MINOR: Add Functional Interface annotation to interfaces used by Lambdas (#19234)

Adds the FunctionalInterface annotation to relevant Kafka Streams
classes. While this is not strictly required for Java, it's still best
practice and also useful for better integration with other JVM
languages, for example Clojure, to allow using these interfaces as
lambdas.

Reviewers: Matthias J. Sax <[email protected]>

Author: Tombert

clients/src/main/java/org/apache/kafka/clients/consumer/OffsetCommitCallback.java

@@ -26,6 +26,7 @@
  * A callback interface that the user can implement to trigger custom actions when a commit request completes. The callback
  * may be executed in any thread calling {@link Consumer#poll(java.time.Duration) poll()}.
  */
+@FunctionalInterface
 public interface OffsetCommitCallback {
 
     /**

streams/src/main/java/org/apache/kafka/streams/kstream/Aggregator.java

@@ -38,6 +38,7 @@
  * @see SessionWindowedKStream#aggregate(Initializer, Aggregator, Merger, Materialized)
  * @see Reducer
  */
+@FunctionalInterface
 public interface Aggregator<K, V, VAgg> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/kstream/ForeachAction.java

@@ -30,6 +30,7 @@
  *
  * @see KStream#foreach(ForeachAction)
  */
+@FunctionalInterface
 public interface ForeachAction<K, V> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/kstream/Initializer.java

@@ -30,6 +30,7 @@
  * @see SessionWindowedKStream#aggregate(Initializer, Aggregator, Merger)
  * @see SessionWindowedKStream#aggregate(Initializer, Aggregator, Merger, Materialized)
  */
+@FunctionalInterface
 public interface Initializer<VAgg> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/kstream/KeyValueMapper.java

@@ -44,6 +44,7 @@
  * @see KTable#groupBy(KeyValueMapper, Grouped)
  * @see KTable#toStream(KeyValueMapper)
  */
+@FunctionalInterface
 public interface KeyValueMapper<K, V, VR> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/kstream/Merger.java

@@ -23,6 +23,7 @@
  * @param <K>   key type
  * @param <V>   aggregate value type
  */
+@FunctionalInterface
 public interface Merger<K, V> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/kstream/Predicate.java

@@ -31,6 +31,7 @@
  * @see KTable#filter(Predicate)
  * @see KTable#filterNot(Predicate)
  */
+@FunctionalInterface
 public interface Predicate<K, V> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/kstream/Reducer.java

@@ -36,6 +36,7 @@
  * @see SessionWindowedKStream#reduce(Reducer, Materialized)
  * @see Aggregator
  */
+@FunctionalInterface
 public interface Reducer<V> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/kstream/TransformerSupplier.java

@@ -38,6 +38,7 @@
  * @deprecated Since 4.0. Use {@link org.apache.kafka.streams.processor.api.ProcessorSupplier api.ProcessorSupplier} instead.
  */
 @Deprecated
+@FunctionalInterface
 public interface TransformerSupplier<K, V, R> extends ConnectedStoreProvider, Supplier<Transformer<K, V, R>> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/kstream/ValueJoiner.java

@@ -40,6 +40,7 @@
  * @see KTable#leftJoin(KTable, ValueJoiner)
  * @see KTable#outerJoin(KTable, ValueJoiner)
  */
+@FunctionalInterface
 public interface ValueJoiner<V1, V2, VR> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/kstream/ValueJoinerWithKey.java

@@ -44,6 +44,7 @@
  * @see KStream#leftJoin(GlobalKTable, KeyValueMapper, ValueJoinerWithKey)
  * @see KStream#leftJoin(GlobalKTable, KeyValueMapper, ValueJoinerWithKey, Named)
  */
+@FunctionalInterface
 public interface ValueJoinerWithKey<K1, V1, V2, VR> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/kstream/ValueMapper.java

@@ -37,6 +37,7 @@
  * @see KTable#mapValues(ValueMapper)
  * @see KTable#mapValues(ValueMapperWithKey)
  */
+@FunctionalInterface
 public interface ValueMapper<V, VR> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/kstream/ValueMapperWithKey.java

@@ -38,6 +38,7 @@
  * @see KTable#mapValues(ValueMapperWithKey)
  */
 
+@FunctionalInterface
 public interface ValueMapperWithKey<K, V, VR> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/kstream/ValueTransformerSupplier.java

@@ -36,6 +36,7 @@
  * @deprecated Since 4.0. Use {@link FixedKeyProcessorSupplier} instead.
  */
 @Deprecated
+@FunctionalInterface
 public interface ValueTransformerSupplier<V, VR> extends ConnectedStoreProvider {
 
     /**

streams/src/main/java/org/apache/kafka/streams/kstream/ValueTransformerWithKeySupplier.java

@@ -35,6 +35,7 @@
  * @see Transformer
  * @see TransformerSupplier
  */
+@FunctionalInterface
 public interface ValueTransformerWithKeySupplier<K, V, VR> extends ConnectedStoreProvider, Supplier<ValueTransformerWithKey<K, V, VR>> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/processor/BatchingStateRestoreCallback.java

@@ -28,6 +28,7 @@
  * It is expected that implementations of this class will not call the {@link StateRestoreCallback#restore(byte[],
  * byte[])} method.
  */
+@FunctionalInterface
 public interface BatchingStateRestoreCallback extends StateRestoreCallback {
 
     /**

streams/src/main/java/org/apache/kafka/streams/processor/Cancellable.java

@@ -23,6 +23,7 @@
  *
  * @see Punctuator
  */
+@FunctionalInterface
 public interface Cancellable {
 
     /**

streams/src/main/java/org/apache/kafka/streams/processor/Punctuator.java

@@ -26,6 +26,7 @@
  *
  * @see Cancellable
  */
+@FunctionalInterface
 public interface Punctuator {
 
     /**

streams/src/main/java/org/apache/kafka/streams/processor/StateRestoreCallback.java

@@ -20,6 +20,7 @@
  * Restoration logic for log-backed state stores upon restart,
  * it takes one record at a time from the logs to apply to the restoring state.
  */
+@FunctionalInterface
 public interface StateRestoreCallback {
 
     void restore(byte[] key, byte[] value);

streams/src/main/java/org/apache/kafka/streams/processor/StreamPartitioner.java

@@ -50,6 +50,7 @@
  *      org.apache.kafka.common.serialization.Serializer, StreamPartitioner, String...)
  * @see Topology#addSink(String, String, StreamPartitioner, String...)
  */
+@FunctionalInterface
 public interface StreamPartitioner<K, V> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/processor/TimestampExtractor.java

@@ -23,6 +23,7 @@
  * An interface that allows the Kafka Streams framework to extract a timestamp from an instance of {@link ConsumerRecord}.
  * The extracted timestamp is defined as milliseconds.
  */
+@FunctionalInterface
 public interface TimestampExtractor {
 
     /**

streams/src/main/java/org/apache/kafka/streams/processor/TopicNameExtractor.java

@@ -19,6 +19,7 @@
 /**
  * An interface that allows to dynamically determine the name of the Kafka topic to send at the sink node of the topology.
  */
+@FunctionalInterface
 public interface TopicNameExtractor<K, V> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/processor/api/FixedKeyProcessor.java

@@ -29,6 +29,7 @@
  * @param <VIn> the type of input values
  * @param <VOut> the type of output values
  */
+@FunctionalInterface
 public interface FixedKeyProcessor<KIn, VIn, VOut> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/processor/api/Processor.java

@@ -30,6 +30,7 @@
  * @param <KOut> the type of output keys
  * @param <VOut> the type of output values
  */
+@FunctionalInterface
 public interface Processor<KIn, VIn, KOut, VOut> {
 
     /**

streams/src/main/java/org/apache/kafka/streams/processor/api/WrappedFixedKeyProcessorSupplier.java

@@ -24,6 +24,7 @@
  * To convert a {@link FixedKeyProcessorSupplier} instance into a {@link WrappedFixedKeyProcessorSupplier},
  * use the {@link ProcessorWrapper#asWrappedFixedKey(FixedKeyProcessorSupplier)} method
  */
+@FunctionalInterface
 public interface WrappedFixedKeyProcessorSupplier<KIn, VIn, VOut> extends FixedKeyProcessorSupplier<KIn, VIn, VOut> {
 
 }

streams/src/main/java/org/apache/kafka/streams/processor/api/WrappedProcessorSupplier.java

@@ -24,6 +24,7 @@
  * To convert a {@link ProcessorSupplier} instance into a {@link WrappedProcessorSupplier},
  * use the {@link ProcessorWrapper#asWrapped(ProcessorSupplier)} method
  */
+@FunctionalInterface
 public interface WrappedProcessorSupplier<KIn, VIn, KOut, VOut> extends ProcessorSupplier<KIn, VIn, KOut, VOut> {
 
 }