Something

private static void doProcessing()
            throws
            GeoAlgorithmExecutionException,
            IOException {

            GridCoverage2D gc = OpenRasterLayer(infile); //infile is defined in main class
            GTRasterLayer dem = new GTRasterLayer();
            dem.create(gc);

            GridCoverage2D gcw = OpenRasterLayer("weight_1.asc"); //a Raster with all values to one
            GTRasterLayer weight = new GTRasterLayer();
            weight.create(gcw);

            System.out.println("Fillsinking");
            IRasterLayer dem_fillsinked = execFillSinks(dem);
            System.out.println("FlowAccum");
            execFlowAcc(dem_fillsinked,weight);
    }