How do I center the vertical and horizontal scrollbars in a JScrollPane?

Question

I have a JPanel with a JLabel in it, added to a JScrollPane. I have an actionListener that calls JLabel.setIcon("file.jpg");. The image is displayed in the JScrollPane correctly and is full size. The scrollbars appear perfectly. I am trying to position the vertical and horizontal scrollbars in the center by default so you are looking at the center of the image by default.

Is there a JScrollPane method that will position the viewport on the center of the image? Or could I manually set the position of each scrollbar to max size divided by 2?

I have tried

JScrollPane.getVerticalScrollBar().setValue(JScrollPane.getVerticalScrollBar().getMaximum() / 2);

While it compiles it does not center the scrollbar. I have also tried setting the layout manager of my JPanel to GridBagLayout but that doesn't work either.

Answer 1

Basically, you need to know the size of the viewport's viewable area.

Rectangle bounds = scrollPane.getViewport().getViewRect();

Then you need the size of the component, but once it's added to the scroll pane, you can get this from the view port...

Dimension size = scrollPane.getViewport().getViewSize();

Now you need to calculate the centre position...

int x = (size.width - bounds.width) / 2;
int y = (size.height - bounds.height) / 2;

Then you need to simply adjust the view port position...

scrollPane.getViewport().setViewPosition(new Point(x, y));

Now, remember, this is only going to work once the scroll pane has being realised on the screen (or at least it has being laid out within it's parent container)