TODO SWING CASILLAS DE VERIFICACION SUMA Y RESTA en 8:59

NUEVO EJEMPLO
import javax.swing.*;
import java.awt.*;
import java.applet.Applet;
import java.awt.event.ItemListener;
import java.awt.event.ItemEvent;
public class SUMA_CHE extends JApplet implements ItemListener
{
    JCheckBox suma,resta,lim;
    JTextField t1,t2,t3;
    public void init()
    {
        Container c=getContentPane();
        c.setLayout(new FlowLayout());
        suma=new JCheckBox("SUMAR");
        resta=new JCheckBox("RESTAR");
        lim=new JCheckBox("LIMPIAR");
        t1=new JTextField(30);
        t2=new JTextField(30);
        t3=new JTextField(30);
        c.add(suma);
        c.add(resta);
        c.add(lim);
        c.add(t1);
        c.add(t2);
        c.add(t3);
        suma.addItemListener(this);
        resta.addItemListener(this);
        lim.addItemListener(this);
    }
    public void itemStateChanged(ItemEvent e)
    {
        double a,b,r;
        if(e.getItemSelectable()==suma)
        {
            a=Double.parseDouble(t1.getText());
            b=Double.parseDouble(t2.getText());
            r=a+b;
            t3.setText(String.valueOf(r));
        }
        if(e.getItemSelectable()==resta)
        {
            a=Double.parseDouble(t1.getText());
            b=Double.parseDouble(t2.getText());
            r=a-b;
            t3.setText(String.valueOf(r));
        }
        if(e.getItemSelectable()==lim)
        {
            t1.setText("");
            t2.setText("");
            t3.setText("");
           
        }
    }

}