class Singleton {
    private class A {
        final int value;
    }

    private static A state = null;
    private static boolean exists = true;
    private Mutex mutex;

    Signleton() {}

    public A getState() {
        if (exists) {
            return state;
        }

        mutex.lock();
        try {
            if (state == null) {
                state = new A();
                if (state != null) {
                    exists = true;
                }
            }
        } finally {
            mutex.unlock();
        }

        return state;
    }
}