Thread: Strange Image IO

I am testing the code for a 4X4 gif image named abc which I made with MSpaint which contained 2 black pixels and rest as white
Once I ran the code (given below) the output was
-1 -1 -1 -1
-1 -1 -16777216 -1
-1 -1 -1 -1
-16777216 -1 -1 -1

I compiled and ran the same program again now output is

-1 -1 -1 -16777216
-1 -1 -1 -16777216
-1 -1 -1 -16777216
-16777216 -1 -1 -16777216

Isn't this strange because my program reads and doesn't modify any pixel data?

After the first run the image is unmodified in MS Paint also
but after the second run the modification is visible in MS Paint also

How can I prevent this from happening?
Is something wrong with my output code?

public static void main(String args[])throws Exception
{
BufferedImage img;
File ptr =new File("abc.gif");
img = ImageIO.read(ptr);
 
int height=img.getHeight();
int width=img.getWidth();
 
int rgb;
 
for(int i=0;i<width;i++)
{
for(int j=0;j<height;j++)
{
rgb =img.getRGB(i,j);
System.out.print(rgb+" ");
}
System.out.print("\n");
}
 
ImageIO.write(img,"gif",ptr);
}