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;
  
  
  /**
   * The {@code Aggregator} interface for aggregating values of the given key.
   * This is a generalization of {@link Reducer} and allows to have different types for input value and aggregation
   * result.
   * {@code Aggregator} is used in combination with {@link Initializer} that provides an initial aggregation value.
   * <p>
   * {@code Aggregator} can be used to implement aggregation functions like count.
   *
   * @param <K> key type
   * @param <V> input value type
   * @param <VAgg> aggregate value type
   *
   * @see Initializer
   * @see KGroupedStream#aggregate(Initializer, Aggregator)
   * @see KGroupedStream#aggregate(Initializer, Aggregator, Materialized)
   * @see TimeWindowedKStream#aggregate(Initializer, Aggregator)
   * @see TimeWindowedKStream#aggregate(Initializer, Aggregator, Materialized)
   * @see SessionWindowedKStream#aggregate(Initializer, Aggregator, Merger)
   * @see SessionWindowedKStream#aggregate(Initializer, Aggregator, Merger, Materialized)
   * @see Reducer
   */
  @FunctionalInterface
  public interface Aggregator<K, V, VAgg> {
  
      /**
       * Compute a new aggregate from the key and value of a record and the current aggregate of the same key.
       *
       * @param key
       *        the key of the record
       * @param value
       *        the value of the record
       * @param aggregate
       *        the current aggregate value
       *
       * @return the new aggregate value
       */
      VAgg apply(final K key, final V value, final VAgg aggregate);
  }
  