1 /*
   2  * Copyright (c) 2010, 2011, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 package java.util.functions;
  26 
  27 /**
  28  * Reduce an input object by combining with a base object.
  29  *
  30  * <p>All reducer implementations are expected to:
  31  * <ul>
  32  *  <li>Provide stable results such that for any {@code a} and {@code b} the
  33  * result of two {@code reduce} operations are always equivalent. ie.<pre>
  34  * Foo one = reducer.reduce(a,b);
  35  * Foo two = reducer.reduce(a,b);
  36  *
  37  * assert one.equals(two) && two.equals(one);
  38  * </pre></li>
  39  * <li>The reducer should not modify the input object in any way that would
  40  * change the result.</li>
  41  * <li>When used for aggregate operations upon many elements reducers
  42  * should not assume that the {@code reduce} operation will be called upon
  43  * elements in any specific orderie.<pre>
  44  * Foo one = reducer.reduce(a,reducer.reduce(b,c));
  45  * Foo two = reducer.reduce(b,reducer.reduce(a,c));
  46  *
  47  * assert one.equals(two) && two.equals(one);
  48  * </pre>/li>
  49  * </ul>
  50 
  51  * @param <T> the type of input objects provided to {@code reduce}.
  52  * @param <U> the type of the base value and output objects from {@code reduce}.
  53  * This may be the same type as {@code <T>}.
  54  */
  55 public interface Reducer<T, U> {
  56 
  57     /**
  58      * Produce a reduced output using the provided input and base objects.
  59      *
  60      * @param base the reduction base.
  61      * @param t the input object.
  62      * @return reduced output.
  63      */
  64     U reduce(U base, T t);
  65 
  66     /**
  67      * Returns a reducer which conditionally performs reduce on the input from
  68      * {@code <U>} to {@code <T>}. The reduce is only performed if the provided
  69      * predicate returns {@code true} for the input {@code <U>}.
  70      * @param predicate A Predicate for {@code <U>} values.
  71      * @return A reducer which conditionally performing reduction from {@code <U>} to
  72      * {@code <T>} if the {@code <U>} input value satisfies a predicate.
  73      */
  74     Reducer<T, U> compose(Predicate<? super T> predicate) default Reducers.compose;
  75 
  76     /**
  77      * Returns a reducer which performs a mapping of inputs from {@code <V>} to
  78      * {@code <T>} before reducing to the result.
  79      * @param <V> Type of input objects.
  80      * @param mapper A mapper from {@code <V>} to {@code <U>}.
  81      * @return A reducer which performing a mapping from {@code <U>} to
  82      * {@code <V>}prior to reducing from {@code <U>} to {@code <T>}.
  83      */
  84      <V> Reducer<V, U> compose(Mapper<? super V, ? extends T> mapper) default Reducers.compose;
  85 }