feat: add FunctionalInterface annotation

api-common-java/src/main/java/com/google/api/core/ApiAsyncFunction.java

@@ -34,6 +34,7 @@
  *
  * <p>It is similar to Guava's {@code AsyncFunction}, redeclared so that Guava can be shaded.
  */
+@FunctionalInterface
 public interface ApiAsyncFunction<I, O> {
   /**
    * Returns an output Future to use in place of the given input. The output Future need not be

api-common-java/src/main/java/com/google/api/core/ApiFunction.java

@@ -34,6 +34,7 @@
  *
  * <p>It is similar to Guava's {@code Function}, redeclared so that Guava can be shaded.
  */
+@FunctionalInterface
 public interface ApiFunction<F, T> {
   T apply(F input);
 }

api-common-java/src/main/java/com/google/api/pathtemplate/TemplatedResourceName.java

@@ -66,6 +66,7 @@ public class TemplatedResourceName implements Map<String, String> {
   // =====================
 
   /** Represents a resource name resolver which can be registered with this class. */
+  @FunctionalInterface
   public interface Resolver {
     /** Resolves the resource name into a resource by calling the underlying API. */
     <T> T resolve(Class<T> resourceType, TemplatedResourceName name, @Nullable String version);

api-common-java/src/main/java/com/google/api/pathtemplate/ValidationException.java

@@ -39,6 +39,7 @@
  */
 public class ValidationException extends IllegalArgumentException {
 
+  @FunctionalInterface
   public interface Supplier<T> {
     T get();
   }

gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/GrpcInterceptorProvider.java

@@ -33,6 +33,7 @@
 import java.util.List;
 
 /** Provider of custom gRPC ClientInterceptors. */
+@FunctionalInterface
 public interface GrpcInterceptorProvider {
 
   /**

gax-java/gax-httpjson/src/main/java/com/google/api/gax/httpjson/FieldsExtractor.java

@@ -33,6 +33,7 @@
  * A functional interface to be implemented for each request message to extract specific fields from
  * it. For advanced usage only.
  */
+@FunctionalInterface
 public interface FieldsExtractor<RequestT, ParamsT> {
   ParamsT extract(RequestT request);
 }

gax-java/gax-httpjson/src/main/java/com/google/api/gax/httpjson/HttpJsonInterceptorProvider.java

@@ -35,6 +35,7 @@
 /** Provider of custom REST ClientInterceptors. */
 @BetaApi(
     "The surface for adding custom interceptors is not stable yet and may change in the future.")
+@FunctionalInterface
 public interface HttpJsonInterceptorProvider {
 
   /**

gax-java/gax/src/main/java/com/google/api/gax/batching/BatchMerger.java

@@ -29,6 +29,7 @@
  */
 package com.google.api.gax.batching;
 
+@FunctionalInterface
 public interface BatchMerger<B> {
   void merge(B batch, B newBatch);
 }

gax-java/gax/src/main/java/com/google/api/gax/batching/ElementCounter.java

@@ -33,6 +33,7 @@
  * Interface representing an object that provides a numerical count given an object of the
  * parameterized type.
  */
+@FunctionalInterface
 public interface ElementCounter<E> {
   /** Provides the numerical count associated with the given object. */
   long count(E element);

gax-java/gax/src/main/java/com/google/api/gax/core/CredentialsProvider.java

@@ -35,6 +35,7 @@
 /**
  * Provides an interface to hold and acquire the credentials that will be used to call the service.
  */
+@FunctionalInterface
 public interface CredentialsProvider {
   /**
    * Gets the credentials which will be used to call the service. If the credentials have not been

gax-java/gax/src/main/java/com/google/api/gax/rpc/ClientStreamReadyObserver.java

@@ -30,6 +30,7 @@
 package com.google.api.gax.rpc;
 
 /** A callback used to report that the {@link ClientStream} is ready to send more messages. */
+@FunctionalInterface
 public interface ClientStreamReadyObserver<V> {
   void onReady(ClientStream<V> stream);
 }

gax-java/gax/src/main/java/com/google/api/gax/rpc/HeaderProvider.java

@@ -32,6 +32,7 @@
 import java.util.Map;
 
 /** Provider of headers to put on http requests. */
+@FunctionalInterface
 public interface HeaderProvider {
 
   /** Get the headers to put on http requests. */