Extends function

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

@@ -15,20 +15,24 @@
 
 import io.reactivex.rxjava3.annotations.NonNull;
 
+import java.util.function.Function;
+
 /**
  * Convenience interface and callback used by the {@link Completable#to} operator to turn a Completable into another
  * value fluently.
  * <p>History: 2.1.7 - experimental
  * @param <R> the output type
  * @since 2.2
  */
-public interface CompletableConverter<R> {
+@FunctionalInterface
+public interface CompletableConverter<R> extends Function<Completable, R> {
     /**
      * Applies a function to the upstream Completable and returns a converted value of type {@code R}.
      *
      * @param upstream the upstream Completable instance
      * @return the converted value
      */
     @NonNull
+    @Override
     R apply(@NonNull Completable upstream);
 }

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

@@ -15,16 +15,20 @@
 
 import io.reactivex.rxjava3.annotations.NonNull;
 
+import java.util.function.Function;
+
 /**
  * Convenience interface and callback used by the compose operator to turn a Completable into another
  * Completable fluently.
  */
-public interface CompletableTransformer {
+@FunctionalInterface
+public interface CompletableTransformer extends Function<Completable, CompletableSource> {
     /**
      * Applies a function to the upstream Completable and returns a CompletableSource.
      * @param upstream the upstream Completable instance
      * @return the transformed CompletableSource instance
      */
     @NonNull
+    @Override
     CompletableSource apply(@NonNull Completable upstream);
 }

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

@@ -15,6 +15,8 @@
 
 import io.reactivex.rxjava3.annotations.NonNull;
 
+import java.util.function.Function;
+
 /**
  * Convenience interface and callback used by the {@link Flowable#to} operator to turn a Flowable into another
  * value fluently.
@@ -23,13 +25,15 @@
  * @param <R> the output type
  * @since 2.2
  */
-public interface FlowableConverter<T, R> {
+@FunctionalInterface
+public interface FlowableConverter<T, R> extends Function<Flowable<T>, R> {
     /**
      * Applies a function to the upstream Flowable and returns a converted value of type {@code R}.
      *
      * @param upstream the upstream Flowable instance
      * @return the converted value
      */
     @NonNull
+    @Override
     R apply(@NonNull Flowable<T> upstream);
 }

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

@@ -17,19 +17,23 @@
 
 import io.reactivex.rxjava3.annotations.NonNull;
 
+import java.util.function.Function;
+
 /**
  * Interface to compose Flowables.
  *
  * @param <Upstream> the upstream value type
  * @param <Downstream> the downstream value type
  */
-public interface FlowableTransformer<Upstream, Downstream> {
+@FunctionalInterface
+public interface FlowableTransformer<Upstream, Downstream> extends Function<Flowable<Upstream>, Publisher<Downstream>> {
     /**
      * Applies a function to the upstream Flowable and returns a Publisher with
      * optionally different element type.
      * @param upstream the upstream Flowable instance
      * @return the transformed Publisher instance
      */
     @NonNull
+    @Override
     Publisher<Downstream> apply(@NonNull Flowable<Upstream> upstream);
 }

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

@@ -15,6 +15,8 @@
 
 import io.reactivex.rxjava3.annotations.NonNull;
 
+import java.util.function.Function;
+
 /**
  * Convenience interface and callback used by the {@link Maybe#to} operator to turn a Maybe into another
  * value fluently.
@@ -23,13 +25,15 @@
  * @param <R> the output type
  * @since 2.2
  */
-public interface MaybeConverter<T, R> {
+@FunctionalInterface
+public interface MaybeConverter<T, R> extends Function<Maybe<T>, R> {
     /**
      * Applies a function to the upstream Maybe and returns a converted value of type {@code R}.
      *
      * @param upstream the upstream Maybe instance
      * @return the converted value
      */
     @NonNull
+    @Override
     R apply(@NonNull Maybe<T> upstream);
 }

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

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

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

@@ -20,6 +20,7 @@
  * @param <Downstream> the value type of the downstream
  * @param <Upstream> the value type of the upstream
  */
+@FunctionalInterface
 public interface MaybeOperator<Downstream, Upstream> {
     /**
      * Applies a function to the child MaybeObserver and returns a new parent MaybeObserver.

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

@@ -24,6 +24,7 @@
  * @param <T> the element type
  * @since 2.0
  */
+@FunctionalInterface
 public interface MaybeSource<T> {
 
     /**

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

@@ -15,19 +15,23 @@
 
 import io.reactivex.rxjava3.annotations.NonNull;
 
+import java.util.function.Function;
+
 /**
  * Interface to compose Maybes.
  *
  * @param <Upstream> the upstream value type
  * @param <Downstream> the downstream value type
  */
-public interface MaybeTransformer<Upstream, Downstream> {
+@FunctionalInterface
+public interface MaybeTransformer<Upstream, Downstream> extends Function<Maybe<Upstream>, MaybeSource<Downstream>> {
     /**
      * Applies a function to the upstream Maybe and returns a MaybeSource with
      * optionally different element type.
      * @param upstream the upstream Maybe instance
      * @return the transformed MaybeSource instance
      */
     @NonNull
+    @Override
     MaybeSource<Downstream> apply(@NonNull Maybe<Upstream> upstream);
 }

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

@@ -15,6 +15,8 @@
 
 import io.reactivex.rxjava3.annotations.NonNull;
 
+import java.util.function.Function;
+
 /**
  * Convenience interface and callback used by the {@link Observable#to} operator to turn an Observable into another
  * value fluently.
@@ -23,13 +25,15 @@
  * @param <R> the output type
  * @since 2.2
  */
-public interface ObservableConverter<T, R> {
+@FunctionalInterface
+public interface ObservableConverter<T, R> extends Function<Observable<T>, R> {
     /**
      * Applies a function to the upstream Observable and returns a converted value of type {@code R}.
      *
      * @param upstream the upstream Observable instance
      * @return the converted value
      */
     @NonNull
+    @Override
     R apply(@NonNull Observable<T> upstream);
 }

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

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

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

@@ -21,6 +21,7 @@
  * @param <Downstream> the value type of the downstream
  * @param <Upstream> the value type of the upstream
  */
+@FunctionalInterface
 public interface ObservableOperator<Downstream, Upstream> {
     /**
      * Applies a function to the child Observer and returns a new parent Observer.

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

@@ -21,6 +21,7 @@
  * @param <T> the element type
  * @since 2.0
  */
+@FunctionalInterface
 public interface ObservableSource<T> {
 
     /**

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

@@ -15,19 +15,23 @@
 
 import io.reactivex.rxjava3.annotations.NonNull;
 
+import java.util.function.Function;
+
 /**
  * Interface to compose Observables.
  *
  * @param <Upstream> the upstream value type
  * @param <Downstream> the downstream value type
  */
-public interface ObservableTransformer<Upstream, Downstream> {
+@FunctionalInterface
+public interface ObservableTransformer<Upstream, Downstream> extends Function<Observable<Upstream>, ObservableSource<Downstream>> {
     /**
      * Applies a function to the upstream Observable and returns an ObservableSource with
      * optionally different element type.
      * @param upstream the upstream Observable instance
      * @return the transformed ObservableSource instance
      */
     @NonNull
+    @Override
     ObservableSource<Downstream> apply(@NonNull Observable<Upstream> upstream);
 }

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

@@ -15,6 +15,8 @@
 
 import io.reactivex.rxjava3.annotations.NonNull;
 
+import java.util.function.Function;
+
 /**
  * Convenience interface and callback used by the {@link Single#to} operator to turn a Single into another
  * value fluently.
@@ -23,13 +25,15 @@
  * @param <R> the output type
  * @since 2.2
  */
-public interface SingleConverter<T, R> {
+@FunctionalInterface
+public interface SingleConverter<T, R> extends Function<Single<T>, R> {
     /**
      * Applies a function to the upstream Single and returns a converted value of type {@code R}.
      *
      * @param upstream the upstream Single instance
      * @return the converted value
      */
     @NonNull
+    @Override
     R apply(@NonNull Single<T> upstream);
 }

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

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

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

@@ -21,6 +21,7 @@
  * @param <Downstream> the value type of the downstream
  * @param <Upstream> the value type of the upstream
  */
+@FunctionalInterface
 public interface SingleOperator<Downstream, Upstream> {
     /**
      * Applies a function to the child SingleObserver and returns a new parent SingleObserver.

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

@@ -24,6 +24,7 @@
  * @param <T> the element type
  * @since 2.0
  */
+@FunctionalInterface
 public interface SingleSource<T> {
 
     /**

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

@@ -15,19 +15,23 @@
 
 import io.reactivex.rxjava3.annotations.NonNull;
 
+import java.util.function.Function;
+
 /**
  * Interface to compose Singles.
  *
  * @param <Upstream> the upstream value type
  * @param <Downstream> the downstream value type
  */
-public interface SingleTransformer<Upstream, Downstream> {
+@FunctionalInterface
+public interface SingleTransformer<Upstream, Downstream> extends Function<Single<Upstream>, SingleSource<Downstream>> {
     /**
      * Applies a function to the upstream Single and returns a SingleSource with
      * optionally different element type.
      * @param upstream the upstream Single instance
      * @return the transformed SingleSource instance
      */
     @NonNull
+    @Override
     SingleSource<Downstream> apply(@NonNull Single<Upstream> upstream);
 }

src/main/java/io/reactivex/rxjava3/functions/Action.java

@@ -16,6 +16,7 @@
 /**
  * A functional interface similar to Runnable but allows throwing a checked exception.
  */
+@FunctionalInterface
 public interface Action {
     /**
      * Runs the action and optionally throws a checked exception.

src/main/java/io/reactivex/rxjava3/functions/BiConsumer.java

@@ -18,6 +18,7 @@
  * @param <T1> the first value type
  * @param <T2> the second value type
  */
+@FunctionalInterface
 public interface BiConsumer<T1, T2> {
 
     /**

src/main/java/io/reactivex/rxjava3/functions/BiFunction.java

@@ -21,6 +21,7 @@
  * @param <T2> the second value type
  * @param <R> the result type
  */
+@FunctionalInterface
 public interface BiFunction<T1, T2, R> {
 
     /**

src/main/java/io/reactivex/rxjava3/functions/BiPredicate.java

@@ -20,6 +20,7 @@
  * @param <T1> the first value
  * @param <T2> the second value
  */
+@FunctionalInterface
 public interface BiPredicate<T1, T2> {
 
     /**