How to implement a FSM - Finite State Machine in Java

Question

I have something to do for work and I need your help. We want to implement a FSM - Finite State Machine, to identify char sequence(like: A, B, C, A, C), and tel

Answer 1

Here is a SUPER SIMPLE implementation/example of a FSM using just "if-else"s which avoids all of the above subclassing answers (taken from Using Finite State Machines for Pattern Matching in Java, where he is looking for a string which ends with "@" followed by numbers followed by "#"--see state graph here):

public static void main(String[] args) {
    String s = "A1@312#";
    String digits = "0123456789";
    int state = 0;
    for (int ind = 0; ind < s.length(); ind++) {
        if (state == 0) {
            if (s.charAt(ind) == '@')
                state = 1;
        } else {
            boolean isNumber = digits.indexOf(s.charAt(ind)) != -1;
            if (state == 1) {
                if (isNumber)
                    state = 2;
                else if (s.charAt(ind) == '@')
                    state = 1;
                else
                    state = 0;
            } else if (state == 2) {
                if (s.charAt(ind) == '#') {
                    state = 3;
                } else if (isNumber) {
                    state = 2;
                } else if (s.charAt(ind) == '@')
                    state = 1;
                else
                    state = 0;
            } else if (state == 3) {
                if (s.charAt(ind) == '@')
                    state = 1;
                else
                    state = 0;
            }
        }
    } //end for loop

    if (state == 3)
        System.out.println("It matches");
    else
        System.out.println("It does not match");
}

P.S: Does not answer your question directly, but shows you how to implement a FSM very easily in Java.