Convert color image indexed in java

Finally I have managed to do it with the help of this code. Thank you very much Jorge, it helped me a lot:

public static BufferedImage rgbaToIndexedBufferedImage(BufferedImage sourceBufferedImage) {
    //With this constructor we create an indexed bufferedimage with the same dimensiosn and with a default 256 color model
    BufferedImage indexedImage= new BufferedImage(sourceBufferedImage.getWidth(),sourceBufferedImage.getHeight(),BufferedImage.TYPE_BYTE_INDEXED);


    ColorModel cm = indexedImage.getColorModel();
    IndexColorModel icm=(IndexColorModel) cm;

    int size=icm.getMapSize();

    byte[] reds = new byte[size];
    byte[] greens = new byte[size];
    byte[] blues = new byte[size];
    icm.getReds(reds);
    icm.getGreens(greens);
    icm.getBlues(blues);

    WritableRaster raster=indexedImage.getRaster();
    int pixel = raster.getSample(0, 0, 0); 
    IndexColorModel icm2 = new IndexColorModel(8, size, reds, greens, blues,pixel);
    indexedImage=new BufferedImage(icm2, raster,sourceBufferedImage.isAlphaPremultiplied(), null);
    indexedImage.getGraphics().drawImage(sourceBufferedImage, 0, 0, null);
    return indexedImage;
}