src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Observable<T> just(T item) {
        ObjectHelper.requireNonNull(item, "item is null");
        return RxJavaPlugins.onAssembly(new ObservableJust<T>(item));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Observable<T> just(T item) {
        ObjectHelper.requireNonNull(item, "item is null");
        return RxJavaPlugins.onAssembly(new ObservableJust<T>(item));
    }
Returns an Observable that signals the given (constant reference) item and then completes.
<p>
<img width="640" height="290" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/just.item.png" alt="">
<p>
Note that the item is taken and re-emitted as is and not computed by any means by {@code just}. Use {@link #fromCallable(Callable)}
to generate a single item on demand (when {@code Observer}s subscribe to it).
<p>
See the multi-parameter overloads of {@code just} to emit more than one (constant reference) items one after the other.
Use {@link #fromArray(Object...)} to emit an arbitrary number of items that are known upfront.
<p>
To emit the items of an {@link Iterable} sequence (such as a {@link java.util.List}), use {@link #fromIterable(Iterable)}.
<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 an Observable that emits {@code value} as a single item and then completes
@see <a href="http://reactivex.io/documentation/operators/just.html">ReactiveX operators documentation: Just</a>
@see #just(Object, Object)
@see #fromCallable(Callable)
@see #fromArray(Object...)
@see #fromIterable(Iterable)