public class TestPanel extends JPanel {
    private static void createAndShowGUI() {
        JFrame myFrame = new JFrame("");
        myFrame.setBounds(new Rectangle(100, 100, 240, 180));
        myFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JPanel box = new JPanel(null);
        box.setBackground(Color.BLUE);
        JPanel left = new JPanel();
        left.setOpaque(false);
        JPanel right = new JPanel();
        right.setOpaque(false);
        JSplitPane splitPane = new JSplitPane(JSplitPane.HORIZONTAL_SPLIT, left, right);
        splitPane.setResizeWeight(0.5);
        splitPane.setOpaque(false);
        box.setLayout(new BorderLayout());
        box.setBorder(new EmptyBorder(10, 10, 10, 10));
        box.add(splitPane);
        myFrame.getContentPane().add(box, BorderLayout.CENTER);
        myFrame.setVisible(true);
    }
    public static void main(String[] args) {
        SwingUtilities.invokeLater(TestPanel::createAndShowGUI);
    }
}