flink-core/src/main/java/org/apache/flink/api/common/functions/FoldFunction.java

/*
 * 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.flink.api.common.functions;

import org.apache.flink.annotation.Public;

import java.io.Serializable;

/**
 * Base interface for Fold functions. Fold functions combine groups of elements to
 * a single value, by applying a binary operation to an initial accumulator element every element from a group elements.
 * <p>
 * The basic syntax for using a FoldFunction is as follows:
 * <pre>{@code
 * DataSet<X> input = ...;
 *
 * X initialValue = ...;
 * DataSet<X> result = input.fold(new MyFoldFunction(), initialValue);
 * }</pre>
 * <p>
 * Like all functions, the FoldFunction needs to be serializable, as defined in {@link java.io.Serializable}.
 *
 * @param <T> Type of the initial input and the returned element
 * @param <O> Type of the elements that the group/list/stream contains
 *
 * @deprecated use {@link AggregateFunction} instead
 */
@Public
@Deprecated
public interface FoldFunction<O, T> extends Function, Serializable {
	/**
	 * The core method of FoldFunction, combining two values into one value of the same type.
	 * The fold function is consecutively applied to all values of a group until only a single value remains.
	 *
	 * @param accumulator The initial value, and accumulator.
	 * @param value The value from the group to "fold" into the accumulator.
	 * @return The accumulator that is at the end of the "folding" the group.
	 *
	 * @throws Exception This method may throw exceptions. Throwing an exception will cause the operation
	 *                   to fail and may trigger recovery.
	 */
	T fold(T accumulator, O value) throws Exception;
}