ExpressionTokenizer.java.html

  1  /**
  2     This class breaks up a string describing an expression
  3     into tokens: numbers, parentheses, and operators.
  4  */
  5  public class ExpressionTokenizer
  6  {
  7     private String input;
  8     private int start; // The start of the current token
  9     private int end; // The position after the end of the current token
 10  
 11     /**
 12        Constructs a tokenizer.
 13        @param anInput the string to tokenize
 14     */
 15     public ExpressionTokenizer(String anInput)
 16     {
 17        input = anInput;
 18        start = 0;
 19        end = 0;
 20        nextToken(); // Find the first token
 21     }
 22  
 23     /**
 24        Peeks at the next token without consuming it.
 25        @return the next token or null if there are no more tokens
 26     */
 27     public String peekToken()
 28     {
 29        if (start >= input.length()) { return null; }
 30        else { return input.substring(start, end); }
 31     }
 32  
 33     /**
 34        Gets the next token and moves the tokenizer to the following token.
 35        @return the next token or null if there are no more tokens
 36     */
 37     public String nextToken()
 38     {
 39        String r = peekToken();
 40        start = end;
 41        if (start >= input.length()) { return r; }
 42        if (Character.isDigit(input.charAt(start)))
 43        {
 44           end = start + 1;
 45           while (end < input.length() 
 46                 && Character.isDigit(input.charAt(end)))
 47           {
 48              end++;
 49           }
 50        }
 51        else
 52        {
 53           end = start + 1;
 54        }
 55        return r;      
 56     }
 57  }