001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.functor.aggregator;
018
019import org.apache.commons.functor.NullaryFunction;
020
021/**
022 * Interface which offers a means of "aggregating" data. It offers functions
023 * which allows for data to be added to a "series" and then aggregate them. The
024 * term "aggregation" is possibly a bit loose here as this interface doesn't
025 * enforce summing up or averaging or any other way of processing the data -- it
026 * simply allows for the values to be processed in such a way that it generates
027 * a result. The aggregation of data will be done in the <code>evaluate()</code>
028 * function. This is the function at the core of the aggregator package as it
029 * allows for a way to process the data series and get a result of the same type
030 * out of this operation. Note that some aggregators will compute the data at
031 * this point by iterating through the series of data previously stored (if any)
032 * while others might prefer to compute this value on the fly, every time
033 * {@link #add(Object)} is called. This interface doesn't make any assumption as
034 * to when the aggregation result should be computed.
035 *
036 * @param <T>
037 *            type of data to aggregate
038 */
039public interface Aggregator<T> extends NullaryFunction<T> {
040    /**
041     * Adds data to the series which will be aggregated. It doesn't enforce any
042     * limitations on how much data can be stored (or in fact whether it should
043     * be stored) nor does it make any assumptions on synchronization.
044     *
045     * @param data
046     *            Data to be added to the series which this aggregator will
047     *            process/aggregate.
048     */
049    void add(T data);
050
051    /**
052     * Resets any series of data previously stored and returns the aggregator in
053     * the initial state.
054     */
055    void reset();
056}