Erro com setCommandListerner

import javax.microedition.lcdui.Choice;
import javax.microedition.lcdui.ChoiceGroup;
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.DateField;
import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.Form;
import javax.microedition.lcdui.TextBox;
import javax.microedition.lcdui.TextField;
import javax.microedition.midlet.MIDlet;


public class Aula2 extends MIDlet implements CommandListener{
	private Display display;
	private Form frmNeri;
	public TextField tf_nome, tf_idade;
	public ChoiceGroup cgSexo;
	public DateField dfNasc;
	public Command cmdSalvar, cmdAbrir;
	public TextBox txTeste;

	
	public Aula2(){
		display 	= Display.getDisplay(this);
		cmdSalvar 	= new Command("Salvar",Command.SCREEN,1);
		cmdAbrir 	= new Command("Terminar",Command.SCREEN,2);
		tf_nome 	= new TextField("Nome:","",40,TextField.ANY);
		tf_idade 	= new TextField("Idade","",3,TextField.NUMERIC);
		txTeste 	= new TextBox("Primeira Midlet - Digite o texto","", 200, TextField.ANY);
		cgSexo 		= new ChoiceGroup("Sexo",Choice.EXCLUSIVE);
		dfNasc 		= new DateField("Data Nascimento",DateField.DATE);
		frmNeri 	= new Form("Primeira Form - Jeisel");
		cgSexo.append("Feminino",null);
		cgSexo.append("Masculino",null);
		frmNeri.append(tf_nome);
		frmNeri.append(tf_idade);
		frmNeri.append(cgSexo);
		frmNeri.append(dfNasc);
		frmNeri.addCommand(cmdSalvar);
		frmNeri.addCommand(cmdAbrir);
		frmNeri.setCommandListener(this);
		
	}
	
	public void startApp(){
		display.setCurrent(frmNeri);
	}

	public void pauseApp(){

	}

	public void destroyApp(boolean unconditional){
	}

	public void commandAction(Command c, Displayable d){
			
	}
}