From 9de06ae09de71512f81ce42f86ad20e6fae12f3f Mon Sep 17 00:00:00 2001
From: Chao Sun <sunchao@apple.com>
Date: Fri, 3 Mar 2023 09:55:37 -0800
Subject: [PATCH] Fix valueCount when loading NullVector

We should set `valueCount` when loading `NullVector`. Unfortunately there's no `nullCount` field.
---
 .../org/apache/arrow/c/RoundtripTest.java     |  7 ++++++
 .../org/apache/arrow/vector/NullVector.java   | 23 ++++++++++++++++++-
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/java/c/src/test/java/org/apache/arrow/c/RoundtripTest.java b/java/c/src/test/java/org/apache/arrow/c/RoundtripTest.java
index 6a2b476b0c395..b9ea67b7190ed 100644
--- a/java/c/src/test/java/org/apache/arrow/c/RoundtripTest.java
+++ b/java/c/src/test/java/org/apache/arrow/c/RoundtripTest.java
@@ -462,6 +462,13 @@ public void testUInt8Vector() {
     }
   }
 
+  @Test
+  public void testNullVector() {
+    try (final NullVector vector = new NullVector("v", 1024)) {
+      assertTrue(roundtrip(vector, NullVector.class));
+    }
+  }
+
   @Test
   public void testVarBinaryVector() {
     try (final VarBinaryVector vector = new VarBinaryVector("v", allocator)) {
diff --git a/java/vector/src/main/java/org/apache/arrow/vector/NullVector.java b/java/vector/src/main/java/org/apache/arrow/vector/NullVector.java
index 1010d8d475f05..203b53895fafc 100644
--- a/java/vector/src/main/java/org/apache/arrow/vector/NullVector.java
+++ b/java/vector/src/main/java/org/apache/arrow/vector/NullVector.java
@@ -57,6 +57,16 @@ public NullVector(String name) {
     this(name, FieldType.nullable(Types.MinorType.NULL.getType()));
   }
 
+  /**
+   * Instantiate a NullVector with the given number of values
+   *
+   * @param name name of the vector
+   * @param valueCount number of values (i.e., nulls) in this vector
+   */
+  public NullVector(String name, int valueCount) {
+    this(new Field(name, FieldType.nullable(Types.MinorType.NULL.getType()), null), valueCount);
+  }
+
   /**
    * Instantiate a NullVector.
    *
@@ -73,8 +83,18 @@ public NullVector(String name, FieldType fieldType) {
    * @param field field materialized by this vector.
    */
   public NullVector(Field field) {
-    this.valueCount = 0;
+    this(field, 0);
+  }
+
+  /**
+   * Instantiate a NullVector with the given number of values
+   *
+   * @param field field materialized by this vector.
+   * @param valueCount number of values (i.e., nulls) in this vector
+   */
+  public NullVector(Field field, int valueCount) {
     this.field = field;
+    this.valueCount = valueCount;
   }
 
   @Deprecated
@@ -192,6 +212,7 @@ public List<FieldVector> getChildrenFromFields() {
   @Override
   public void loadFieldBuffers(ArrowFieldNode fieldNode, List<ArrowBuf> ownBuffers) {
     Preconditions.checkArgument(ownBuffers.isEmpty(), "Null vector has no buffers");
+    this.valueCount = fieldNode.getLength();
   }
 
   @Override