2.x: Document and test amb subscription ordering.

src/main/java/io/reactivex/Completable.java

@@ -46,7 +46,8 @@ public abstract class Completable implements CompletableSource {
      *  <dt><b>Scheduler:</b></dt>
      *  <dd>{@code ambArray} does not operate by default on a particular {@link Scheduler}.</dd>
      * </dl>
-     * @param sources the array of source Completables
+     * @param sources the array of source Completables. A subscription to each source will
+     *            occur in the same order as in this array.
      * @return the new Completable instance
      * @throws NullPointerException if sources is null
      */
@@ -71,7 +72,8 @@ public static Completable ambArray(final CompletableSource... sources) {
      *  <dt><b>Scheduler:</b></dt>
      *  <dd>{@code amb} does not operate by default on a particular {@link Scheduler}.</dd>
      * </dl>
-     * @param sources the array of source Completables
+     * @param sources the array of source Completables. A subscription to each source will
+     *            occur in the same order as in this Iterable.
      * @return the new Completable instance
      * @throws NullPointerException if sources is null
      */
@@ -776,7 +778,8 @@ public static Completable wrap(CompletableSource source) {
      *  <dt><b>Scheduler:</b></dt>
      *  <dd>{@code ambWith} does not operate by default on a particular {@link Scheduler}.</dd>
      * </dl>
-     * @param other the other Completable, not null
+     * @param other the other Completable, not null. A subscription to this provided source will occur after subscribing
+     *            to the current source.
      * @return the new Completable instance
      * @throws NullPointerException if other is null
      */

src/main/java/io/reactivex/Flowable.java

@@ -78,7 +78,8 @@ public abstract class Flowable<T> implements Publisher<T> {
      *
      * @param <T> the common element type
      * @param sources
-     *            an Iterable of Publishers sources competing to react first
+     *            an Iterable of Publishers sources competing to react first. A subscription to each Publisher will
+     *            occur in the same order as in this Iterable.
      * @return a Flowable that emits the same sequence as whichever of the source Publishers first
      *         emitted an item or sent a termination notification
      * @see <a href="http://reactivex.io/documentation/operators/amb.html">ReactiveX operators documentation: Amb</a>
@@ -106,7 +107,8 @@ public static <T> Flowable<T> amb(Iterable<? extends Publisher<? extends T>> sou
      *
      * @param <T> the common element type
      * @param sources
-     *            an array of Publisher sources competing to react first
+     *            an array of Publisher sources competing to react first. A subscription to each Publisher will
+     *            occur in the same order as in this Iterable.
      * @return a Flowable that emits the same sequence as whichever of the source Publishers first
      *         emitted an item or sent a termination notification
      * @see <a href="http://reactivex.io/documentation/operators/amb.html">ReactiveX operators documentation: Amb</a>
@@ -5131,7 +5133,8 @@ public final Single<Boolean> all(Predicate<? super T> predicate) {
      * </dl>
      *
      * @param other
-     *            a Publisher competing to react first
+     *            a Publisher competing to react first. A subscription to this provided Publisher will occur after subscribing
+     *            to the current Publisher.
      * @return a Flowable that emits the same sequence as whichever of the source Publishers first
      *         emitted an item or sent a termination notification
      * @see <a href="http://reactivex.io/documentation/operators/amb.html">ReactiveX operators documentation: Amb</a>

src/main/java/io/reactivex/Maybe.java

@@ -54,7 +54,8 @@ public abstract class Maybe<T> implements MaybeSource<T> {
      * <dd>{@code amb} does not operate by default on a particular {@link Scheduler}.</dd>
      * </dl>
      * @param <T> the value type
-     * @param sources the Iterable sequence of sources
+     * @param sources the Iterable sequence of sources. A subscription to each source will
+     *            occur in the same order as in the Iterable.
      * @return the new Maybe instance
      */
     @CheckReturnValue
@@ -72,7 +73,8 @@ public static <T> Maybe<T> amb(final Iterable<? extends MaybeSource<? extends T>
      * <dd>{@code ambArray} does not operate by default on a particular {@link Scheduler}.</dd>
      * </dl>
      * @param <T> the value type
-     * @param sources the array of sources
+     * @param sources the array of sources. A subscription to each source will
+     *            occur in the same order as in the array.
      * @return the new Maybe instance
      */
     @CheckReturnValue
@@ -1966,7 +1968,8 @@ public static <T, R> Maybe<R> zipArray(Function<? super Object[], ? extends R> z
      * </dl>
      *
      * @param other
-     *            a MaybeSource competing to react first
+     *            a MaybeSource competing to react first. A subscription to this provided source will occur after
+     *            subscribing to the current source.
      * @return a Maybe that emits the same sequence as whichever of the source MaybeSources first
      *         signalled
      * @see <a href="http://reactivex.io/documentation/operators/amb.html">ReactiveX operators documentation: Amb</a>

src/main/java/io/reactivex/Observable.java

@@ -69,7 +69,8 @@ public abstract class Observable<T> implements ObservableSource<T> {
      *
      * @param <T> the common element type
      * @param sources
-     *            an Iterable of ObservableSources sources competing to react first
+     *            an Iterable of ObservableSource sources competing to react first. A subscription to each source will
+     *            occur in the same order as in the Iterable.
      * @return an Observable that emits the same sequence as whichever of the source ObservableSources first
      *         emitted an item or sent a termination notification
      * @see <a href="http://reactivex.io/documentation/operators/amb.html">ReactiveX operators documentation: Amb</a>
@@ -93,7 +94,8 @@ public static <T> Observable<T> amb(Iterable<? extends ObservableSource<? extend
      *
      * @param <T> the common element type
      * @param sources
-     *            an array of ObservableSource sources competing to react first
+     *            an array of ObservableSource sources competing to react first. A subscription to each source will
+     *            occur in the same order as in the array.
      * @return an Observable that emits the same sequence as whichever of the source ObservableSources first
      *         emitted an item or sent a termination notification
      * @see <a href="http://reactivex.io/documentation/operators/amb.html">ReactiveX operators documentation: Amb</a>
@@ -4547,7 +4549,8 @@ public final Single<Boolean> all(Predicate<? super T> predicate) {
      * </dl>
      *
      * @param other
-     *            an ObservableSource competing to react first
+     *            an ObservableSource competing to react first. A subscription to this provided source will occur after
+     *            subscribing to the current source.
      * @return an Observable that emits the same sequence as whichever of the source ObservableSources first
      *         emitted an item or sent a termination notification
      * @see <a href="http://reactivex.io/documentation/operators/amb.html">ReactiveX operators documentation: Amb</a>

src/main/java/io/reactivex/Single.java

@@ -67,7 +67,8 @@ public abstract class Single<T> implements SingleSource<T> {
      * <dd>{@code amb} does not operate by default on a particular {@link Scheduler}.</dd>
      * </dl>
      * @param <T> the value type
-     * @param sources the Iterable sequence of sources
+     * @param sources the Iterable sequence of sources. A subscription to each source will
+     *            occur in the same order as in this Iterable.
      * @return the new Single instance
      * @since 2.0
      */
@@ -86,7 +87,8 @@ public static <T> Single<T> amb(final Iterable<? extends SingleSource<? extends
      * <dd>{@code ambArray} does not operate by default on a particular {@link Scheduler}.</dd>
      * </dl>
      * @param <T> the value type
-     * @param sources the array of sources
+     * @param sources the array of sources. A subscription to each source will
+     *            occur in the same order as in this array.
      * @return the new Single instance
      * @since 2.0
      */
@@ -1493,7 +1495,8 @@ public static <T, R> Single<R> zipArray(Function<? super Object[], ? extends R>
      * <dd>{@code ambWith} does not operate by default on a particular {@link Scheduler}.</dd>
      * </dl>
      * @param other the other SingleSource to race for the first emission of success or error
-     * @return the new Single instance
+     * @return the new Single instance. A subscription to this provided source will occur after subscribing
+     *            to the current source.
      * @since 2.0
      */
     @CheckReturnValue

src/test/java/io/reactivex/internal/operators/completable/CompletableAmbTest.java

@@ -145,4 +145,22 @@ public void run() {
         }
     }
 
+    @Test
+    public void ambWithOrder() {
+        Completable error = Completable.error(new RuntimeException());
+        Completable.complete().ambWith(error).test().assertComplete();
+    }
+
+    @Test
+    public void ambIterableOrder() {
+        Completable error = Completable.error(new RuntimeException());
+        Completable.amb(Arrays.asList(Completable.complete(), error)).test().assertComplete();
+    }
+
+    @Test
+    public void ambArrayOrder() {
+        Completable error = Completable.error(new RuntimeException());
+        Completable.ambArray(Completable.complete(), error).test().assertComplete();
+    }
+
 }

src/test/java/io/reactivex/internal/operators/flowable/FlowableAmbTest.java

@@ -697,4 +697,24 @@ public Flowable<Integer> apply(Integer v) throws Exception {
         .test()
         .assertFailureAndMessage(TestException.class, "next()");
     }
+
+    @Test
+    public void ambWithOrder() {
+        Flowable<Integer> error = Flowable.error(new RuntimeException());
+        Flowable.just(1).ambWith(error).test().assertValue(1).assertComplete();
+    }
+
+    @SuppressWarnings("unchecked")
+    @Test
+    public void ambIterableOrder() {
+        Flowable<Integer> error = Flowable.error(new RuntimeException());
+        Flowable.amb(Arrays.asList(Flowable.just(1), error)).test().assertValue(1).assertComplete();
+    }
+
+    @SuppressWarnings("unchecked")
+    @Test
+    public void ambArrayOrder() {
+        Flowable<Integer> error = Flowable.error(new RuntimeException());
+        Flowable.ambArray(Flowable.just(1), error).test().assertValue(1).assertComplete();
+    }
 }

src/test/java/io/reactivex/internal/operators/observable/ObservableAmbTest.java

@@ -363,4 +363,24 @@ public void run() {
             }
         }
     }
+
+    @Test
+    public void ambWithOrder() {
+        Observable<Integer> error = Observable.error(new RuntimeException());
+        Observable.just(1).ambWith(error).test().assertValue(1).assertComplete();
+    }
+
+    @SuppressWarnings("unchecked")
+    @Test
+    public void ambIterableOrder() {
+        Observable<Integer> error = Observable.error(new RuntimeException());
+        Observable.amb(Arrays.asList(Observable.just(1), error)).test().assertValue(1).assertComplete();
+    }
+
+    @SuppressWarnings("unchecked")
+    @Test
+    public void ambArrayOrder() {
+        Observable<Integer> error = Observable.error(new RuntimeException());
+        Observable.ambArray(Observable.just(1), error).test().assertValue(1).assertComplete();
+    }
 }

src/test/java/io/reactivex/internal/operators/single/SingleAmbTest.java

@@ -261,4 +261,24 @@ public void manySources() {
         .test()
         .assertResult(31);
     }
+
+    @Test
+    public void ambWithOrder() {
+        Single<Integer> error = Single.error(new RuntimeException());
+        Single.just(1).ambWith(error).test().assertValue(1);
+    }
+
+    @SuppressWarnings("unchecked")
+    @Test
+    public void ambIterableOrder() {
+        Single<Integer> error = Single.error(new RuntimeException());
+        Single.amb(Arrays.asList(Single.just(1), error)).test().assertValue(1);
+    }
+
+    @SuppressWarnings("unchecked")
+    @Test
+    public void ambArrayOrder() {
+        Single<Integer> error = Single.error(new RuntimeException());
+        Single.ambArray(Single.just(1), error).test().assertValue(1);
+    }
 }

src/test/java/io/reactivex/maybe/MaybeTest.java

@@ -1580,6 +1580,26 @@ public void ambArrayOne() {
         assertSame(Maybe.never(), Maybe.ambArray(Maybe.never()));
     }
 
+    @Test
+    public void ambWithOrder() {
+        Maybe<Integer> error = Maybe.error(new RuntimeException());
+        Maybe.just(1).ambWith(error).test().assertValue(1);
+    }
+
+    @SuppressWarnings("unchecked")
+    @Test
+    public void ambIterableOrder() {
+        Maybe<Integer> error = Maybe.error(new RuntimeException());
+        Maybe.amb(Arrays.asList(Maybe.just(1), error)).test().assertValue(1);
+    }
+
+    @SuppressWarnings("unchecked")
+    @Test
+    public void ambArrayOrder() {
+        Maybe<Integer> error = Maybe.error(new RuntimeException());
+        Maybe.ambArray(Maybe.just(1), error).test().assertValue(1);
+    }
+
     @SuppressWarnings("unchecked")
     @Test
     public void ambArray1SignalsSuccess() {