content: /*
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements. See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to You under the Apache License, Version 2.0
   * (the "License"); you may not use this file except in compliance with
   * the License. You may obtain a copy of the License at
   *
   *    http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.kafka.streams.kstream;
  
  import org.apache.kafka.streams.KeyValue;
  import org.apache.kafka.streams.processor.ProcessorContext;
  import org.apache.kafka.streams.processor.PunctuationType;
  import org.apache.kafka.streams.processor.Punctuator;
  import org.apache.kafka.streams.processor.StateStore;
  import org.apache.kafka.streams.processor.To;
  
  import java.time.Duration;
  
  /**
   * The {@code Transformer} interface is for stateful mapping of an input record to zero, one, or multiple new output
   * records (both key and value type can be altered arbitrarily).
   * This is a stateful record-by-record operation, i.e, {@link #transform(Object, Object)} is invoked individually for
   * each record of a stream and can access and modify a state that is available beyond a single call of
   * {@link #transform(Object, Object)} (cf. {@link KeyValueMapper} for stateless record transformation).
   * Additionally, this {@code Transformer} can {@link ProcessorContext#schedule(Duration, PunctuationType, Punctuator) schedule}
   * a method to be {@link Punctuator#punctuate(long) called periodically} with the provided context.
   * <p>
   * Use {@link TransformerSupplier} to provide new instances of {@code Transformer} to Kafka Stream's runtime.
   * <p>
   * If only a record's value should be modified {@link ValueTransformer} can be used.
   *
   * @param <K> key type
   * @param <V> value type
   * @param <R> {@link KeyValue} return type (both key and value type can be set
   *            arbitrarily)
   * @see TransformerSupplier
   * @see ValueTransformer
   * @see KStream#map(KeyValueMapper)
   * @see KStream#flatMap(KeyValueMapper)
   * @deprecated Since 4.0. Use {@link org.apache.kafka.streams.processor.api.Processor api.Processor} instead.
   */
  @Deprecated
  public interface Transformer<K, V, R> {
  
      /**
       * Initialize this transformer.
       * This is called once per instance when the topology gets initialized.
       * When the framework is done with the transformer, {@link #close()} will be called on it; the
       * framework may later re-use the transformer by calling {@link #init(ProcessorContext)} again.
       * <p>
       * The provided {@link ProcessorContext context} can be used to access topology and record meta data, to
       * {@link ProcessorContext#schedule(Duration, PunctuationType, Punctuator) schedule} a method to be
       * {@link Punctuator#punctuate(long) called periodically} and to access attached {@link StateStore}s.
       * <p>
       * Note, that {@link ProcessorContext} is updated in the background with the current record's meta data.
       * Thus, it only contains valid record meta data when accessed within {@link #transform(Object, Object)}.
       *
       * @param context the context
       */
      void init(final ProcessorContext context);
  
      /**
       * Transform the record with the given key and value.
       * Additionally, any {@link StateStore state} that is attached to this operator can be accessed and modified
       * arbitrarily (cf. {@link ProcessorContext#getStateStore(String)}).
       * <p>
       * If only one record should be forward downstream, {@code transform} can return a new {@link KeyValue}. If
       * more than one output record should be forwarded downstream, {@link ProcessorContext#forward(Object, Object)}
       * and {@link ProcessorContext#forward(Object, Object, To)} can be used.
       * If no record should be forwarded downstream, {@code transform} can return {@code null}.
       *
       * Note that returning a new {@link KeyValue} is merely for convenience. The same can be achieved by using
       * {@link ProcessorContext#forward(Object, Object)} and returning {@code null}.
       *
       * @param key the key for the record
       * @param value the value for the record
       * @return new {@link KeyValue} pair&mdash;if {@code null} no key-value pair will
       * be forwarded to down stream
       */
      R transform(final K key, final V value);
  
      /**
       * Close this transformer and clean up any resources. The framework may
       * later re-use this transformer by calling {@link #init(ProcessorContext)} on it again.
       * <p>
       * To generate new {@link KeyValue} pairs {@link ProcessorContext#forward(Object, Object)} and
       * {@link ProcessorContext#forward(Object, Object, To)} can be used.
       */
      void close();
  
  }
  