///////   URLDecoder.java   /////// 
import java.io.*;

/**
decode - translate String from x-www-form-urlencoded format by
         replacing + by space and %xx by character
         with hex code xx
  
  Parameters:
  s - String to be decoded
  Returns:
  the decoded String.
*/
  
public class URLDecoder
{ public static String decode(String s)
  {  int a, b, len = s.length();
     char c, x1, x2, space = ' ';
     StringBuffer buf = new StringBuffer(); // (1)
     for (int i=0; i < len; i++)
     {  c = s.charAt(i);
        if ( c == '+' )  c = space;
        else if ( c == '%' )
        {  x1 = s.charAt(++i);              // (2)
           x2 = s.charAt(++i);              // (3)
           a = Character.digit(x1,16);
           b = Character.digit(x2,16);
           if ( a < 0 || b < 0 )  
           {   System.err.println(
                  "Invalid code %" + x1 + x2);
               buf.append(c); buf.append(x1);
               c = x2;
           }
           else
               c = (char) (16*a+b);
        }
        buf.append(c);
     }  // end of for
     return new String(buf);                 // (4)
  }

  public static void main (String args[])
  {   if ( args.length != 1 )
      { System.err.println( "Usage: URLDecoder string");
        System.exit(1);
      }
      System.out.println(decode(args[0]));
  }
}


// try URLDecoder %3A%2Dabcd+%3F+%3Dzyx++Uvw