What does .defer() do in RxJava?

Question

I am new to rx java and I cannot understand what Completable.defer brings and why it's a good practice to use it.

so, what's the difference between:

public Completable someMethod1() {
    return Completable.defer(() -> someMethod2());
}

vs

public Completable someMethod1() {
    return someMethod2();
}

I can see that in the method's implementation there is some exception handling but this is surpassing me. Appreciate it.

Answer 1

Defer will makes sure each subscriber can get its own source sequence, independent of the other subscribers. Let me illustrate it with two examples:

AtomicInteger index = new AtomicInteger();

Flowable<String> source =
    Flowable.just("a", "b", "c", "d", "e", "f")
    .map(v -> index.incrementAndGet() + "-" + v)
    ;

source.subscribe(System.out:println);

source.subscribe(System.out:println);

prints

1-a
2-b
3-c
4-d
5-e
6-f
7-a
8-b
9-c
10-d
11-e
12-f

versus

Flowable<String> source =
    Flowable.defer(() -> {
        AtomicInteger index = new AtomicInteger();

        return Flowable.just("a", "b", "c", "d", "e", "f")
               .map(v -> index.incrementAndGet() + "-" + v)
               ;
    })
    ;

source.subscribe(System.out:println);

source.subscribe(System.out:println);

prints

1-a
2-b
3-c
4-d
5-e
6-f
1-a
2-b
3-c
4-d
5-e
6-f

In the second example, there is a per subscriber state that would have been otherwise shared across all subscribers. Now since each subscriber gets its own individual sequence created, both index items as one would generally expect.