Complete Program

Here is the complete program, suitable for copying and pasting to a source file, and then compiling and running. Play with the code for a while. Perhaps you can improve it somehow.

class MyDate
{
  int day, month, year;
  MyDate ( int d, int m, int y )
  {day = d; month = m; year = y;}

  public String toString()
  {return day+"/"+month+"/"+year ;}
 }

class FSADate
{
  MyDate convert( String str )
  {
    int  reject= 10;       // rejection state
    int  state = 1;        // the current state
    char current;          // the current character
    int  index = 0 ;       // index of the current character
    String buffer = null;
    int  day=0, month=0, year=0;
    
    while ( index < str.length()  && state != reject  )
    {
      current = str.charAt( index++ ) ;

      if      ( state==1 && current >=  '0' && current <= '9')
      {
        state = 2; buffer = "" + current ; 
      }
      else if ( state==2 && current=='/' )
      {
        state = 4; day = Integer.parseInt( buffer ); 
      }
      else if ( state==2 && current >=  '0' && current <= '9' )
      {
        state = 3; buffer +=  current ;  
      }
      else if ( state==3 && current=='/' )
      {
        state = 4; day = Integer.parseInt( buffer ); 
      }
      else if ( state==4 && current >=  '0' && current <= '9' )
      {
        state = 5; buffer = "" + current ;
      }
      else if ( state==5 && current=='/' )
      {
        state = 7; month = Integer.parseInt(  buffer ); 
      }
      else if ( state==5 && current >=  '0' && current <= '9' )
      {
        state = 6; buffer +=  current ;
      }
      else if ( state==6 && current=='/' )
      {
        state = 7;  month = Integer.parseInt(   buffer ); 
      }
      else if ( state==7 && current >=  '0' && current <= '9' )
      {
        state = 8;  buffer = "" + current;
      }
      else if ( state==8 && current >=  '0' && current <= '9' )
      {
        state = 9;  buffer += current  ;
        year = Integer.parseInt( buffer ); 
      }
      else
        state = reject ;
    }

  if ( index == str.length()  && state ==9  )
    return new MyDate( day, month, year )  ;
  else
    return null ;
  }
}

class DateTester
{
  public static void main (String[] args)
  {
    String  inString  = null;
    FSADate fst       = new FSADate();
    MyDate  date      = null;

    if ( args.length == 1 )
     inString = args[0];
    else
    {
      System.out.println("java DateTester dd/mm/yy");
      return;
    }

    if ( (date = fst.convert( inString )) != null )
      System.out.println("The string is transformed to:" + date );
    else
      System.out.println("The string is rejected.");
  }
}