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 Reducer} interface for combining two values of the same type into a new value.
   * In contrast to {@link Aggregator} the result type must be the same as the input type.
   * <p>
   * The provided values can be either original values from input {@link KeyValue} pair records or be a previously
   * computed result from {@link Reducer#apply(Object, Object)}.
   * <p>
   * {@code Reducer} can be used to implement aggregation functions like sum, min, or max.
   *
   * @param <V> value type
   * @see KGroupedStream#reduce(Reducer)
   * @see KGroupedStream#reduce(Reducer, Materialized)
   * @see TimeWindowedKStream#reduce(Reducer)
   * @see TimeWindowedKStream#reduce(Reducer, Materialized)
   * @see SessionWindowedKStream#reduce(Reducer)
   * @see SessionWindowedKStream#reduce(Reducer, Materialized)
   * @see Aggregator
   */
  @FunctionalInterface
  public interface Reducer<V> {
  
      /**
       * Aggregate the two given values into a single one.
       *
       * @param value1 the first value for the aggregation
       * @param value2 the second value for the aggregation
       * @return the aggregated value
       */
      V apply(final V value1, final V value2);
  }
  