Problemas com janela. Ajuda urgende, plz!

import java.awt.Dimension;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.*;

public class tela_principal extends JFrame implements ActionListener {
	
	public JMenuBar barra = null;
	public JMenu menu01 = null;
	public JMenuItem item01, item02 = null;
	
	public tela_principal() {
		super();
		iniciar();
	}
	
	public void iniciar(){
		this.setTitle("Programa");  
        this.setJMenuBar(getbarraMenu());  
        this.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);  
        this.setSize(800,800);  
        this.setVisible(true);
	}
	
	private JMenuBar getbarraMenu() {  
        if (barra == null){  
            barra = new JMenuBar();  
            barra.add(getmenuArquivo());  
        }  
        return barra;}
        
        private JMenu getmenuArquivo() {  
            if (menu01 == null){  
                menu01 = new JMenu();  
                menu01.setText("Arquivo");  
                menu01.add(getitemMenu());
                menu01.addSeparator();
                menu01.add(getitemMenu2());
            }  
             return menu01;  
         }
        
        private JMenuItem getitemMenu() {  
            if (item01 == null){  
                item01 = new JMenuItem();  
                item01.setText("Novo");
                item01.addActionListener(this);
            }  
            return item01;  
        }
        
        private JMenuItem getitemMenu2() {  
            if (item02 == null){  
                item02 = new JMenuItem();  
                item02.setText("Sair");
                item02.addActionListener(this);
            }  
            return item02;  
        }
        
        public void actionPerformed(ActionEvent e) {
            tela_principal t_p = new tela_principal();
           
            if(e.getSource() == item02) {
               System.exit(0);
            }
            
            if(e.getSource() == item01) {
               tela_01 emitirCert = new tela_01();
               emitirCert.tela_01();
            }
        }
    
	public static void main(String[] args) {
		new tela_principal();

	}

}

Tela secundária (ainda em desenvolvimento)

[/code]

import javax.swing.JDialog;

import javax.swing.<em>;

import java.awt.</em>;

public void tela_01()  {
    this.setTitle("Cert");
    this.setSize(700,350);
    this.setResizable(false);
    this.alignCenter();
    this.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
    getContentPane().setLayout(null);
}


public void t_01() {
	tela_01 emititirCert = new tela_01();
	emititirCert.setTitle("Emitir certificado");
	emititirCert.setSize(550,250);
	emititirCert.setResizable(false);
	emititirCert.setModal(true);
	emititirCert.alignCenter();
	emititirCert.getContentPane().setLayout(null);
}

public void alignCenter() {
    Dimension t = Toolkit.getDefaultToolkit().getScreenSize();
    Dimension j = getSize();
    setLocation((t.width-j.width)/2,(t.height-j.height)/2);
}

}