How to implement average and uniform sharpening filter 3x3 without using predefined methods?

I have an assignment to do average and uniform sharpening filter on 3x3 png image.

My code was

 public static void AverageFilter(BufferedImage img) throws IOException{


    Kernel kernel = new Kernel(3, 3, new float[] { 1f/9f, 1f/9f, 1f/9f, 1f/9f, 1f/9f, 1f/9f, 1f/9f,
            1f/9f, 1f/9f });
        BufferedImageOp op = new ConvolveOp(kernel);
        img = op.filter(img, null);

        DisplayImage(img);


  } 

  public static void Sharpening(BufferedImage img) throws IOException{


    Kernel kernel = new Kernel(3, 3, new float[] { -1, -1, -1, -1, 9, -1, -1,
            -1, -1 });
        BufferedImageOp op = new ConvolveOp(kernel);
        img = op.filter(img, null);
        DisplayImage(img);


   }

How to implement this code using a predefined method such as loops and if conditions to handle all cases for zero padding ?