code: protected void moveLeftToRight() {
          // Loop over the selected items and locate them in the data model, Add
          // these to the selection.
          Object[] sourceSelected = sourceList.getSelectedValues();
          int nSourceSelected = sourceSelected.length;
          int[] currentSelection = helperList.getSelectedIndices();
          int[] newSelection = new int[currentSelection.length + nSourceSelected];
          System.arraycopy(currentSelection, 0, newSelection, 0, currentSelection.length);
          int destPos = currentSelection.length;
  
          for (int i = 0; i < sourceSelected.length; i++) {
              newSelection[destPos++] = indexOf(sourceSelected[i]);
          }
  
          helperList.setSelectedIndices(newSelection);
          update();
      }
docs: Move the selected items in the source list to the chosen list. I.e., add
  the items to our selection model.