fix wrong value returned by the method

Author: zeapo

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

@@ -384,6 +384,7 @@ public final void get(int index, NullableBitHolder holder) {
      * Get the number of bits set to 1
      * @return the number of bits set to 1
      */
+    @Override
     public final int getNullCount() {
       int count = 0;
       for (int i = 0; i < allocationSizeInBytes; ++i) {
@@ -392,7 +393,7 @@ public final int getNullCount() {
         // will have 32bits set to 1. Masking the MSB and then adding it back solves the issue.
         count += Integer.bitCount(byteValue & 0x7F) - (byteValue >> 7);
       }
-      return count;
+      return (allocationSizeInBytes * 8) - count;
     }
   }
 

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

@@ -301,7 +301,7 @@ public void testBitVector() {
       assertEquals(0, accessor.get(100));
       assertEquals(1, accessor.get(1022));
 
-      assertEquals(2, accessor.getNullCount());
+      assertEquals(1022, accessor.getNullCount());
 
       // test setting the same value twice
       m.set(0, 1);
@@ -318,21 +318,21 @@ public void testBitVector() {
       assertEquals(1, accessor.get(1));
 
       // should not change
-      assertEquals(2, accessor.getNullCount());
+      assertEquals(1022, accessor.getNullCount());
 
       // Ensure unallocated space returns 0
       assertEquals(0, accessor.get(3));
 
       m.set(1, 0);
       m.set(1022, 0);
 
-      assertEquals(0, accessor.getNullCount());
+      assertEquals(1024, accessor.getNullCount());
 
       for (int i = 0; i < 1024; ++i) {
         m.set(i, 1);
       }
 
-      assertEquals(1024, accessor.getNullCount());
+      assertEquals(0, accessor.getNullCount());
     }
   }