public class TestingPanel {
    private JButton button1;
    public JPanel panel1;
    private JButton button2;
    private JButton button3;
}

JPanel testingPanel = new TestingPanel().panel1;

public class TestingPanel extends JPanel {
    private JButton button1;
    private JButton button2;
    private JButton button3;

    public void onCreate() {
         //change name, add listeners etc.
    }

}