Recursion - Java Concepts - page 842

2 This class breaks up a string describing an expression
3 into tokens: numbers, parentheses, and operators.
4 */
5 public class ExpressionTokenizer
6 {
7 /**
8 Constructs a tokenizer.
9 @param anInput the string to tokenize
10 */
11 public ExpressionTokenizer(String anInput)
12 {
13 input = anInput;
14 start = 0 ;
15 end = 0 ;
16 nextToken();
17 }
18
19 /**
20 Peeks at the next token without consuming it.
21 @return the next token or null if there are no more
tokens
22 */
23 public String peekToken()
24 {
25 if (start >= input.length()) return
null ;
26 else return input.substring(start,
end);
27 }
28
29 /**
30 Gets the next token and moves the tokenizer to the following token.
31 @return the next token or null if there are no more
tokens
32 */
33 public String nextToken()
34 {
35 String r = peekToken();
36 start = end;
37 if (start >= input.length()) return r;