Calculadora java

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Prueba extends JFrame implements ActionListener {
boolean nuevo = true;
float resultado_total = 0;
Stringultimo = "=";
Label pantalla = null;
Button b;
JPanel panel, panel2;

private void Pantalla() {

panel = new JPanel();
panel.setLayout(new GridLayout(1, 1));
pantalla = new Label();pantalla.setText("0");
pantalla.setAlignment(Label.RIGHT);
pantalla.setForeground(Color.black);
pantalla.setBackground(Color.white);

panel.add(pantalla);
add("North", panel);
}

public voidTeclado () {
panel2 = new JPanel();
panel2.setLayout(new GridLayout(4, 4));

addBoton("7", Color.blue);
addBoton("8", Color.blue);
addBoton("9", Color.blue);
addBoton("/", Color.red);addBoton("C", Color.red);
addBoton("4", Color.blue);
addBoton("5", Color.blue);
addBoton("6", Color.blue);
addBoton("*", Color.red);
addBoton("√", Color.red);
addBoton("1", Color.blue);
addBoton("2",Color.blue);
addBoton("3", Color.blue);
addBoton("-", Color.red);
addBoton("AC", Color.red);
addBoton("0", Color.blue);
addBoton("+/-", Color.red);
addBoton(".", Color.red);
addBoton("+",Color.red);
addBoton("=", Color.red);

add("Center", panel2);
}

private void addBoton(String n, Color color) {
b = new Button(n);
b.setForeground(color);
panel2.add(b);
b.addActionListener( this);
}

public void actionPerformed(ActionEvent event) {

String digit = event.getActionCommand();
String s = pantalla.getText();
float valor = 0;
try {
valor = newFloat(s).floatValue();
} catch (Exception e) {
if (!digit.equals("C")) return;
}

if ("0123456789".indexOf(digit) != -1) {

if (nuevo) {
nuevo = false;
pantalla.setText(digit);
} else {
pantalla.setText(s +digit);
}

} else if (digit.equals(".")) {

if (nuevo) {
nuevo = false;
pantalla.setText("0.");
} else {
pantalla.setText(s + digit);
}

} else if (digit.equals("sqrt")) {

valor =...