Ayuda con interfaz Swing basica

Question

Hola buenas estoy aprendiendo java y al hacer una pequeña interfaz de un programa que concatene palabras en Swing y no consigo que funciona, al ejecutar me da un Null pointer exception... a ver si me podeis ayudar.  Gracias de antemano.


import javax.swing.*;

import java.awt.GridLayout;

import java.awt.Toolkit;

import java.awt.event.ActionEvent;

import java.awt.event.ActionListener;





@SuppressWarnings("serial")

public class Main extends JFrame implements ActionListener{

private int anchoVentana = 300;

private int altoVentana = 400;

private JLabel j;

private JPanel pf1;

private JPanel pf2;

private JPanel pf3;

private JPanel pf4;

private JTextField t1;

private JTextField t2;

private JButton bUno;

private JButton bDos;

private JButton bUnoDos;

private JButton bDosUno;



private void inicializarVentana() {

int anchoPantalla = Toolkit.getDefaultToolkit().getScreenSize().width;

int altoPantalla = Toolkit.getDefaultToolkit().getScreenSize().height;

setSize(anchoVentana, altoVentana);

setLocation(anchoPantalla / 2 - anchoVentana / 2, altoPantalla / 2

- altoVentana / 2);

setTitle("Ejercicio 04");

setLayout(new GridLayout(4,1));

setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);







//Creación de los paneles principales







//Panel1

this.pf1=new JPanel();

pf1.setLayout(new GridLayout(1,1));

this.add(pf1);





//JLabel en panel 1

JLabel j=new JLabel();

pf1.add(j);









//Panel2

this.pf2=new JPanel();

pf2.setLayout(new GridLayout(1,2));

this.add(pf2);





//JTextfields en panel 2



this.t1=new JTextField(); pf2.add(t1);

this.t2=new JTextField(); pf2.add(t2);







// Panel 3

pf3=new JPanel();

pf3.setLayout(new GridLayout(1,2));

this.add(pf3);



// JButtons en panel 3

bUnoDos=new JButton("Uno=>Dos"); pf3.add(bUnoDos);

bUnoDos.addActionListener(this);

bUnoDos.setActionCommand("bUnoDos");





bDosUno=new JButton("Dos=>Uno");pf3.add(bDosUno);

bDosUno.addActionListener(this);

bDosUno.setActionCommand("bDosUno");





// Panel 4

pf4=new JPanel();

pf4.setLayout(new GridLayout(1,2));

this.add(pf4);



//JButtons en panel 4

bUno=new JButton ("Uno"); pf4.add(bUno);

bUno.addActionListener(this);

bUno.setActionCommand("bUno");





bDos= new JButton ("Dos"); pf4.add(bDos);

bDos.addActionListener(this);

bDos.setActionCommand("bDos");



setVisible(true);

}



public Main() {

inicializarVentana();



}



public static void main(String[] args) {

new Main();



}



public void actionPerformed(ActionEvent e) {



//Da NULL Pointer Exception en tiempo de ejecución



switch (e.getActionCommand()) {



case "bUnoDos": j.setText(t1.getText()+t2.getText() );break;

case "bDosUno": j.setText(t2.getText()+t1.getText() );break;

case "bUno":    j.setText(t1.getText() );break;

case "bDos":    j.setText(t2.getText() );break;

}



}



}