doubts about setPixels() in WritableRaster

I was trying to get familiar with Raster,BufferedImage classes
etc..Encountered some doubts there..

1).
In Raster.getPixels() method why do we have to pass an optionally
predefined double array?

2).
I implemented a method which normalises the image data..However, it
doesn't do what I expected .For some reason ,the
BufferedImage.setData() doesn't give the expected result..

I have listed the output of this program below..

original image data:
23.0 32.0 13.0 55.0 65.0 36.0 46.0 64.0 27.0 43.0 71.0 58.0 38.0 25.0
62.0 47.0 19.0 72.0 37.0 55.0
image data after normalisation:
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
1.0 0.0 0.0

I can't make out why these zeroes are getting printed..It should have
been
0.3194 0.4444 0.18055 0.7638....etc

If anyone can help me figure out where I have messed up ..please do.
thanks,
mark

<code>

import java.awt.image.BufferedImage;
import java.awt.image.ColorConvertOp;
import java.awt.image.DataBuffer;
import java.awt.image.Raster;
import java.awt.image.WritableRaster;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;

public class MyImage {

    private BufferedImage bi; //of type BufferedImage.TYPE_BYTE_GRAY
    private String fileName;
    public MyImage(String imageFileName) throws IOException{
        this.fileName = imageFileName;
        File imageFile = new File(imageFileName);
        this.bi=makeGrayScaleImage(ImageIO.read(imageFile));
    }
    public MyImage(String imageName,int width,int height,double[] data){
        if (data.length != width*height){
            throw new IllegalArgumentException("data size must be equal to
"+width*height);
        }
        this.bi = new BufferedImage(width, height,
BufferedImage.TYPE_BYTE_GRAY);
        this.bi.getRaster().setPixels(0, 0, width, height, data);
        this.fileName = imageName;
    }
    private BufferedImage makeGrayScaleImage(BufferedImage img) {
        BufferedImage gray = null;
        try{
            gray = new BufferedImage(img.getWidth(),img.getHeight(),
                BufferedImage.TYPE_BYTE_GRAY);
                ColorConvertOp ccop = new ColorConvertOp(
                img.getColorModel().getColorSpace(),
                gray.getColorModel().getColorSpace(),null);
            ccop.filter(img,gray);

            }catch(Exception e){
                System.err.println("grayscale conversion failed");
            }
            return gray;
    }
    public int getWidth(){
        return this.bi.getWidth();
    }
    public int getHeight(){
        return this.bi.getHeight();
    }
    public double[] getData(){
        int h = getHeight();
        int w = getWidth();
        double[] data = new double[h*w];
        double[] pixeldata=this.bi.getData().getPixels(0,0,w,h,data);
        return pixeldata;
    }
    public void normaliseImageData(){
        double[] ndata = getNormalisedData();
        WritableRaster wr = this.bi.getRaster();
        wr.setPixels(0,0,this.getWidth(),this.getHeight(),ndata);
        this.bi.setData(wr);
    }
    private double[] getNormalisedData(){
        double[] d = getData();
        double maxval = max(d);
        for (int i = 0; i<d.length; i++){
            d[i] /= maxval;
        }
        return d;
    }
    private static double max(double[] arr){
        double m=Double.MIN_VALUE;
        for(int i = 0; i<arr.length; i++){
            m = Math.max(m,arr[i]);
        }
        return m;
    }
    private static void printArray(double[] a){
        for (double x:a){
            System.out.print(x+" ");
        }
        System.out.println();
    }
    private static void debug(String msg) {
        System.out.println(msg);
    }
    public static void main(String[] args) {
        MyImage my = new MyImage("dummy.png", 4, 5, new double[]
{23,32,13,55,65,36,46,64,27,43,71,58,38,25,62,47,19,72,37,55});
        double[] data = my.getData();
        debug("original image data:");
        printArray(data);
        my.normaliseImageData();
        debug("image data after normalisation:");
        double[] newdata = my.getData();
        printArray(newdata);

    }
}
</code>