Hello World Threads!

hello_world_threads.java

package pl.angulski.example.threads;

enum State {
    HELLO("Hello "),
    WORLD("world"),
    EXCLAMATION("!\n");

    private final String text;

    State(String text) {
        this.text = text;
    }

    boolean isAllowed(State to) {
        return to.ordinal() - 1 == this.ordinal() || this == EXCLAMATION && to == HELLO;
    }

    String getText() {
        return text;
    }
}

class StateMachine {
    private static StateMachine INSTANCE = null;

    private State state = State.EXCLAMATION;

    private StateMachine() {
    }

    static synchronized StateMachine getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new StateMachine();
        }
        return INSTANCE;
    }

    private boolean isNotAllowedToPrint(State state) {
        return !this.state.isAllowed(state);
    }

    private void transition() {
        State[] values = State.values();
        state = values[state.ordinal() + 1 % values.length];
    }

    Thread thread(int times, State state) {
        return new Thread(() -> {
            for (int i = 0; i < times; i++) {
                synchronized (INSTANCE) {
                    while (isNotAllowedToPrint(state)) {
                        try {
                            INSTANCE.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                            throw new RuntimeException(e);
                        }
                    }
                    System.out.print(state.getText());
                    transition();
                    INSTANCE.notifyAll();
                }
            }
        });
    }
}

class HelloWorldCommand {
    static void run(int times) {
        StateMachine sm = StateMachine.getInstance();

        Thread helloThread = sm.thread(times, State.HELLO);
        helloThread.start();

        Thread worldThread = sm.thread(times, State.WORLD);
        worldThread.start();

        Thread exclamationThread = sm.thread(times, State.EXCLAMATION);
        exclamationThread.start();
    }
}