Como detectar configurações do proxy para conexões de Internet

public class testProxy {
    
    public static void main(String[] args) {
        try {
            System.setProperty("java.net.useSystemProxies","true");
            List l = ProxySelector.getDefault().select(
                        new URI(http://www.yahoo.com/));            
            for (Iterator iter = l.iterator(); iter.hasNext(); ) {

                
                Proxy proxy = (Proxy) iter.next();                
                System.out.println("proxy hostname : " + proxy.type());

                
                InetSocketAddress addr = (InetSocketAddress)
                    proxy.address();
                
                if(addr == null) {                    
                    System.out.println("Sem Proxy");                    
                } else {                    
                    System.out.println("proxy hostname : " + 
                            addr.getHostName());                    
                    System.out.println("proxy port : " + 
                            addr.getPort());
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}