code: public List<Column> getColumns() {
          final List<Column> columns = new ArrayList<>();
          final Component[] components = _sourceComboBoxPanel.getComponents();
          for (final Component component : components) {
              if (component instanceof SourceColumnComboBox) {
                  final SourceColumnComboBox sourceColumnComboBox = (SourceColumnComboBox) component;
                  columns.add(sourceColumnComboBox.getSelectedItem());
              }
          }
          return columns;
      }
docs: Returns the columns selected as Column[].