java - Adding JPanel Object into a separate JPanel -

i'm working on menu, want make object orientated possible i've made separate class menu jpanel object. problem doesn't want add main jpanel. doing wrong , how should fix it?

main class:

package stackoverflow; import java.awt.cardlayout; import javax.swing.*;  public class main {      private jframe frame = new jframe();     private jpanel mainpanel = new jpanel();     private cardlayout cl = new cardlayout();     private gamepanel gp = new gamepanel();      public main(){         frame.setlocation(100, 100);         frame.setsize(1200, 700);         frame.settitle("rain | pre-alpha");         frame.setdefaultcloseoperation(jframe.exit_on_close);          mainpanel.setlayout(cl);          mainpanel.add(gp, "1");          frame.add(mainpanel);          cl.show(mainpanel, "1");          frame.setvisible(true);     }      public static void main(string[] args) {         new main();     } }     

gamepanel class:

package stackoverflow; import java.awt.color; import javax.swing.jpanel;  public class gamepanel {     private jpanel gamepanel = new jpanel();      public gamepanel(){         gamepanel.setbackground(color.green);     } } 

you can't add class jframe, jframe accept component :

public component add(component comp, int index) 

so have many ways solve problem :

option 1

extends jpanel instead :

public class gamepanel extends jpanel {      public gamepanel() {         super.setbackground(color.green);     } } 

option 2

you can use getters , setters :

public class gamepanel {      private jpanel gamepanel = new jpanel();      public jpanel getgamepanel() {         return gamepanel;     }      public void setgamepanel(jpanel gamepanel) {         this.gamepanel = gamepanel;     }      public gamepanel() {         gamepanel.setbackground(color.green);     }  } 

and can add jpanel :

mainpanel.add(gp.getgamepanel(), "1");