<applet code="JAppletURL.class" width="200" height="30">
   <param name="etic1" value="Java">
   <param name="link1" value="http://www.valentinocondoluci.it/appunti/java">
   <param name="etic2" value="Html">
   <param name="link2" value="http://www.valcon.it/htmleco">
   <param name="etic3" value="IOI">
   <param name="link3" value="http://www.valcon.it/ioi">
</applet>

<applet code="JAppletURL.class" width="450" height="40">
   <param name="etic1" value="la Repubblica">
   <param name="link1" value="http://www.repubblica.it">
   <param name="etic2" value="Corriere della Sera">
   <param name="link2" value="http://www.corriere.it">
   <param name="etic3" value="LA STAMPA">
   <param name="link3" value="http://www.lastampa.it">
</applet>

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

public class JAppletURL extends    JApplet
                        implements ActionListener
{ 
   String l1, l2, l3, //link
          e1, e2, e3; //etichette
   URL    u1, u2, u3; //url

   public void init()
   {
      l1=getParameter("link1"); e1=getParameter("etic1");
      l2=getParameter("link2"); e2=getParameter("etic2");
      l3=getParameter("link3"); e3=getParameter("etic3"); 

      try
      {
         u1=new URL(l1);
         u2=new URL(l2);
         u3=new URL(l3);
      }
      catch(MalformedURLException ex)
      {
         System.out.println("Problemi con i parametri?");
      }

      JPanel   p=new JPanel();
      JButton b1=new JButton(e1); b1.addActionListener(this); p.add(b1);
      JButton b2=new JButton(e2); b2.addActionListener(this); p.add(b2);
      JButton b3=new JButton(e3); b3.addActionListener(this); p.add(b3);
      add(p);
   }

   public void actionPerformed(ActionEvent ae)
   {
      String nome=ae.getActionCommand(); 

           if(nome.equals(e1)) getAppletContext().showDocument(u1);
      else if(nome.equals(e2)) getAppletContext().showDocument(u2);
      else if(nome.equals(e3)) getAppletContext().showDocument(u3);
      else                     System.out.println("Problemi con i pulsanti?");
   }
}

1. <param name="..." value="...">

2. getParameter()

3. URL, MalformedURLException

4. getAppletContext().showDocument()