为什么Java突然在图像上返回空指针异常？

Question

I have been trying to create a resizable image by drawing BufferedImage on a panel and redrawing it whenever componentResized happened. However, despite the image loading fine in earlier versions (which either didn't resize at all or didn't do it properly) now Java claims the image isn't there. The code is as follows

    public class Image extends JPanel{
    BufferedImage img=null;

    public Image{
    try {
        img = ImageIO.read(new File("Untitled.png"));
    }
    catch (IOException e) {
    }
    Dimension d=getSize();
    Graphics g=getGraphics();
    g.drawImage(img, 0, 0, d.width, d.height, null);

even without the component listener, it returns NullPointerException on drawImage. But I know the image isn't null, since it worked previously, which leads me to thinking there's something wrong with the code here

Answer 1

If something goes wrong in here: img = ImageIO.read(new File("Untitled.png")); Then you are drawing on a null reference object here:

g.drawImage(img, 0, 0, d.width, d.height, null);

That is the reason of the NPE

Answer 2

This is due to that the Graphics object g may be null. Graphics g = getGraphics();