Add FunctionalInterface annotations. (#6773)

Author: He-Pin

src/main/java/io/reactivex/rxjava3/core/CompletableConverter.java

@@ -22,6 +22,7 @@
  * @param <R> the output type
  * @since 2.2
  */
+@FunctionalInterface
 public interface CompletableConverter<R> {
     /**
      * Applies a function to the upstream Completable and returns a converted value of type {@code R}.

src/main/java/io/reactivex/rxjava3/core/CompletableOnSubscribe.java

@@ -19,6 +19,7 @@
  * an instance of a {@link CompletableEmitter} instance that allows pushing
  * an event in a cancellation-safe manner.
  */
+@FunctionalInterface
 public interface CompletableOnSubscribe {
 
     /**

src/main/java/io/reactivex/rxjava3/core/CompletableOperator.java

@@ -18,6 +18,7 @@
 /**
  * Interface to map/wrap a downstream observer to an upstream observer.
  */
+@FunctionalInterface
 public interface CompletableOperator {
     /**
      * Applies a function to the child CompletableObserver and returns a new parent CompletableObserver.

src/main/java/io/reactivex/rxjava3/core/CompletableSource.java

@@ -20,6 +20,7 @@
  *
  * @since 2.0
  */
+@FunctionalInterface
 public interface CompletableSource {
 
     /**

src/main/java/io/reactivex/rxjava3/core/CompletableTransformer.java

@@ -19,6 +19,7 @@
  * Convenience interface and callback used by the compose operator to turn a Completable into another
  * Completable fluently.
  */
+@FunctionalInterface
 public interface CompletableTransformer {
     /**
      * Applies a function to the upstream Completable and returns a CompletableSource.

src/main/java/io/reactivex/rxjava3/core/FlowableConverter.java

@@ -23,6 +23,7 @@
  * @param <R> the output type
  * @since 2.2
  */
+@FunctionalInterface
 public interface FlowableConverter<T, R> {
     /**
      * Applies a function to the upstream Flowable and returns a converted value of type {@code R}.

src/main/java/io/reactivex/rxjava3/core/FlowableOnSubscribe.java

@@ -21,6 +21,7 @@
  *
  * @param <T> the value type pushed
  */
+@FunctionalInterface
 public interface FlowableOnSubscribe<T> {
 
     /**

src/main/java/io/reactivex/rxjava3/core/FlowableOperator.java

@@ -23,6 +23,7 @@
  * @param <Downstream> the value type of the downstream
  * @param <Upstream> the value type of the upstream
  */
+@FunctionalInterface
 public interface FlowableOperator<Downstream, Upstream> {
     /**
      * Applies a function to the child Subscriber and returns a new parent Subscriber.

src/main/java/io/reactivex/rxjava3/core/FlowableTransformer.java

@@ -23,6 +23,7 @@
  * @param <Upstream> the upstream value type
  * @param <Downstream> the downstream value type
  */
+@FunctionalInterface
 public interface FlowableTransformer<Upstream, Downstream> {
     /**
      * Applies a function to the upstream Flowable and returns a Publisher with

src/main/java/io/reactivex/rxjava3/core/MaybeConverter.java

@@ -23,6 +23,7 @@
  * @param <R> the output type
  * @since 2.2
  */
+@FunctionalInterface
 public interface MaybeConverter<T, R> {
     /**
      * Applies a function to the upstream Maybe and returns a converted value of type {@code R}.