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

class MMTMU extends    JFrame
            implements ActionListener
{ 
   private JTextField txt1, txt2, txt3, txt4;
   private JLabel     lOut; 

   public MMTMU()
   { 
      JPanel pNORTH=new JPanel();
         txt1=new JTextField("-20", 10); pNORTH.add(txt1);
         txt2=new JTextField("-1" , 10); pNORTH.add(txt2);
         txt3=new JTextField("+1" , 10); pNORTH.add(txt3);
         txt4=new JTextField("+20", 10); pNORTH.add(txt4);
      this.add(pNORTH, BorderLayout.NORTH);

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

      JPanel pSOUTH=new JPanel();
         JButton bMIN=new JButton("Minimo");  bMIN.addActionListener(this); pSOUTH.add(bMIN);
         JButton bMAX=new JButton("Massimo"); bMAX.addActionListener(this); pSOUTH.add(bMAX);
         JButton bTOT=new JButton("Totale");  bTOT.addActionListener(this); pSOUTH.add(bTOT);
         JButton bMED=new JButton("Media");   bMED.addActionListener(this); pSOUTH.add(bMED);
         JButton bESC=new JButton("Uscita");  bESC.addActionListener(this); pSOUTH.add(bESC);
           bESC.setBackground(Color.black);   bESC.setForeground(Color.white); 
      this.add(pSOUTH, BorderLayout.SOUTH);

      this.setTitle("Calcolo minimo/massimo/totale/media");
      this.pack();
      this.setVisible(true);
   }

   public void actionPerformed(ActionEvent evento)
   {
      String nome=evento.getActionCommand();
      if(nome.equals("Uscita"))
         System.exit(0);
      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);
   }

   public static void main(String[] args)
   {
      MMTMU f = new MMTMU();
   }

}

1. JLabel, JButton, JTextField, JPanel, JFrame