So far we've learned how to create observables and how to extract relevant data from observables. In this chapter we will go beyond what is necessary for simple examples and discuss more advanced functionality, as well as some good practices for using Rx in bigger applications.

});

indexed.subscribe(w -> System.out.println(index.getCount() + ": " + w));

indexed.subscribe(w -> System.out.println("1st observer: " + index.getCount() + ": " + w));

indexed.subscribe(w -> System.out.println("2nd observer: " + index.getCount() + ": " + w));

indexed.subscribe(w -> System.out.println("1st observer: " + w.index + ": " + w.item));

indexed.subscribe(w -> System.out.println("2nd observer: " + w.index + ": " + w.item));

.map(s -> s.toUpperCase())

.subscribe(new PrintSubscriber("Process"));

.filter(s -> s.length() > 5)

.subscribe(new PrintSubscriber("Process"));

subject.onNext(3);

subject.onCompleted();

Now we have properly protected our `Subject`. This protection is not only against malicious attacks but also against mistakes. We have mentioned before that subjects should be avoided when alternatives exist, and now we've seen examples of why. Subjects introduce state to our observables. Calls to `onNext`, `onCompleted` and `onError` alter the sequence that consumers will see. Observables that are constructed with any of the factory methods or operators exposed on `Observable` are immutable, provided that we don't introduce side-effects ourselves, as we saw in [Issues with side effects](/Part%203%20-%20Taming%20the%20sequence/1.%20Side%20effects.md#issues-with-side-effects).

As one might expect, an Rx pipeline forwards references to objects and doesn't create copies (unless we do so ourselves in the functions we supply). Modifications to the objects will be visible to every position in the pipeline that uses them. Consider the following mutable class:

data.subscribe(d -> d.name = "Garbage");

data.subscribe(d -> System.out.println(d.id + ": " + d.name));