Operator - first

emit only the first item (or the first item that meets some condition) emitted by an Observable

If you are only interested in the first item emitted by an Observable, or the first item that meets some criteria, you can filter the Observable with the First operator.

In some implementations, First is not implemented as a filtering operator that returns an Observable, but as a blocking function that returns a particular item at such time as the source Observable emits that item. In those implementations, if you instead want a filtering operator, you may have better luck with Take(1) or ElementAt(0).

In some implementations there is also a Single operator. It behaves similarly to First except that it waits until the source Observable terminates in order to guarantee that it only emits a single item (otherwise, rather than emitting that item, it terminates with an error). You can use this to not only take the first item from the source Observable but to also guarantee that there was only one item.

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 Observable

.just(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

.first(0)

.subscribe(System.out::println);

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