当JPanel位于JScrollPane中时，绘图如何工作？

Question

So I have a JPanel inside a JScrollPane (let's call them panel and pane for reference) So I added created the pane as so :

JScrollPane pane = new JScrollPane(panel);

Then I added my pane to the frame. So now I want to draw in my panel. I first tried doing this by overriding the paintComponent method in my panel class. I thought that my whole panel would be drawn and then I would be able to scroll through it because I added the panel to the pane. But every time I move the bar, it keeps trying to redraw. Is my logic not right? Should I be drawing in another manner?

Answer 1

I thought that my whole panel would be drawn and then I would be able to scroll through it because I added the panel to the pane. But every time I move the bar, it keeps trying to redraw.

This is just how JScrollPane s work, see the documentation for JViewport for a description of the algorithm.

If your drawing algorithm involves heavy processing, or you otherwise want to avoid having to redraw the panel so frequently, you can draw to an BufferedImage instead, and then use drawImage() from Graphics2d to draw the panel more efficiently on scroll updates.