Java: JPanel in JFrame without main( )

package home.aquaballoon;

// Chage from constructor to sub class
class CS3_Linux_Sub extends JPanel {

    public CS3_Linux_Sub() {
        initComponents();
    }

    private void initComponents() {
    }
}

// Change from main( ) to constructor
public class CS3_Linux {
    public CS3_Linux(){
        JFrame frame = new JFrame();
        frame.setContentPane(new CS3_Linux_Sub());
        frame.pack();
        frame.setVisible(true);
    }
}