private static BufferedImage histogramEqualization(BufferedImage original) {

        int red;
        int green;
        int blue;
        int alpha;
        int newPixel = 0;

        // Get the Lookup table for histogram equalization
        ArrayList<int[]> histLUT = histogramEqualizationLUT(original);

        BufferedImage histogramEQ = new BufferedImage(original.getWidth(), original.getHeight(), original.getType());

        for(int i=0; i<original.getWidth(); i++) {
            for(int j=0; j<original.getHeight(); j++) {

                // Get pixels by R, G, B
                alpha = new Color(original.getRGB (i, j)).getAlpha();
                red = new Color(original.getRGB (i, j)).getRed();
                green = new Color(original.getRGB (i, j)).getGreen();
                blue = new Color(original.getRGB (i, j)).getBlue();

                // Set new pixel values using the histogram lookup table
                red = histLUT.get(0)[red];
                green = histLUT.get(1)[green];
                blue = histLUT.get(2)[blue];

                // Return back to original format
                newPixel = colorToRGB(alpha, red, green, blue);

                // Write pixels into image
                histogramEQ.setRGB(i, j, newPixel);

            }
        }

        return histogramEQ;

    }

    // Get the histogram equalization lookup table for separate R, G, B channels
    public static ArrayList<int[]> histogramEqualizationLUT(BufferedImage input) {

        // Get an image histogram - calculated values by R, G, B channels
        ArrayList<int[]> imageHist = imageHistogram(input);

        // Create the lookup table
        ArrayList<int[]> imageLUT = new ArrayList<int[]>();

        // Fill the lookup table
        int[] rhistogram = new int[256];
        int[] ghistogram = new int[256];
        int[] bhistogram = new int[256];

        for(int i=0; i<rhistogram.length; i++) rhistogram[i] = 0;
        for(int i=0; i<ghistogram.length; i++) ghistogram[i] = 0;
        for(int i=0; i<bhistogram.length; i++) bhistogram[i] = 0;

        long sumr = 0;
        long sumg = 0;
        long sumb = 0;

        // Calculate the scale factor
        float scale_factor = (float) (255.0 / (input.getWidth() * input.getHeight()));

        for(int i=0; i<rhistogram.length; i++) {
            sumr += imageHist.get(0)[i];
            int valr = (int) (sumr * scale_factor);
            if(valr > 255) {
                rhistogram[i] = 255;
            }
            else rhistogram[i] = valr;

            sumg += imageHist.get(1)[i];
            int valg = (int) (sumg * scale_factor);
            if(valg > 255) {
                ghistogram[i] = 255;
            }
            else ghistogram[i] = valg;

            sumb += imageHist.get(2)[i];
            int valb = (int) (sumb * scale_factor);
            if(valb > 255) {
                bhistogram[i] = 255;
            }
            else bhistogram[i] = valb;
        }

        imageLUT.add(rhistogram);
        imageLUT.add(ghistogram);
        imageLUT.add(bhistogram);

        return imageLUT;

    }

    private static int colorToRGB(int alpha, int red, int green, int blue) {

        int newPixel = 0;
        newPixel += alpha; newPixel = newPixel << 8;
        newPixel += red; newPixel = newPixel << 8;
        newPixel += green; newPixel = newPixel << 8;
        newPixel += blue;

        return newPixel;

    }

I kinda didn't have time to play with JAI (Java Advanced Imaging), so I've made my own histogram equalization algorithm. It's a small step towards making a simple image editing program.