/**
   This class breaks up a string describing an expression
   into tokens: numbers, parentheses, and operators.
*/
public class ExpressionTokenizer
{
   private String input;
   private int start; // The start of the current token
   private int end; // The position after the end of the current token

   /**
      Constructs a tokenizer.
      @param anInput the string to tokenize
   */
   public ExpressionTokenizer(String anInput)
   {
      input = anInput;
      start = 0;
      end = 0;
      nextToken(); // Find the first token
   }

   /**
      Peeks at the next token without consuming it.
      @return the next token or null if there are no more tokens
   */
   public String peekToken()
   {
      if (start >= input.length()) { return null; }
      else { return input.substring(start, end); }
   }

   /**
      Gets the next token and moves the tokenizer to the following token.
      @return the next token or null if there are no more tokens
   */
   public String nextToken()
   {
      String r = peekToken();
      start = end;
      if (start >= input.length()) { return r; }
      if (Character.isDigit(input.charAt(start)))
      {
         end = start + 1;
         while (end < input.length() 
               && Character.isDigit(input.charAt(end)))
         {
            end++;
         }
      }
      else
      {
         end = start + 1;
      }
      return r;      
   }
}