<applet code="MMTM.class" width="480" height="100">...</applet>

/*  M       M        T       M
 *  Minimo, Massimo, Totale, Media
 */

import java.awt.*;
import java.awt.color.*;
import java.awt.event.*;
import java.applet.*;

public class MMTM extends    Applet
                  implements ActionListener
{ 
   private TextField txt1, txt2, txt3, txt4;
   private Label     lOut; 

   public void init()
   {
      this.setLayout(new BorderLayout());

      Panel pNORTH=new Panel();
         txt1=new TextField("-20", 10); pNORTH.add(txt1);
         txt2=new TextField("-1" , 10); pNORTH.add(txt2);
         txt3=new TextField("+1" , 10); pNORTH.add(txt3);
         txt4=new TextField("+20", 10); pNORTH.add(txt4);
      this.add(pNORTH, BorderLayout.NORTH);

      lOut=new Label("???", Label.CENTER);
      this.add(lOut, BorderLayout.CENTER);

      Panel pSOUTH=new Panel();
         Button bMIN=new Button("Minimo");  bMIN.addActionListener(this); pSOUTH.add(bMIN);
         Button bMAX=new Button("Massimo"); bMAX.addActionListener(this); pSOUTH.add(bMAX);
         Button bTOT=new Button("Totale");  bTOT.addActionListener(this); pSOUTH.add(bTOT);
         Button bMED=new Button("Media");   bMED.addActionListener(this); pSOUTH.add(bMED); 
      this.add(pSOUTH, BorderLayout.SOUTH); 
   }

   public void actionPerformed(ActionEvent evento)
   {
      String nome=evento.getActionCommand(); 
      String msg =new String();

      try
      {
         double in1=Double.parseDouble(txt1.getText());
         double in2=Double.parseDouble(txt2.getText());
         double in3=Double.parseDouble(txt3.getText());
         double in4=Double.parseDouble(txt4.getText());
         double risultato=0; 

         if(nome.equals("Minimo"))
         {
            risultato=Math.min(Math.min(in1, in2),
                               Math.min(in3, in4));
            msg="Minimo = " + risultato;
         }
         else if(nome.equals("Massimo"))
         {
            risultato=Math.max(Math.max(in1, in2),
                               Math.max(in3, in4));
            msg="Massimo = " + risultato;
         }
         else if(nome.equals("Totale"))
         {
            risultato=in1+in2+in3+in4;
            msg="Totale = "+risultato;
         }
         else if(nome.equals("Media"))
         {
            risultato=(in1+in2+in3+in4)/4.0;
            msg="Media = "+risultato;
         }
      }
      catch(Exception eccezione)
      {
         msg="???";
      }

      lOut.setText(msg);
   }
}

1. this.setLayout(new BorderLayout());

2. Double.parseDouble()

3. try
   {
     ...
   }
   catch(...)
   {
     ...
   }