ARROW-5518: [Java] Set VectorSchemaRoot rowCount to 0 on allocateNew …

…and clear

Set VectorSchemaRoot::rowCount to 0 in allocateNew() and clear(). This makes the behaviour of VectorSchemaRoot consistent with ValueVector implementations which set their valueCount to 0 on clear().

Author: Johannes Luong <[email protected]>

Closes apache#4481 from jmaschad/master and squashes the following commits:

3570c96 <Johannes Luong> Remove unused variable
1a38403 <Johannes Luong> Test clear as well
5cdffe0 <Johannes Luong> Reset VectorSchemaRoot row count

java/vector/src/main/java/org/apache/arrow/vector/VectorSchemaRoot.java

@@ -121,6 +121,7 @@ public void allocateNew() {
     for (FieldVector v : fieldVectors) {
       v.allocateNew();
     }
+    rowCount = 0;
   }
 
   /**
@@ -130,6 +131,7 @@ public void clear() {
     for (FieldVector v : fieldVectors) {
       v.clear();
     }
+    rowCount = 0;
   }
 
   public List<FieldVector> getFieldVectors() {

java/vector/src/test/java/org/apache/arrow/vector/TestVectorSchemaRoot.java

@@ -0,0 +1,78 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.arrow.vector;
+
+import static org.junit.Assert.assertEquals;
+
+import org.apache.arrow.memory.BufferAllocator;
+import org.apache.arrow.memory.RootAllocator;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+
+public class TestVectorSchemaRoot {
+  private BufferAllocator allocator;
+
+  @Before
+  public void init() {
+    allocator = new RootAllocator(Long.MAX_VALUE);
+  }
+
+  @After
+  public void terminate() {
+    allocator.close();
+  }
+
+  @Test
+  public void testResetRowCount() {
+    final int size = 20;
+    try (final BitVector vec1 = new BitVector("bit", allocator);
+         final IntVector vec2 = new IntVector("int", allocator)) {
+      VectorSchemaRoot vsr = VectorSchemaRoot.of(vec1, vec2);
+
+      vsr.allocateNew();
+      assertEquals(vsr.getRowCount(), 0);
+
+      for (int i = 0; i < size; i++) {
+        vec1.setSafe(i, i % 2);
+        vec2.setSafe(i, i);
+      }
+      vsr.setRowCount(size);
+      checkCount(vec1, vec2, vsr, size);
+
+      vsr.allocateNew();
+      checkCount(vec1, vec2, vsr, 0);
+
+      for (int i = 0; i < size; i++) {
+        vec1.setSafe(i, i % 2);
+        vec2.setSafe(i, i);
+      }
+      vsr.setRowCount(size);
+      checkCount(vec1, vec2, vsr, size);
+
+      vsr.clear();
+      checkCount(vec1, vec2, vsr, 0);
+    }
+  }
+
+  private void checkCount(BitVector vec1, IntVector vec2, VectorSchemaRoot vsr, int count) {
+    assertEquals(vec1.getValueCount(), count);
+    assertEquals(vec2.getValueCount(), count);
+    assertEquals(vsr.getRowCount(), count);
+  }
+}