InvalidFieldsStream.java

Observable<Boolean> invalidFieldsStream = Observable.combineLatest(
        emailStream,
        passwordStream,
        passwordConfirmationStream,
        emptyFieldStream, new Func4<Boolean, Boolean, Boolean, Boolean, Boolean>() {
    @Override
    public Boolean call(Boolean emailInvalid, Boolean passwordInvalid, Boolean passwordConfirmationInvalid, Boolean emptyFieldExist) {
        return !emailInvalid && !passwordInvalid && !passwordConfirmationInvalid && !emptyFieldExist;
    }
});