Karoch

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Properties;

public class Karoch {
    // Data Source interface and plain simple factory, console input by default
    static interface DataSource {
        static final String CONFIG_KEY = "ds";
        static final String TYPE_CONSOLE = "console";
        static final String TYPE_STATIC = "static";

        String readLine() throws IOException;
    }

    static DataSource getDataSource(Properties properties) throws Exception {
        switch (properties.getProperty(DataSource.CONFIG_KEY, DataSource.TYPE_CONSOLE)) {
            case DataSource.TYPE_CONSOLE:
                return () -> new BufferedReader(new InputStreamReader(System.in)).readLine();

            case DataSource.TYPE_STATIC:
                return () -> {
                    System.out.println("0");
                    return "0";
                };

            default:
                throw new Exception("Data Source type is unknown");
        }
    }

    // Data Validator with plain simple factory, only positive number are valid by default
    static interface Validator {
        static final String CONFIG_KEY = "validator";
        static final String TYPE_POSITIVE_NUMBER = "posNumber";
        boolean isValid(String s);
    }

    static Validator getValidator(Properties properties) {
        switch (properties.getProperty(Validator.CONFIG_KEY, Validator.TYPE_POSITIVE_NUMBER)) {
            case Validator.TYPE_POSITIVE_NUMBER:
                return (String s) -> {
                    try {
                        return Double.parseDouble(s) >= 0;
                    } catch (NumberFormatException e) {
                        return false;
                    }
                };

            default:
                return (String s) -> true;
        }
    }

    static class Context {
        public Properties properties;
        public String data;
    }

    // States
    static interface State {
        State proceed(Context context) throws Exception;
    }

    static class GreetingsState implements State {
        @Override
        public State proceed(Context context) {
            System.out.println("Enter value for A:");
            return new InputState();
        }
    }

    static class InputState implements State {
        @Override
        public State proceed(Context context) throws Exception {
            DataSource dataSource = getDataSource(context.properties);
            context.data = dataSource.readLine();
            return new ValidateState();
        }
    }

    static class ValidateState implements State {
        @Override
        public State proceed(Context context) {
            return getValidator(context.properties).isValid(context.data) ? new FinishState() : new InvalidState();
        }
    }

    static class InvalidState implements State {
        @Override
        public State proceed(Context context) {
            System.out.println("Try again:");
            return new InputState();
        }
    }

    static class FinishState implements State {
        @Override
        public State proceed(Context context) {
            System.out.println("A is " + context.data);
            return null;
        }
    }

    static class ErrorState implements State {
        private final Exception exception;

        public ErrorState(Exception exception) {
            this.exception = exception;
        }

        @Override
        public State proceed(Context context) {
            System.out.println("Something went wrong:\n" + exception.getMessage());
            return null;
        }
    }


    public static void main(String[] args) {
        Context context = new Context();
        context.properties = new Properties(); // todo: create INI file with configuration, load it here

        // Run states machine
        State state = new GreetingsState();
        while (state != null) {
            try {
                state = state.proceed(context);
            } catch (Exception e) {
                state = new ErrorState(e);
            }
        }
    }
}