JTabbedPane problems

Hey guys! Im making a small little database program and i want it to have tabs hence the use of a JTabbedPane. I have a porblem though, whenever i make it the tabbedpane in my jframe is in essence a small box thats centered in the middle of the JFrame. This is the code

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTabbedPane;
import javax.swing.JTextField;
import net.miginfocom.swing.MigLayout;

/**
 *
 * @author Harry
 */
public class tabbedPane  extends JPanel {

    /**
     * Creates new form UIf
     */
    //NewData create;
    JTabbedPane tabbedPane;
    JPanel customerPane,employeePane, guarantorPane;
    
    public tabbedPane() {
       // create = new NewData();
        
        tabbedPane = new JTabbedPane();
        add(tabbedPane);
        customerPane = new JPanel(new MigLayout());
        
         customerPane.add(new JButton("dfasd"));
         customerPane.add(new JLabel("First Name:"));
    
    
        employeePane = new JPanel();
        guarantorPane = new JPanel();
        tabbedPane.addTab("customer", customerPane);
        tabbedPane.addTab("employee", employeePane);
        tabbedPane.addTab("guarantor",guarantorPane);
        
        //pack();
        //add(tabbedPane);
    }
    
    public static void main(String[] args){
        tabbedPane a = new tabbedPane();
         JFrame frame = new JFrame("Game");
         
       frame.add(a);
      
        frame.pack();
        frame.setVisible(true);
       frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}

The problem comes because the class tabbedPane inherits from JPanel. If i were to make it inherit from JFrame it would work fine by making the jframe actually look like a program with a tabs. Anyone know why this happens?