Java: problem when bitmap.setPixel() replaced by bitmap.setPixels()

I have the following code:

 Bitmap bitmap = Bitmap.createBitmap(WIDTH, HEIGHT, Bitmap.Config.ARGB_4444); 

    for (int y = 0; y < HEIGHT; y++) {
        for (int x = 0; x < WIDTH; x++) {
            int index = y * WIDTH + x;
            bitmap.setPixel(x, y, Color.argb(255, 0, mask[index],0)); // mask is an array of int between 0 and 255 
        }
    }

It works properly: I get my bitmap but...this code is really extremely slow.

I tried to replace it with:

 Bitmap bitmap = Bitmap.createBitmap(WIDTH, HEIGHT, Bitmap.Config.ARGB_4444);
bitmap.setPixels(mask, 0, WIDTH, 0, 0, WIDTH, HEIGHT);

but this is not working. I get a black image.

Anybody can help ?

Thanks !