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;
  
  /**
   * The {@code KeyValueMapper} interface for mapping a {@link KeyValue key-value pair} to a new value of arbitrary type.
   * For example, it can be used to
   * <ul>
   * <li>map from an input {@link KeyValue} pair to an output {@link KeyValue} pair with different key and/or value type
   *     (for this case output type {@code VR == }{@link KeyValue KeyValue&lt;NewKeyType,NewValueType&gt;})</li>
   * <li>map from an input record to a new key (with arbitrary key type as specified by {@code VR})</li>
   * </ul>
   * This is a stateless record-by-record operation, i.e, {@link #apply(Object, Object)} is invoked individually for each
   * record of a stream (cf. {@link org.apache.kafka.streams.processor.api.Processor api.Processor} for stateful record transformation).
   * {@code KeyValueMapper} is a generalization of {@link ValueMapper}.
   *
   * @param <K>  key type
   * @param <V>  value type
   * @param <VR> mapped value type
   * @see ValueMapper
   * @see org.apache.kafka.streams.processor.api.Processor
   * @see KStream#map(KeyValueMapper)
   * @see KStream#flatMap(KeyValueMapper)
   * @see KStream#selectKey(KeyValueMapper)
   * @see KStream#groupBy(KeyValueMapper)
   * @see KStream#groupBy(KeyValueMapper, Grouped)
   * @see KTable#groupBy(KeyValueMapper)
   * @see KTable#groupBy(KeyValueMapper, Grouped)
   * @see KTable#toStream(KeyValueMapper)
   */
  @FunctionalInterface
  public interface KeyValueMapper<K, V, VR> {
  
      /**
       * Map a record with the given key and value to a new value.
       *
       * @param key   the key of the record
       * @param value the value of the record
       * @return the new value
       */
      VR apply(final K key, final V value);
  }
  