Add FunctionalInterface annotations.

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}.

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

@@ -21,6 +21,7 @@
  * @param <Upstream> the upstream value type
  * @param <Downstream> the downstream value type
  */
+@FunctionalInterface
 public interface MaybeTransformer<Upstream, Downstream> {
     /**
      * Applies a function to the upstream Maybe and returns a MaybeSource with

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

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

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

@@ -21,6 +21,7 @@
  * @param <Upstream> the upstream value type
  * @param <Downstream> the downstream value type
  */
+@FunctionalInterface
 public interface ObservableTransformer<Upstream, Downstream> {
     /**
      * Applies a function to the upstream Observable and returns an ObservableSource with

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

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

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

@@ -21,6 +21,7 @@
  * @param <Upstream> the upstream value type
  * @param <Downstream> the downstream value type
  */
+@FunctionalInterface
 public interface SingleTransformer<Upstream, Downstream> {
     /**
      * Applies a function to the upstream Single and returns a SingleSource with

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> {
 
     /**

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.

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 {
 
     /**

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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> {
 
     /**

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

src/main/java/io/reactivex/rxjava3/parallel/ParallelFlowableConverter.java

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

src/main/java/io/reactivex/rxjava3/parallel/ParallelTransformer.java

@@ -22,6 +22,7 @@
  * @param <Downstream> the downstream value type
  * @since 2.2
  */
+@FunctionalInterface
 public interface ParallelTransformer<Upstream, Downstream> {
     /**
      * Applies a function to the upstream ParallelFlowable and returns a ParallelFlowable with