private static final int THUMBNAIL_SIZE = 150;
private static final BufferedImage SCRATCH_BUFFER = new BufferedImage(THUMBNAIL_SIZE,
                                                                          THUMBNAIL_SIZE,
                                                                          BufferedImage.TYPE_INT_RGB);
    
private static final Graphics2D GRAPHICS = SCRATCH_BUFFER.createGraphics();
    
// this is an inner class in the java file
private final class GenThumbnailTask implements Runnable {
        private final String source_;
        private final String destination_;
        private final String extension_;
        
        private GenThumbnailTask(String source, String destination, String extension) {
            source_ = source;
            destination_ = destination;
            extension_ = extension;
        }
        
        private void generateThumbnail() throws IOException {
            File f = new File(source_);
            BufferedImage im = ImageIO.read(f);
            int width = im.getWidth();
            int height = im.getHeight();
            
            if (width > THUMBNAIL_SIZE && height > THUMBNAIL_SIZE) {
                //clear the previous image
                int dimensionToUse = width;
                if (height < width) {
                    dimensionToUse = height;
                }
                GRAPHICS.setColor(Color.BLACK);
                GRAPHICS.fillRect(0, 0, THUMBNAIL_SIZE, THUMBNAIL_SIZE);
                GRAPHICS.drawImage(im, 0, 0, THUMBNAIL_SIZE, THUMBNAIL_SIZE, 0, 0, dimensionToUse, dimensionToUse, null);
                ImageIO.write(SCRATCH_BUFFER, extension_, new File(destination_));
            }
        }
        
        /** {@inheritDoc} */
        public void run() {
            try {
                File sourceFile = new File(source_);
                if (sourceFile.exists()) {
                    File thumb = new File(destination_);
                    thumb.getParentFile().mkdirs();
                    
                    
                    generateThumbnail();
                    
                    
                }
            } catch (IOException e) {
                e.printStackTrace();
            }            
        }
    }