#vulnerable code 
                              @Test
  public void handleAllFieldsMappingSettingAndTheMappingsProvided() {
    List<DbTableColumn> columns = Lists.newArrayList(
        new DbTableColumn("col1", true, false, 1),
        new DbTableColumn("col2", false, false, 1),
        new DbTableColumn("col3", false, false, 1));
    DbTable table = new DbTable("tableA", columns);

    Map<String, FieldAlias> aliasMap = new HashMap<>();
    aliasMap.put("f1", new FieldAlias("col3"));
    FieldsMappings mappings = new FieldsMappings("tableA", "topic1", true, aliasMap);

    FieldsMappings newMappings = PreparedStatementBuilderHelper.validateAndMerge(mappings, table);
    assertEquals(newMappings.getTableName(), mappings.getTableName());
    assertEquals(newMappings.getIncomingTopic(), mappings.getIncomingTopic());
    assertEquals(newMappings.areAllFieldsIncluded(), false);

    Map<String, FieldAlias> newAliasMap = newMappings.getMappings();
    assertEquals(4, newAliasMap.size()); //+ the specific mapping
    assertTrue(newAliasMap.containsKey("col1"));
    assertEquals(newAliasMap.get("col1").getName(), "col1");
    assertEquals(newAliasMap.get("col1").isPrimaryKey(), true);

    assertTrue(newAliasMap.containsKey("col2"));
    assertEquals(newAliasMap.get("col2").getName(), "col2");
    assertEquals(newAliasMap.get("col2").isPrimaryKey(), false);

    assertTrue(newAliasMap.containsKey("col3"));
    assertEquals(newAliasMap.get("col3").getName(), "col3");
    assertEquals(newAliasMap.get("col3").isPrimaryKey(), false);

    assertTrue(newAliasMap.containsKey("f1"));
    assertEquals(newAliasMap.get("f1").getName(), "col3");
    assertEquals(newAliasMap.get("f1").isPrimaryKey(), false);
  } 
                            #location 21 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              @Test
  public void handleAllFieldsMappingSettingAndTheMappingsProvided() {
    List<DbTableColumn> columns = Lists.newArrayList(
        new DbTableColumn("col1", true, false, 1),
        new DbTableColumn("col2", false, false, 1),
        new DbTableColumn("col3", false, false, 1));
    DbTable table = new DbTable("tableA", columns);

    Map<String, FieldAlias> aliasMap = new HashMap<>();
    aliasMap.put("f1", new FieldAlias("col3"));
    FieldsMappings mappings = new FieldsMappings("tableA", "topic1", true, aliasMap);

    FieldsMappings newMappings = PreparedStatementBuilderHelper.validateAndMerge(mappings, table, InsertModeEnum.INSERT);
    assertEquals(newMappings.getTableName(), mappings.getTableName());
    assertEquals(newMappings.getIncomingTopic(), mappings.getIncomingTopic());
    assertEquals(newMappings.areAllFieldsIncluded(), false);

    Map<String, FieldAlias> newAliasMap = newMappings.getMappings();
    assertEquals(4, newAliasMap.size()); //+ the specific mapping
    assertTrue(newAliasMap.containsKey("col1"));
    assertEquals(newAliasMap.get("col1").getName(), "col1");
    assertEquals(newAliasMap.get("col1").isPrimaryKey(), true);

    assertTrue(newAliasMap.containsKey("col2"));
    assertEquals(newAliasMap.get("col2").getName(), "col2");
    assertEquals(newAliasMap.get("col2").isPrimaryKey(), false);

    assertTrue(newAliasMap.containsKey("col3"));
    assertEquals(newAliasMap.get("col3").getName(), "col3");
    assertEquals(newAliasMap.get("col3").isPrimaryKey(), false);

    assertTrue(newAliasMap.containsKey("f1"));
    assertEquals(newAliasMap.get("f1").getName(), "col3");
    assertEquals(newAliasMap.get("f1").isPrimaryKey(), false);
  }