Switch Maybe and Single to use their Transformers in compose() (ReactiveX#4651)

* Switch Maybe and Single to use their Transformers in compose()

Resolves ReactiveX#4650

* Update compose() tests

Author: ZacSweers

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

@@ -1967,13 +1967,12 @@ public final <U> Maybe<U> cast(final Class<? extends U> clazz) {
      * </dl>
      *
      * @param <R> the value type of the Maybe returned by the transformer function
-     * @param transformer
-     *            implements the function that transforms the source Maybe
+     * @param transformer the transformer function, not null
      * @return a Maybe, transformed by the transformer function
      * @see <a href="https://github.com/ReactiveX/RxJava/wiki/Implementing-Your-Own-Operators">RxJava wiki: Implementing Your Own Operators</a>
      */
     @SchedulerSupport(SchedulerSupport.NONE)
-    public final <R> Maybe<R> compose(Function<? super Maybe<T>, ? extends MaybeSource<R>> transformer) {
+    public final <R> Maybe<R> compose(MaybeTransformer<T, R> transformer) {
         return wrap(to(transformer));
     }
 

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

@@ -1467,13 +1467,12 @@ public final Single<T> hide() {
      * </dl>
      *
      * @param <R> the value type of the single returned by the transformer function
-     * @param transformer
-     *            implements the function that transforms the source Single
+     * @param transformer the transformer function, not null
      * @return the source Single, transformed by the transformer function
      * @see <a href="https://github.com/ReactiveX/RxJava/wiki/Implementing-Your-Own-Operators">RxJava wiki: Implementing Your Own Operators</a>
      */
     @SchedulerSupport(SchedulerSupport.NONE)
-    public final <R> Single<R> compose(Function<? super Single<T>, ? extends SingleSource<R>> transformer) {
+    public final <R> Single<R> compose(SingleTransformer<T, R> transformer) {
         return wrap(to(transformer));
     }
 

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

@@ -13,18 +13,23 @@
 
 package io.reactivex.internal.operators.single;
 
-import static org.junit.Assert.*;
-
-import java.util.concurrent.*;
-import java.util.concurrent.atomic.AtomicBoolean;
-
-import org.junit.Test;
-
-import io.reactivex.*;
+import io.reactivex.Single;
+import io.reactivex.SingleObserver;
+import io.reactivex.SingleSource;
+import io.reactivex.SingleTransformer;
 import io.reactivex.disposables.Disposables;
 import io.reactivex.exceptions.TestException;
 import io.reactivex.functions.*;
 import io.reactivex.schedulers.Schedulers;
+import org.junit.Test;
+
+import java.util.concurrent.Callable;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static org.junit.Assert.assertNotSame;
+import static org.junit.Assert.assertSame;
 
 public class SingleMiscTest {
     @Test
@@ -78,7 +83,7 @@ public void contains() {
     public void compose() {
 
         Single.just(1)
-        .compose(new Function<Single<Integer>, SingleSource<Object>>() {
+        .compose(new SingleTransformer<Integer, Object>() {
             @Override
             public SingleSource<Object> apply(Single<Integer> f) throws Exception {
                 return f.map(new Function<Integer, Object>() {

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

@@ -388,7 +388,7 @@ public void toNull() {
 
     @Test
     public void compose() {
-        Maybe.just(1).compose(new Function<Maybe<Integer>, MaybeSource<Integer>>() {
+        Maybe.just(1).compose(new MaybeTransformer<Integer, Integer>() {
             @Override
             public MaybeSource<Integer> apply(Maybe<Integer> m) throws Exception {
                 return m.map(new Function<Integer, Integer>() {