diff --git a/src/main/java/io/reactivex/rxjava3/core/CompletableConverter.java b/src/main/java/io/reactivex/rxjava3/core/CompletableConverter.java
index 6ef04e2608..c1229ff7a2 100644
--- a/src/main/java/io/reactivex/rxjava3/core/CompletableConverter.java
+++ b/src/main/java/io/reactivex/rxjava3/core/CompletableConverter.java
@@ -15,6 +15,8 @@
 
 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.
@@ -22,7 +24,8 @@
  * @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}.
      *
@@ -30,5 +33,6 @@ public interface CompletableConverter<R> {
      * @return the converted value
      */
     @NonNull
+    @Override
     R apply(@NonNull Completable upstream);
 }
diff --git a/src/main/java/io/reactivex/rxjava3/core/CompletableOnSubscribe.java b/src/main/java/io/reactivex/rxjava3/core/CompletableOnSubscribe.java
index 8dc2531997..6aea056be7 100644
--- a/src/main/java/io/reactivex/rxjava3/core/CompletableOnSubscribe.java
+++ b/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 {
 
     /**
diff --git a/src/main/java/io/reactivex/rxjava3/core/CompletableOperator.java b/src/main/java/io/reactivex/rxjava3/core/CompletableOperator.java
index 3a45c9085f..763cd670c6 100644
--- a/src/main/java/io/reactivex/rxjava3/core/CompletableOperator.java
+++ b/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.
diff --git a/src/main/java/io/reactivex/rxjava3/core/CompletableSource.java b/src/main/java/io/reactivex/rxjava3/core/CompletableSource.java
index 390be60b2e..9b754a698a 100644
--- a/src/main/java/io/reactivex/rxjava3/core/CompletableSource.java
+++ b/src/main/java/io/reactivex/rxjava3/core/CompletableSource.java
@@ -20,6 +20,7 @@
  *
  * @since 2.0
  */
+@FunctionalInterface
 public interface CompletableSource {
 
     /**
diff --git a/src/main/java/io/reactivex/rxjava3/core/CompletableTransformer.java b/src/main/java/io/reactivex/rxjava3/core/CompletableTransformer.java
index b6acc252b0..7eb4ee65de 100644
--- a/src/main/java/io/reactivex/rxjava3/core/CompletableTransformer.java
+++ b/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);
 }
diff --git a/src/main/java/io/reactivex/rxjava3/core/FlowableConverter.java b/src/main/java/io/reactivex/rxjava3/core/FlowableConverter.java
index 0736dc189d..a3907843dd 100644
--- a/src/main/java/io/reactivex/rxjava3/core/FlowableConverter.java
+++ b/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.
@@ -23,7 +25,8 @@
  * @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}.
      *
@@ -31,5 +34,6 @@ public interface FlowableConverter<T, R> {
      * @return the converted value
      */
     @NonNull
+    @Override
     R apply(@NonNull Flowable<T> upstream);
 }
diff --git a/src/main/java/io/reactivex/rxjava3/core/FlowableOnSubscribe.java b/src/main/java/io/reactivex/rxjava3/core/FlowableOnSubscribe.java
index 93d125de2f..2bbc8b57a3 100644
--- a/src/main/java/io/reactivex/rxjava3/core/FlowableOnSubscribe.java
+++ b/src/main/java/io/reactivex/rxjava3/core/FlowableOnSubscribe.java
@@ -21,6 +21,7 @@
  *
  * @param <T> the value type pushed
  */
+@FunctionalInterface
 public interface FlowableOnSubscribe<T> {
 
     /**
diff --git a/src/main/java/io/reactivex/rxjava3/core/FlowableOperator.java b/src/main/java/io/reactivex/rxjava3/core/FlowableOperator.java
index 8ce0ab8f7d..6d241f7498 100644
--- a/src/main/java/io/reactivex/rxjava3/core/FlowableOperator.java
+++ b/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.
diff --git a/src/main/java/io/reactivex/rxjava3/core/FlowableTransformer.java b/src/main/java/io/reactivex/rxjava3/core/FlowableTransformer.java
index ecee751040..4eff5e8c9a 100644
--- a/src/main/java/io/reactivex/rxjava3/core/FlowableTransformer.java
+++ b/src/main/java/io/reactivex/rxjava3/core/FlowableTransformer.java
@@ -17,13 +17,16 @@
 
 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.
@@ -31,5 +34,6 @@ public interface FlowableTransformer<Upstream, Downstream> {
      * @return the transformed Publisher instance
      */
     @NonNull
+    @Override
     Publisher<Downstream> apply(@NonNull Flowable<Upstream> upstream);
 }
diff --git a/src/main/java/io/reactivex/rxjava3/core/MaybeConverter.java b/src/main/java/io/reactivex/rxjava3/core/MaybeConverter.java
index de70b81814..fd79eb3afe 100644
--- a/src/main/java/io/reactivex/rxjava3/core/MaybeConverter.java
+++ b/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.
@@ -23,7 +25,8 @@
  * @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}.
      *
@@ -31,5 +34,6 @@ public interface MaybeConverter<T, R> {
      * @return the converted value
      */
     @NonNull
+    @Override
     R apply(@NonNull Maybe<T> upstream);
 }
diff --git a/src/main/java/io/reactivex/rxjava3/core/MaybeOnSubscribe.java b/src/main/java/io/reactivex/rxjava3/core/MaybeOnSubscribe.java
index dbccf870f9..2dde118757 100644
--- a/src/main/java/io/reactivex/rxjava3/core/MaybeOnSubscribe.java
+++ b/src/main/java/io/reactivex/rxjava3/core/MaybeOnSubscribe.java
@@ -21,6 +21,7 @@
  *
  * @param <T> the value type pushed
  */
+@FunctionalInterface
 public interface MaybeOnSubscribe<T> {
 
     /**
diff --git a/src/main/java/io/reactivex/rxjava3/core/MaybeOperator.java b/src/main/java/io/reactivex/rxjava3/core/MaybeOperator.java
index b3173eb833..48039a9bd0 100644
--- a/src/main/java/io/reactivex/rxjava3/core/MaybeOperator.java
+++ b/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.
diff --git a/src/main/java/io/reactivex/rxjava3/core/MaybeSource.java b/src/main/java/io/reactivex/rxjava3/core/MaybeSource.java
index dd037cc864..17e2b7f694 100644
--- a/src/main/java/io/reactivex/rxjava3/core/MaybeSource.java
+++ b/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> {
 
     /**
diff --git a/src/main/java/io/reactivex/rxjava3/core/MaybeTransformer.java b/src/main/java/io/reactivex/rxjava3/core/MaybeTransformer.java
index d53bfda71d..115386aaa7 100644
--- a/src/main/java/io/reactivex/rxjava3/core/MaybeTransformer.java
+++ b/src/main/java/io/reactivex/rxjava3/core/MaybeTransformer.java
@@ -15,13 +15,16 @@
 
 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.
@@ -29,5 +32,6 @@ public interface MaybeTransformer<Upstream, Downstream> {
      * @return the transformed MaybeSource instance
      */
     @NonNull
+    @Override
     MaybeSource<Downstream> apply(@NonNull Maybe<Upstream> upstream);
 }
diff --git a/src/main/java/io/reactivex/rxjava3/core/ObservableConverter.java b/src/main/java/io/reactivex/rxjava3/core/ObservableConverter.java
index 744c0ef48e..f52a1a086c 100644
--- a/src/main/java/io/reactivex/rxjava3/core/ObservableConverter.java
+++ b/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.
@@ -23,7 +25,8 @@
  * @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}.
      *
@@ -31,5 +34,6 @@ public interface ObservableConverter<T, R> {
      * @return the converted value
      */
     @NonNull
+    @Override
     R apply(@NonNull Observable<T> upstream);
 }
diff --git a/src/main/java/io/reactivex/rxjava3/core/ObservableOnSubscribe.java b/src/main/java/io/reactivex/rxjava3/core/ObservableOnSubscribe.java
index 1f98911285..43d6252849 100644
--- a/src/main/java/io/reactivex/rxjava3/core/ObservableOnSubscribe.java
+++ b/src/main/java/io/reactivex/rxjava3/core/ObservableOnSubscribe.java
@@ -21,6 +21,7 @@
  *
  * @param <T> the value type pushed
  */
+@FunctionalInterface
 public interface ObservableOnSubscribe<T> {
 
     /**
diff --git a/src/main/java/io/reactivex/rxjava3/core/ObservableOperator.java b/src/main/java/io/reactivex/rxjava3/core/ObservableOperator.java
index f7335b6221..76ec06e295 100644
--- a/src/main/java/io/reactivex/rxjava3/core/ObservableOperator.java
+++ b/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.
diff --git a/src/main/java/io/reactivex/rxjava3/core/ObservableSource.java b/src/main/java/io/reactivex/rxjava3/core/ObservableSource.java
index a48011e438..ad5d5c79c3 100644
--- a/src/main/java/io/reactivex/rxjava3/core/ObservableSource.java
+++ b/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> {
 
     /**
diff --git a/src/main/java/io/reactivex/rxjava3/core/ObservableTransformer.java b/src/main/java/io/reactivex/rxjava3/core/ObservableTransformer.java
index 63dc87ca8b..5144d16cf6 100644
--- a/src/main/java/io/reactivex/rxjava3/core/ObservableTransformer.java
+++ b/src/main/java/io/reactivex/rxjava3/core/ObservableTransformer.java
@@ -15,13 +15,16 @@
 
 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.
@@ -29,5 +32,6 @@ public interface ObservableTransformer<Upstream, Downstream> {
      * @return the transformed ObservableSource instance
      */
     @NonNull
+    @Override
     ObservableSource<Downstream> apply(@NonNull Observable<Upstream> upstream);
 }
diff --git a/src/main/java/io/reactivex/rxjava3/core/SingleConverter.java b/src/main/java/io/reactivex/rxjava3/core/SingleConverter.java
index 5309dbc160..26de3e79ed 100644
--- a/src/main/java/io/reactivex/rxjava3/core/SingleConverter.java
+++ b/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.
@@ -23,7 +25,8 @@
  * @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}.
      *
@@ -31,5 +34,6 @@ public interface SingleConverter<T, R> {
      * @return the converted value
      */
     @NonNull
+    @Override
     R apply(@NonNull Single<T> upstream);
 }
diff --git a/src/main/java/io/reactivex/rxjava3/core/SingleOnSubscribe.java b/src/main/java/io/reactivex/rxjava3/core/SingleOnSubscribe.java
index 157a8e42dc..46cc6a6f56 100644
--- a/src/main/java/io/reactivex/rxjava3/core/SingleOnSubscribe.java
+++ b/src/main/java/io/reactivex/rxjava3/core/SingleOnSubscribe.java
@@ -21,6 +21,7 @@
  *
  * @param <T> the value type pushed
  */
+@FunctionalInterface
 public interface SingleOnSubscribe<T> {
 
     /**
diff --git a/src/main/java/io/reactivex/rxjava3/core/SingleOperator.java b/src/main/java/io/reactivex/rxjava3/core/SingleOperator.java
index 58ea9d338e..2fc2d80ab6 100644
--- a/src/main/java/io/reactivex/rxjava3/core/SingleOperator.java
+++ b/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.
diff --git a/src/main/java/io/reactivex/rxjava3/core/SingleSource.java b/src/main/java/io/reactivex/rxjava3/core/SingleSource.java
index 1a3780c3f6..496462d0e1 100644
--- a/src/main/java/io/reactivex/rxjava3/core/SingleSource.java
+++ b/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> {
 
     /**
diff --git a/src/main/java/io/reactivex/rxjava3/core/SingleTransformer.java b/src/main/java/io/reactivex/rxjava3/core/SingleTransformer.java
index 080a2d7af1..f693e36ad7 100644
--- a/src/main/java/io/reactivex/rxjava3/core/SingleTransformer.java
+++ b/src/main/java/io/reactivex/rxjava3/core/SingleTransformer.java
@@ -15,13 +15,16 @@
 
 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.
@@ -29,5 +32,6 @@ public interface SingleTransformer<Upstream, Downstream> {
      * @return the transformed SingleSource instance
      */
     @NonNull
+    @Override
     SingleSource<Downstream> apply(@NonNull Single<Upstream> upstream);
 }
diff --git a/src/main/java/io/reactivex/rxjava3/functions/Action.java b/src/main/java/io/reactivex/rxjava3/functions/Action.java
index 153eff9375..da0e76e0a6 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/Action.java
+++ b/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.
diff --git a/src/main/java/io/reactivex/rxjava3/functions/BiConsumer.java b/src/main/java/io/reactivex/rxjava3/functions/BiConsumer.java
index c060c38331..7fa60ef4c7 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/BiConsumer.java
+++ b/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> {
 
     /**
diff --git a/src/main/java/io/reactivex/rxjava3/functions/BiFunction.java b/src/main/java/io/reactivex/rxjava3/functions/BiFunction.java
index f7e1174c6e..174cc3c6d9 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/BiFunction.java
+++ b/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> {
 
     /**
diff --git a/src/main/java/io/reactivex/rxjava3/functions/BiPredicate.java b/src/main/java/io/reactivex/rxjava3/functions/BiPredicate.java
index 4793e1d086..fc41682ef5 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/BiPredicate.java
+++ b/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> {
 
     /**
diff --git a/src/main/java/io/reactivex/rxjava3/functions/BooleanSupplier.java b/src/main/java/io/reactivex/rxjava3/functions/BooleanSupplier.java
index dc801e5180..b30087fbd7 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/BooleanSupplier.java
+++ b/src/main/java/io/reactivex/rxjava3/functions/BooleanSupplier.java
@@ -16,6 +16,7 @@
 /**
  * A functional interface (callback) that returns a boolean value.
  */
+@FunctionalInterface
 public interface BooleanSupplier {
     /**
      * Returns a boolean value.
diff --git a/src/main/java/io/reactivex/rxjava3/functions/Cancellable.java b/src/main/java/io/reactivex/rxjava3/functions/Cancellable.java
index 47683eafdc..b9f3420c1b 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/Cancellable.java
+++ b/src/main/java/io/reactivex/rxjava3/functions/Cancellable.java
@@ -17,6 +17,7 @@
  * A functional interface that has a single cancel method
  * that can throw.
  */
+@FunctionalInterface
 public interface Cancellable {
 
     /**
diff --git a/src/main/java/io/reactivex/rxjava3/functions/Consumer.java b/src/main/java/io/reactivex/rxjava3/functions/Consumer.java
index 08d94eb928..788622c170 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/Consumer.java
+++ b/src/main/java/io/reactivex/rxjava3/functions/Consumer.java
@@ -17,6 +17,7 @@
  * A functional interface (callback) that accepts a single value.
  * @param <T> the value type
  */
+@FunctionalInterface
 public interface Consumer<T> {
     /**
      * Consume the given value.
diff --git a/src/main/java/io/reactivex/rxjava3/functions/Function.java b/src/main/java/io/reactivex/rxjava3/functions/Function.java
index 23d06b041b..d81a2dec52 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/Function.java
+++ b/src/main/java/io/reactivex/rxjava3/functions/Function.java
@@ -22,6 +22,7 @@
  * @param <T> the input value type
  * @param <R> the output value type
  */
+@FunctionalInterface
 public interface Function<T, R> {
     /**
      * Apply some calculation to the input value and return some other value.
diff --git a/src/main/java/io/reactivex/rxjava3/functions/Function3.java b/src/main/java/io/reactivex/rxjava3/functions/Function3.java
index bdc1b4af2b..cdc65112e7 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/Function3.java
+++ b/src/main/java/io/reactivex/rxjava3/functions/Function3.java
@@ -22,6 +22,7 @@
  * @param <T3> the third value type
  * @param <R> the result type
  */
+@FunctionalInterface
 public interface Function3<T1, T2, T3, R> {
     /**
      * Calculate a value based on the input values.
diff --git a/src/main/java/io/reactivex/rxjava3/functions/Function4.java b/src/main/java/io/reactivex/rxjava3/functions/Function4.java
index 907f742907..84c3ab1449 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/Function4.java
+++ b/src/main/java/io/reactivex/rxjava3/functions/Function4.java
@@ -23,6 +23,7 @@
  * @param <T4> the fourth value type
  * @param <R> the result type
  */
+@FunctionalInterface
 public interface Function4<T1, T2, T3, T4, R> {
     /**
      * Calculate a value based on the input values.
diff --git a/src/main/java/io/reactivex/rxjava3/functions/Function5.java b/src/main/java/io/reactivex/rxjava3/functions/Function5.java
index a8468844b7..77f2ff7220 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/Function5.java
+++ b/src/main/java/io/reactivex/rxjava3/functions/Function5.java
@@ -24,6 +24,7 @@
  * @param <T5> the fifth value type
  * @param <R> the result type
  */
+@FunctionalInterface
 public interface Function5<T1, T2, T3, T4, T5, R> {
     /**
      * Calculate a value based on the input values.
diff --git a/src/main/java/io/reactivex/rxjava3/functions/Function6.java b/src/main/java/io/reactivex/rxjava3/functions/Function6.java
index 6bc5caec1f..50c25f3df9 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/Function6.java
+++ b/src/main/java/io/reactivex/rxjava3/functions/Function6.java
@@ -25,6 +25,7 @@
  * @param <T6> the sixth value type
  * @param <R> the result type
  */
+@FunctionalInterface
 public interface Function6<T1, T2, T3, T4, T5, T6, R> {
     /**
      * Calculate a value based on the input values.
diff --git a/src/main/java/io/reactivex/rxjava3/functions/Function7.java b/src/main/java/io/reactivex/rxjava3/functions/Function7.java
index 32d5a08e41..613800a4c1 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/Function7.java
+++ b/src/main/java/io/reactivex/rxjava3/functions/Function7.java
@@ -26,6 +26,7 @@
  * @param <T7> the seventh value type
  * @param <R> the result type
  */
+@FunctionalInterface
 public interface Function7<T1, T2, T3, T4, T5, T6, T7, R> {
     /**
      * Calculate a value based on the input values.
diff --git a/src/main/java/io/reactivex/rxjava3/functions/Function8.java b/src/main/java/io/reactivex/rxjava3/functions/Function8.java
index 8a7a88f9e8..2fda54ea40 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/Function8.java
+++ b/src/main/java/io/reactivex/rxjava3/functions/Function8.java
@@ -27,6 +27,7 @@
  * @param <T8> the eighth value type
  * @param <R> the result type
  */
+@FunctionalInterface
 public interface Function8<T1, T2, T3, T4, T5, T6, T7, T8, R> {
     /**
      * Calculate a value based on the input values.
diff --git a/src/main/java/io/reactivex/rxjava3/functions/Function9.java b/src/main/java/io/reactivex/rxjava3/functions/Function9.java
index 28319eac14..3eb0779e30 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/Function9.java
+++ b/src/main/java/io/reactivex/rxjava3/functions/Function9.java
@@ -28,6 +28,7 @@
  * @param <T9> the ninth value type
  * @param <R> the result type
  */
+@FunctionalInterface
 public interface Function9<T1, T2, T3, T4, T5, T6, T7, T8, T9, R> {
     /**
      * Calculate a value based on the input values.
diff --git a/src/main/java/io/reactivex/rxjava3/functions/IntFunction.java b/src/main/java/io/reactivex/rxjava3/functions/IntFunction.java
index 8a135ad491..e01cc13ab5 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/IntFunction.java
+++ b/src/main/java/io/reactivex/rxjava3/functions/IntFunction.java
@@ -18,6 +18,7 @@
  * A functional interface (callback) that takes a primitive value and return value of type T.
  * @param <T> the returned value type
  */
+@FunctionalInterface
 public interface IntFunction<T> {
     /**
      * Calculates a value based on a primitive integer input.
diff --git a/src/main/java/io/reactivex/rxjava3/functions/LongConsumer.java b/src/main/java/io/reactivex/rxjava3/functions/LongConsumer.java
index 6b5b72101c..9af7de4f47 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/LongConsumer.java
+++ b/src/main/java/io/reactivex/rxjava3/functions/LongConsumer.java
@@ -15,6 +15,7 @@
 /**
  * A functional interface (callback) that consumes a primitive long value.
  */
+@FunctionalInterface
 public interface LongConsumer {
     /**
      * Consume a primitive long input.
diff --git a/src/main/java/io/reactivex/rxjava3/functions/Predicate.java b/src/main/java/io/reactivex/rxjava3/functions/Predicate.java
index 3197114158..99450f865b 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/Predicate.java
+++ b/src/main/java/io/reactivex/rxjava3/functions/Predicate.java
@@ -19,6 +19,7 @@
  * A functional interface (callback) that returns true or false for the given input value.
  * @param <T> the first value
  */
+@FunctionalInterface
 public interface Predicate<T> {
     /**
      * Test the given input value and return a boolean.
diff --git a/src/main/java/io/reactivex/rxjava3/functions/Supplier.java b/src/main/java/io/reactivex/rxjava3/functions/Supplier.java
index 95ef2e88a1..af5052ab83 100644
--- a/src/main/java/io/reactivex/rxjava3/functions/Supplier.java
+++ b/src/main/java/io/reactivex/rxjava3/functions/Supplier.java
@@ -22,6 +22,7 @@
  * @param <T> the value type returned
  * @since 3.0.0
  */
+@FunctionalInterface
 public interface Supplier<T> {
 
     /**
diff --git a/src/main/java/io/reactivex/rxjava3/internal/fuseable/ScalarSupplier.java b/src/main/java/io/reactivex/rxjava3/internal/fuseable/ScalarSupplier.java
index 039d730be6..fc0d12323f 100644
--- a/src/main/java/io/reactivex/rxjava3/internal/fuseable/ScalarSupplier.java
+++ b/src/main/java/io/reactivex/rxjava3/internal/fuseable/ScalarSupplier.java
@@ -29,6 +29,7 @@
  * <p>
  * @param <T> the scalar value type held by the implementing reactive type
  */
+@FunctionalInterface
 public interface ScalarSupplier<T> extends Supplier<T> {
 
     // overridden to remove the throws Throwable
diff --git a/src/main/java/io/reactivex/rxjava3/parallel/ParallelFlowableConverter.java b/src/main/java/io/reactivex/rxjava3/parallel/ParallelFlowableConverter.java
index ea2d07502a..f6ce1ddde0 100644
--- a/src/main/java/io/reactivex/rxjava3/parallel/ParallelFlowableConverter.java
+++ b/src/main/java/io/reactivex/rxjava3/parallel/ParallelFlowableConverter.java
@@ -15,6 +15,8 @@
 
 import io.reactivex.rxjava3.annotations.NonNull;
 
+import java.util.function.Function;
+
 /**
  * Convenience interface and callback used by the {@link ParallelFlowable#to} operator to turn a ParallelFlowable into
  * another value fluently.
@@ -23,7 +25,8 @@
  * @param <R> the output type
  * @since 2.2
  */
-public interface ParallelFlowableConverter<T, R> {
+@FunctionalInterface
+public interface ParallelFlowableConverter<T, R> extends Function<ParallelFlowable<T>, R> {
     /**
      * Applies a function to the upstream ParallelFlowable and returns a converted value of type {@code R}.
      *
@@ -31,5 +34,6 @@ public interface ParallelFlowableConverter<T, R> {
      * @return the converted value
      */
     @NonNull
+    @Override
     R apply(@NonNull ParallelFlowable<T> upstream);
 }
diff --git a/src/main/java/io/reactivex/rxjava3/parallel/ParallelTransformer.java b/src/main/java/io/reactivex/rxjava3/parallel/ParallelTransformer.java
index 1a918f0835..2a069b8070 100644
--- a/src/main/java/io/reactivex/rxjava3/parallel/ParallelTransformer.java
+++ b/src/main/java/io/reactivex/rxjava3/parallel/ParallelTransformer.java
@@ -15,6 +15,8 @@
 
 import io.reactivex.rxjava3.annotations.NonNull;
 
+import java.util.function.Function;
+
 /**
  * Interface to compose ParallelFlowable.
  * <p>History: 2.0.8 - experimental
@@ -22,7 +24,8 @@
  * @param <Downstream> the downstream value type
  * @since 2.2
  */
-public interface ParallelTransformer<Upstream, Downstream> {
+@FunctionalInterface
+public interface ParallelTransformer<Upstream, Downstream> extends Function<ParallelFlowable<Upstream>, ParallelFlowable<Downstream>> {
     /**
      * Applies a function to the upstream ParallelFlowable and returns a ParallelFlowable with
      * optionally different element type.
@@ -30,5 +33,6 @@ public interface ParallelTransformer<Upstream, Downstream> {
      * @return the transformed ParallelFlowable instance
      */
     @NonNull
+    @Override
     ParallelFlowable<Downstream> apply(@NonNull ParallelFlowable<Upstream> upstream);
 }
\ No newline at end of file