How to create a window with nice rounded corners?

Question

Java

The following code makes the fillets pixel by pixel. Looks ugly

frame.setShape(new RoundRectangle2D.Double(0, 0, 880, 495, 30, 30));

The option to make the frame transparent and place a panel with rounded edges on it using Graphics tools did not satisfy due to the fact that the contents of the window "twitch". Defects appear
Can rounds be rendered smooth and nice, without pixels, using RoundRectangle2D? Maybe there are other options? In which direction to dig?))