(createDatum(null, "root", false, null); 
......
createDatum(null, "root2", false, null);
....

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

import org.eclipse.nebula.widgets.nattable.sort.ISortModel;
import org.eclipse.nebula.widgets.nattable.sort.SortDirectionEnum;
import org.eclipse.nebula.widgets.nattable.util.ComparatorChain;

public class SortableTreeComparator<T> implements Comparator<T> {

    private final Comparator<T> treeComparator;
    private final ISortModel sortModel;

    public SortableTreeComparator(Comparator<T> treeComparator, ISortModel sortModel) {
        this.treeComparator = treeComparator;
        this.sortModel = sortModel;
    }

    @SuppressWarnings({ "rawtypes", "unchecked" })
    @Override
    public int compare(T o1, T o2) {
        int treeComparatorResult = this.treeComparator.compare(o1, o2);
        if (treeComparatorResult == 0) {
            return 0;
        } else {
            List<Integer> sortedColumnIndexes = this.sortModel.getSortedColumnIndexes();
            if (sortedColumnIndexes != null && sortedColumnIndexes.size() > 0) {
                List<Comparator<T>> comparators = new ArrayList<Comparator<T>>();
                for (int sortedColumnIndex : sortedColumnIndexes) {
                    // get comparator for column index... somehow
                    List<Comparator> columnComparators =
                            this.sortModel.getComparatorsForColumnIndex(sortedColumnIndex);

                    if (columnComparators != null) {
                        SortDirectionEnum sortDirection = this.sortModel.getSortDirection(sortedColumnIndex);
                        for (Comparator columnComparator : columnComparators) {
                            switch (sortDirection) {
                                case ASC:
                                    comparators.add(columnComparator);
                                    break;
                                case DESC:
                                    comparators.add(Collections.reverseOrder(columnComparator));
                                    break;
                            }
                        }
                    }
                }
                //*******added line 
                comparators.add(treeComparato)
                return new ComparatorChain<T>(comparators).compare(o1, o2);
            } else {
                return treeComparatorResult;
            }
        }
    }

}