@Override
    public void actionPerformed (ActionEvent e) {
        if (e.getSource() == bRead){
            solids = SolidFileIO.readSolids(fileName);
            doOutput();
        }
        if ((e.getSource() == bSort1) && (solids != null)){
            // sort Volume
            solids = SolidFileIO.readSolids(fileName);
            Arrays.sort(solids);
            doOutput();
        }
        if ((e.getSource() == bSort2) && (solids != null)){
            // sort surface
            solids = SolidFileIO.readSolids(fileName);
            SurfaceComparator c1 = new SurfaceComparator();
            Arrays.sort(solids, c1);
            doOutput();
        }
        if ((e.getSource() == bSort3) && (solids != null)){
            // sort Max dimension
            solids = SolidFileIO.readSolids(fileName);
            MaxDimensionComparator c2 = new MaxDimensionComparator();
            Arrays.sort(solids, c2);
            doOutput();
        }
    }