2.x: add Maybe.flatMapSingleElement returning Maybe

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

@@ -2701,6 +2701,32 @@ public final <R> Single<R> flatMapSingle(final Function<? super T, ? extends Sin
         return RxJavaPlugins.onAssembly(new MaybeFlatMapSingle<T, R>(this, mapper));
     }
 
+    /**
+     * Returns a {@link Maybe} based on applying a specified function to the item emitted by the
+     * source {@link Maybe}, where that function returns a {@link Single}.
+     * When this Maybe just completes the resulting {@code Maybe} completes as well.
+     * <p>
+     * <img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Maybe.flatMapSingle.png" alt="">
+     * <dl>
+     * <dt><b>Scheduler:</b></dt>
+     * <dd>{@code flatMapSingleElement} does not operate by default on a particular {@link Scheduler}.</dd>
+     * </dl>
+     *
+     * @param <R> the result value type
+     * @param mapper
+     *            a function that, when applied to the item emitted by the source Maybe, returns a
+     *            Single
+     * @return the new Maybe instance
+     * @see <a href="http://reactivex.io/documentation/operators/flatmap.html">ReactiveX operators documentation: FlatMap</a>
+     * @since 2.0.2 - experimental
+     */
+    @SchedulerSupport(SchedulerSupport.NONE)
+    @Experimental
+    public final <R> Maybe<R> flatMapSingleElement(final Function<? super T, ? extends SingleSource<? extends R>> mapper) {
+        ObjectHelper.requireNonNull(mapper, "mapper is null");
+        return RxJavaPlugins.onAssembly(new MaybeFlatMapSingleElement<T, R>(this, mapper));
+    }
+
     /**
      * Returns a {@link Completable} that completes based on applying a specified function to the item emitted by the
      * source {@link Maybe}, where that function returns a {@link Completable}.

src/main/java/io/reactivex/internal/operators/maybe/MaybeFlatMapSingleElement.java

@@ -0,0 +1,134 @@
+/**
+ * Copyright 2016 Netflix, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
+ * compliance with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is
+ * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
+ * the License for the specific language governing permissions and limitations under the License.
+ */
+
+package io.reactivex.internal.operators.maybe;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+import io.reactivex.*;
+import io.reactivex.annotations.Experimental;
+import io.reactivex.disposables.Disposable;
+import io.reactivex.exceptions.Exceptions;
+import io.reactivex.functions.Function;
+import io.reactivex.internal.disposables.DisposableHelper;
+import io.reactivex.internal.functions.ObjectHelper;
+
+/**
+ * Maps the success value of the source MaybeSource into a Single.
+ * @param <T> the input value type
+ * @param <R> the result value type
+ * 
+ * @since 2.0.2 - experimental
+ */
+@Experimental
+public final class MaybeFlatMapSingleElement<T, R> extends Maybe<R> {
+
+    final MaybeSource<T> source;
+
+    final Function<? super T, ? extends SingleSource<? extends R>> mapper;
+
+    public MaybeFlatMapSingleElement(MaybeSource<T> source, Function<? super T, ? extends SingleSource<? extends R>> mapper) {
+        this.source = source;
+        this.mapper = mapper;
+    }
+
+    @Override
+    protected void subscribeActual(MaybeObserver<? super R> actual) {
+        source.subscribe(new FlatMapMaybeObserver<T, R>(actual, mapper));
+    }
+
+    static final class FlatMapMaybeObserver<T, R>
+    extends AtomicReference<Disposable>
+    implements MaybeObserver<T>, Disposable {
+
+        private static final long serialVersionUID = 4827726964688405508L;
+
+        final MaybeObserver<? super R> actual;
+
+        final Function<? super T, ? extends SingleSource<? extends R>> mapper;
+
+        FlatMapMaybeObserver(MaybeObserver<? super R> actual, Function<? super T, ? extends SingleSource<? extends R>> mapper) {
+            this.actual = actual;
+            this.mapper = mapper;
+        }
+
+        @Override
+        public void dispose() {
+            DisposableHelper.dispose(this);
+        }
+
+        @Override
+        public boolean isDisposed() {
+            return DisposableHelper.isDisposed(get());
+        }
+
+        @Override
+        public void onSubscribe(Disposable d) {
+            if (DisposableHelper.setOnce(this, d)) {
+                actual.onSubscribe(this);
+            }
+        }
+
+        @Override
+        public void onSuccess(T value) {
+            SingleSource<? extends R> ss;
+
+            try {
+                ss = ObjectHelper.requireNonNull(mapper.apply(value), "The mapper returned a null SingleSource");
+            } catch (Throwable ex) {
+                Exceptions.throwIfFatal(ex);
+                onError(ex);
+                return;
+            }
+
+            ss.subscribe(new FlatMapSingleObserver<R>(this, actual));
+        }
+
+        @Override
+        public void onError(Throwable e) {
+            actual.onError(e);
+        }
+
+        @Override
+        public void onComplete() {
+            actual.onComplete();
+        }
+    }
+
+    static final class FlatMapSingleObserver<R> implements SingleObserver<R> {
+
+        final AtomicReference<Disposable> parent;
+
+        final MaybeObserver<? super R> actual;
+
+        FlatMapSingleObserver(AtomicReference<Disposable> parent, MaybeObserver<? super R> actual) {
+            this.parent = parent;
+            this.actual = actual;
+        }
+
+        @Override
+        public void onSubscribe(final Disposable d) {
+            DisposableHelper.replace(parent, d);
+        }
+
+        @Override
+        public void onSuccess(final R value) {
+            actual.onSuccess(value);
+        }
+
+        @Override
+        public void onError(final Throwable e) {
+            actual.onError(e);
+        }
+    }
+}

src/test/java/io/reactivex/internal/operators/maybe/MaybeFlatMapSingleElementTest.java

@@ -0,0 +1,147 @@
+/**
+ * Copyright 2016 Netflix, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
+ * compliance with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is
+ * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
+ * the License for the specific language governing permissions and limitations under the License.
+ */
+
+package io.reactivex.internal.operators.maybe;
+
+import org.junit.Test;
+
+import io.reactivex.*;
+import io.reactivex.exceptions.TestException;
+import io.reactivex.functions.Function;
+
+public class MaybeFlatMapSingleElementTest {
+    @Test(expected = NullPointerException.class)
+    public void flatMapSingleElementNull() {
+        Maybe.just(1)
+            .flatMapSingleElement(null);
+    }
+
+    @Test
+    public void flatMapSingleElementValue() {
+        Maybe.just(1).flatMapSingleElement(new Function<Integer, SingleSource<Integer>>() {
+            @Override public SingleSource<Integer> apply(final Integer integer) throws Exception {
+                if (integer == 1) {
+                    return Single.just(2);
+                }
+
+                return Single.just(1);
+            }
+        })
+            .test()
+            .assertResult(2);
+    }
+
+    @Test
+    public void flatMapSingleElementValueDifferentType() {
+        Maybe.just(1).flatMapSingleElement(new Function<Integer, SingleSource<String>>() {
+            @Override public SingleSource<String> apply(final Integer integer) throws Exception {
+                if (integer == 1) {
+                    return Single.just("2");
+                }
+
+                return Single.just("1");
+            }
+        })
+            .test()
+            .assertResult("2");
+    }
+
+    @Test
+    public void flatMapSingleElementValueNull() {
+        Maybe.just(1).flatMapSingleElement(new Function<Integer, SingleSource<Integer>>() {
+            @Override public SingleSource<Integer> apply(final Integer integer) throws Exception {
+                return null;
+            }
+        })
+            .test()
+            .assertNoValues()
+            .assertError(NullPointerException.class)
+            .assertErrorMessage("The mapper returned a null SingleSource");
+    }
+
+    @Test
+    public void flatMapSingleElementValueErrorThrown() {
+        Maybe.just(1).flatMapSingleElement(new Function<Integer, SingleSource<Integer>>() {
+            @Override public SingleSource<Integer> apply(final Integer integer) throws Exception {
+                throw new RuntimeException("something went terribly wrong!");
+            }
+        })
+            .test()
+            .assertNoValues()
+            .assertError(RuntimeException.class)
+            .assertErrorMessage("something went terribly wrong!");
+    }
+
+    @Test
+    public void flatMapSingleElementError() {
+        RuntimeException exception = new RuntimeException("test");
+
+        Maybe.error(exception).flatMapSingleElement(new Function<Object, SingleSource<Object>>() {
+            @Override public SingleSource<Object> apply(final Object integer) throws Exception {
+                return Single.just(new Object());
+            }
+        })
+            .test()
+            .assertError(exception);
+    }
+
+    @Test
+    public void flatMapSingleElementEmpty() {
+        Maybe.<Integer>empty().flatMapSingleElement(new Function<Integer, SingleSource<Integer>>() {
+            @Override public SingleSource<Integer> apply(final Integer integer) throws Exception {
+                return Single.just(2);
+            }
+        })
+            .test()
+            .assertNoValues()
+            .assertResult();
+    }
+
+    @Test
+    public void dispose() {
+        TestHelper.checkDisposed(Maybe.just(1).flatMapSingleElement(new Function<Integer, SingleSource<Integer>>() {
+            @Override
+            public SingleSource<Integer> apply(final Integer integer) throws Exception {
+                return Single.just(2);
+            }
+        }));
+    }
+
+    @Test
+    public void doubleOnSubscribe() {
+        TestHelper.checkDoubleOnSubscribeMaybe(new Function<Maybe<Integer>, Maybe<Integer>>() {
+            @Override
+            public Maybe<Integer> apply(Maybe<Integer> m) throws Exception {
+                return m.flatMapSingleElement(new Function<Integer, SingleSource<Integer>>() {
+                    @Override
+                    public SingleSource<Integer> apply(final Integer integer) throws Exception {
+                        return Single.just(2);
+                    }
+                });
+            }
+        });
+    }
+
+    @Test
+    public void singleErrors() {
+        Maybe.just(1)
+        .flatMapSingleElement(new Function<Integer, SingleSource<Integer>>() {
+                    @Override
+                    public SingleSource<Integer> apply(final Integer integer) throws Exception {
+                        return Single.error(new TestException());
+                    }
+                })
+        .test()
+        .assertFailure(TestException.class);
+    }
+}