Simple enum for java automaton

Question

I'm trying to implement this automaton example : http://www.javacodegeeks.com/2012/03/automaton-implementation-in-java.html.

However, an error keeps on being displayed while running the program :

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: 
           String index out of range: 3
at java.lang.String.charAt(String.java:686)
at mealy.Input.read(Input.java:7)
at mealy.States$4.next(Input.java:46)
at mealy.Test.main(Test.java:9)

I tried modifying the lines responsible for the error but nothing changed. Could someone please take a look at this program and help me find a solution?

I have the following .java :

State.java :

interface State {
    public State next(Input in);
}

NB : I had to change the original "public Stat next()" into "public State next(Input in);"

Input.java :

class Input {
    private String input;
    private int current;
    public Input(String input) {this.input = input;}
    char read() { return input.charAt(current++); }
}

enum States implements State {
    Init {
        @Override
        public State next(Input word) {
            switch(word.read()) {
                case 'a': return A;
                default: return Fail;
            }
        }
    },
    A {
        @Override
        public State next(Input word) {
            switch(word.read()) {
                case 'a': return A;
                case 'b': return B;
                case 'c': return C;
                case 'd': return null;
                default: return Fail;
            }
        }
    },
    B {
        @Override
        public State next(Input word) {
            switch(word.read()) {
                case 'b': return B;
                case 'c': return C;
                case 'd': return null;
                default: return Fail;
            }
        }
    },
    C {
        @Override
        public State next(Input word) {
            switch(word.read()) {
                case 'c': return C;
                case 'd': return null;
                default: return Fail;
            }
        }
    },
    Fail {
        @Override
        public State next(Input word) {
               return Fail;
        }
    };

    public abstract State next(Input word);
}

Test.java :

public class Test {
    public static void main(String args[]){
        State s;
        Input in = new Input("abc");

        for(s = States.Init; s != null || s != States.Fail; s = s.next(in)) {}
        if(s == States.Init) {System.out.println("Valid!");}
        else {System.out.println("Failed");}
    }
}

Answer 1

There appears to be a bug in the Input class. When you attempt to read the character after the last one, it throws an exception you are not handling in main. I would change Input so that it return a token you can handle in your state machine.

BTW I suggest you have a look at this for context. http://vanillajava.blogspot.co.uk/2011/06/java-secret-using-enum-as-state-machine.html

I assume Attila was looking to provide a simple, working example. I will see if he can fix his code.

Answer 2

the error is in this line

char read() { return input.charAt(current++); }

You don't check the length of the String input (which i consider a bad name inside a class Input) and after calling read() three times you try to access the 4th letter of a 3 letter String, which then throws the exception you see.

Update:

Addressing your comment I'd suggest changing the return value of read() to a new interface ReadResult:

public interface ReadResult {
    boolean isOkay();
    char getReadCharacter();  
}

with two implementations. one for positive results...

public class ReadOkay implements ReadResult{
    private char readCharacter;

    public ReadOkay(char readCharacter) {
        this.readCharacter = readCharacter;
    }

    @Override
    public boolean isOkay() {
        return true;
    }

    @Override
    public char getReadCharacter() {
        return readCharacter;
    }
}

and one for negative results

public class ReadFailed implements ReadResult {

    @Override
    public boolean isOkay() {
        return false;
    }

    @Override
    public char getReadCharacter() {
        throw new IllegalStateException("Read failed! no character data there to return!");
    }
}

Having this you can change read() to let it return then new interface

public ReadResult read() {
        if (input != null && current >= 0 && current < input.length()) {
            return new ReadOkay(input.charAt(current++));
        } else {
            return new ReadFailed();
        }
    }

and update your States accordingly.

replace:

switch(word.read()) {

with:

ReadResult result = word.read();
if (!result.isOkay()) {
    return Fail;
}
switch (result.getReadCharacter()) {

Answer 3

you need to change read() method like below

char read() { 
    if(current>=input.length())      // this if condition should be checked
          return 'z';                // you need to change your character according to your need
    return input.charAt(current++); 
}