Undecorated JDialog border

You need to change the Border of the root pane:

getRootPane().
   setBorder( BorderFactory.createLineBorder(Color.RED) );

If you want to get rid of it you can use

frame.setUndecorated(true);
frame.getRootPane().setWindowDecorationStyle(JRootPane.NONE);

To change the look of it from the Java style to the windows style you can use

UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());

You can do something like this:

((JPanel)getContentPane()).setBorder(BorderFactory.createLineBorder(Color.BLUE));

enter image description here

You can try to do this in order to change the most outsude border:

getRootPane().setBorder(BorderFactory.createLineBorder(Color.BLUE));

Is this what you want to do?

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