src/main/java/io/reactivex/Single.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Single<T> defer(final Callable<? extends SingleSource<? extends T>> singleSupplier) {
        ObjectHelper.requireNonNull(singleSupplier, "singleSupplier is null");
        return RxJavaPlugins.onAssembly(new SingleDefer<T>(singleSupplier));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Single<T> defer(final Callable<? extends SingleSource<? extends T>> singleSupplier) {
        ObjectHelper.requireNonNull(singleSupplier, "singleSupplier is null");
        return RxJavaPlugins.onAssembly(new SingleDefer<T>(singleSupplier));
    }
Calls a {@link Callable} for each individual {@link SingleObserver} to return the actual {@link SingleSource} to
be subscribed to.
<p>
<img width="640" height="515" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.defer.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code defer} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param <T> the value type
@param singleSupplier the {@code Callable} that is called for each individual {@code SingleObserver} and
returns a SingleSource instance to subscribe to
@return the new Single instance