 +449,7 @@ class ToLogicalPlan(spark: SparkSession = SparkSession.builder().getOrCreate())
   }
 
   def convert(rel: relation.Rel): LogicalPlan = {
-    val logicalPlan = rel.accept(this, null)
+    val logicalPlan = rel.accept(this, EmptyVisitationContext.INSTANCE)
     require(logicalPlan.resolved)
     logicalPlan
   }

spark/src/main/scala/io/substrait/spark/logical/ToSubstraitRel.scala

@@ -43,6 +43,7 @@ import io.substrait.relation.RelProtoConverter
 import io.substrait.relation.Set.SetOp
 import io.substrait.relation.files.{FileFormat, FileOrFiles}
 import io.substrait.relation.files.FileOrFiles.PathType
+import io.substrait.util.EmptyVisitationContext
 import io.substrait.utils.Util
 
 import java.util
@@ -575,7 +576,7 @@ class ToSubstraitRel extends AbstractLogicalPlanVisitor with Logging {
         proto.PlanRel
           .newBuilder()
           .setRel(substraitRel
-            .accept(relProtoConverter, null))
+            .accept(relProtoConverter, EmptyVisitationContext.INSTANCE))
       )
     extensionCollector.addExtensionsToPlan(builder)
     builder.build().toByteArray

spark/src/test/scala/io/substrait/spark/SubstraitPlanTestBase.scala

@@ -29,6 +29,7 @@ import io.substrait.extension.ExtensionCollector
 import io.substrait.plan.{Plan, PlanProtoConverter, ProtoPlanConverter}
 import io.substrait.proto
 import io.substrait.relation.{ProtoRelConverter, RelProtoConverter}
+import io.substrait.util.EmptyVisitationContext
 import org.scalactic.Equality
 import org.scalactic.source.Position
 import org.scalatest.Succeeded
@@ -72,7 +73,7 @@ trait SubstraitPlanTestBase { self: SharedSparkSession =>
 
     // convert substrait back to spark plan
     val toLogicalPlan = new ToLogicalPlan(spark);
-    val sparkPlan2 = substraitRel2.accept(toLogicalPlan, null)
+    val sparkPlan2 = substraitRel2.accept(toLogicalPlan, EmptyVisitationContext.INSTANCE)
     require(sparkPlan2.resolved)
 
     // and back to substrait again

spark/src/test/scala/io/substrait/spark/TypesAndLiteralsSuite.scala

@@ -10,6 +10,8 @@ import org.apache.spark.sql.types._
 import org.apache.spark.substrait.SparkTypeUtil
 import org.apache.spark.unsafe.types.UTF8String
 
+import io.substrait.util.EmptyVisitationContext
+
 import java.time.{Duration, Instant, LocalDate, LocalDateTime, Period}
 
 class TypesAndLiteralsSuite extends SparkFunSuite {
@@ -101,7 +103,9 @@ class TypesAndLiteralsSuite extends SparkFunSuite {
     l => {
       test(s"test literal: $l (${l.dataType})") {
         val substraitLiteral = ToSubstraitLiteral.convert(l).get
-        val sparkLiteral = substraitLiteral.accept(toSparkExpression, null).asInstanceOf[Literal]
+        val sparkLiteral = substraitLiteral
+          .accept(toSparkExpression, EmptyVisitationContext.INSTANCE)
+          .asInstanceOf[Literal]
 
         println("Before: " + l + " " + l.dataType)
         println("After: " + sparkLiteral + " " + sparkLiteral.dataType)
@@ -118,7 +122,9 @@ class TypesAndLiteralsSuite extends SparkFunSuite {
       MapType(IntegerType, StringType, valueContainsNull = false))
 
     val substraitLiteral = ToSubstraitLiteral.convert(l).get
-    val sparkLiteral = substraitLiteral.accept(toSparkExpression, null).asInstanceOf[Literal]
+    val sparkLiteral = substraitLiteral
+      .accept(toSparkExpression, EmptyVisitationContext.INSTANCE)
+      .asInstanceOf[Literal]
 
     println("Before: " + l + " " + l.dataType)
     println("After: " + sparkLiteral + " " + sparkLiteral.dataType)

spark/src/test/scala/io/substrait/spark/expression/SubstraitExpressionTestBase.scala

@@ -21,6 +21,7 @@ import io.substrait.spark.SparkExtension
 import org.apache.spark.sql.catalyst.expressions.Expression
 
 import io.substrait.expression.{Expression => SExpression}
+import io.substrait.util.EmptyVisitationContext
 import org.scalatest.Assertions.assertResult
 
 trait SubstraitExpressionTestBase {
@@ -48,7 +49,8 @@ trait SubstraitExpressionTestBase {
     f(substraitExp)
 
     if (bidirectional) {
-      val convertedExpression = substraitExp.accept(toSparkExpression, null)
+      val convertedExpression =
+        substraitExp.accept(toSparkExpression, EmptyVisitationContext.INSTANCE)
       assertResult(expression)(convertedExpression)
     }
   }