RxJava Single.just() vs Single.fromCallable()?

Question

I wondered if someone can shed some light on this question, when to use

Single.fromCallable( ()-> myObject )

instead of

Single.just(myObject)

---

from the documentation, Single.fromCallable():

 /**
 * Returns a {@link Single} that invokes passed function and emits its result for each new SingleObserver that subscribes.
 * <p>
 * Allows you to defer execution of passed function until SingleObserver subscribes to the {@link Single}.
 * It makes passed function "lazy".
 * Result of the function invocation will be emitted by the {@link Single}.
 * <dl>
 *   <dt><b>Scheduler:</b></dt>
 *   <dd>{@code fromCallable} does not operate by default on a particular {@link Scheduler}.</dd>
 * </dl>
 *
 * @param callable
 *         function which execution should be deferred, it will be invoked when SingleObserver will subscribe to the {@link Single}.
 * @param <T>
 *         the type of the item emitted by the {@link Single}.
 * @return a {@link Single} whose {@link SingleObserver}s' subscriptions trigger an invocation of the given function.
 */

and the documentation for Single.just():

 /**
 * Returns a {@code Single} that emits a specified item.
 * <p>
 * <img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.just.png" alt="">
 * <p>
 * To convert any object into a {@code Single} that emits that object, pass that object into the
 * {@code just} method.
 * <dl>
 * <dt><b>Scheduler:</b></dt>
 * <dd>{@code just} does not operate by default on a particular {@link Scheduler}.</dd>
 * </dl>
 *
 * @param item
 *            the item to emit
 * @param <T>
 *            the type of that item
 * @return a {@code Single} that emits {@code item}
 * @see <a href="http://reactivex.io/documentation/operators/just.html">ReactiveX operators documentation: Just</a>
 */

Answer 1

In the use case you've mentioned, there is actually no major difference.

Now imagine we need the object to be created dynamically through a function call?

fun getTimeObject() {
    val timeInMillis = System.currentTimeMillis()
    return TimeObject(timeInMillis)
}

Then with, Single.just(getTimeObject()) the resulting Single will emit the same Long when it has a new subscriber.

However, with Single.fromcallable(()-> getTimeObject()), the resulting Single will emit a different Long indicating the current time in millis when it has a new subscriber.

That's because fromCallable executes it's lambda everytime it has a new subscriber Lazily.