I'm encountering a problem with Eclipse's Swing WindowBuilder.
When I click on the border options button to edit the border of the jPanel:
I get the following error:
I am running on Mac OS X 10.8.4. With the following Eclipse version, and WindowBuilder version.
WindowBuilder has many, many, many problems. It looks like you've found one of them. I had no problems trying to set the border using the same method as you, even though I am using the same version of eclipse as you.
Workaround:
To add a border, try switching to the code view and changing:
contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
To:
JBorder myBorder = BorderFactory.createLineBorder(Color.black); contentPane.setBorder(myBorder);
Check out http://docs.oracle.com/javase/tutorial/uiswing/components/border.html and http://docs.oracle.com/javase/7/docs/api/javax/swing/BorderFactory.html for more information on creating borders.