Circular dependencies between immutable objects; the Freeze Pattern
This class should instead use an AtomicReference to ensure the state is kept valid. Alternatively, you should incorporate thread-safe handling of the String.

Consider:

private final AtomicReference&lt;String&gt; stateref = new AtomicReference&lt;String&gt;();

public void setState(final String state) {
    // only one initializer will succeed (assuming state is not null)...
    if (!stateref.compareAndSet(null, state)) {
        throw new IllegalStateException("state already set: '" + state + "'.");
    }
}

private void ensureInitialized() {
    if (stateref.get() == null) {
        throw new IllegalStateException(
            "state must be set before this instance is used."
        );
    }
}


This pattern ensures usage is consistent, there can be only one initialization of the instance, and that any thread-unsafe practices are handled well.