initLookAndFeel.java

    private void initLookAndFeel() {
        try {
            if (OSValidator.isWindows())
                UIManager.setLookAndFeel("com.sun.java.swing.plaf.windows.WindowsLookAndFeel");
            else if (OSValidator.isLinux())
                UIManager.setLookAndFeel("com.sun.java.swing.plaf.gtk.GTKLookAndFeel");
            else if (OSValidator.isUnix())
                UIManager.setLookAndFeel("com.sun.java.swing.plaf.motif.MotifLookAndFeel");
            else
                UIManager.setLookAndFeel(UIManager.getCrossPlatformLookAndFeelClassName());
        } catch (ClassNotFoundException | InstantiationException | UnsupportedLookAndFeelException | IllegalAccessException e) {
            showError(e);
        }
    }

OSValidator.java

package hotelManager.utils;

/**
 * OSValidator class
 */
public class OSValidator {

    private static String OS = System.getProperty("os.name").toLowerCase();

    public static boolean isWindows() {
        return (OS.contains("win"));
    }

    public static boolean isMac() {
        return (OS.contains("mac"));
    }

    public static boolean isLinux() {
        return OS.contains("linux");
    }

    public static boolean isUnix() {
        return (OS.contains("nix") || OS.contains("nux") || OS.indexOf("aix") > 0);
    }

    public static boolean isSolaris() {
        return (OS.contains("sunos"));
    }
}