/* * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.util; import java.util.function.DoubleConsumer; /** * A state object for collecting statistics such as count, min, max, sum, and * average. * *

This class is designed to work with (though does not require) * {@linkplain java.util.stream streams}. For example, you can compute * summary statistics on a stream of doubles with: *

 {@code
 * DoubleSummaryStatistics stats = doubleStream.collect(DoubleSummaryStatistics::new,
 *                                                      DoubleSummaryStatistics::accept,
 *                                                      DoubleSummaryStatistics::combine);
 * }
* *

{@code DoubleSummaryStatistics} can be used as a * {@linkplain java.util.stream.Stream#collect(Collector) reduction} * target for a {@linkplain java.util.stream.Stream stream}. For example: * *

 {@code
 * DoubleSummaryStatistics stats = people.stream()
 *     .collect(Collectors.summarizingDouble(Person::getWeight));
 *}
* * This computes, in a single pass, the count of people, as well as the minimum, * maximum, sum, and average of their weights. * * @implNote This implementation is not thread safe. However, it is safe to use * {@link java.util.stream.Collectors#summarizingDouble(java.util.function.ToDoubleFunction) * Collectors.toDoubleStatistics()} on a parallel stream, because the parallel * implementation of {@link java.util.stream.Stream#collect Stream.collect()} * provides the necessary partitioning, isolation, and merging of results for * safe and efficient parallel execution. * @since 1.8 */ public class DoubleSummaryStatistics implements DoubleConsumer { private long count; private double sum; private double sumCompensation; // Low order bits of sum private double simpleSum; // Used to compute right sum for non-finite inputs private double min = Double.POSITIVE_INFINITY; private double max = Double.NEGATIVE_INFINITY; /** * Construct an empty instance with zero count, zero sum, * {@code Double.POSITIVE_INFINITY} min, {@code Double.NEGATIVE_INFINITY} * max and zero average. */ public DoubleSummaryStatistics() { } /** * Records another value into the summary information. * * @param value the input value */ @Override public void accept(double value) { ++count; simpleSum += value; sumWithCompensation(value); min = Math.min(min, value); max = Math.max(max, value); } /** * Combines the state of another {@code DoubleSummaryStatistics} into this * one. * * @param other another {@code DoubleSummaryStatistics} * @throws NullPointerException if {@code other} is null */ public void combine(DoubleSummaryStatistics other) { count += other.count; simpleSum += other.simpleSum; sumWithCompensation(other.sum); sumWithCompensation(other.sumCompensation); min = Math.min(min, other.min); max = Math.max(max, other.max); } /** * Incorporate a new double value using Kahan summation / * compensated summation. */ private void sumWithCompensation(double value) { double tmp = value - sumCompensation; double velvel = sum + tmp; // Little wolf of rounding error sumCompensation = (velvel - sum) - tmp; sum = velvel; } /** * Return the count of values recorded. * * @return the count of values */ public final long getCount() { return count; } /** * Returns the sum of values recorded, or zero if no values have been * recorded. * * If any recorded value is a NaN or the sum is at any point a NaN * then the sum will be NaN. * *

The value of a floating-point sum is a function both of the * input values as well as the order of addition operations. The * order of addition operations of this method is intentionally * not defined to allow for implementation flexibility to improve * the speed and accuracy of the computed result. * * In particular, this method may be implemented using compensated * summation or other technique to reduce the error bound in the * numerical sum compared to a simple summation of {@code double} * values. * * @apiNote Values sorted by increasing absolute magnitude tend to yield * more accurate results. * * @return the sum of values, or zero if none */ public final double getSum() { // Better error bounds to add both terms as the final sum double tmp = sum + sumCompensation; if (Double.isNaN(tmp) && Double.isInfinite(simpleSum)) // If the compensated sum is spuriously NaN from // accumulating one or more same-signed infinite values, // return the correctly-signed infinity stored in // simpleSum. return simpleSum; else return tmp; } /** * Returns the minimum recorded value, {@code Double.NaN} if any recorded * value was NaN or {@code Double.POSITIVE_INFINITY} if no values were * recorded. Unlike the numerical comparison operators, this method * considers negative zero to be strictly smaller than positive zero. * * @return the minimum recorded value, {@code Double.NaN} if any recorded * value was NaN or {@code Double.POSITIVE_INFINITY} if no values were * recorded */ public final double getMin() { return min; } /** * Returns the maximum recorded value, {@code Double.NaN} if any recorded * value was NaN or {@code Double.NEGATIVE_INFINITY} if no values were * recorded. Unlike the numerical comparison operators, this method * considers negative zero to be strictly smaller than positive zero. * * @return the maximum recorded value, {@code Double.NaN} if any recorded * value was NaN or {@code Double.NEGATIVE_INFINITY} if no values were * recorded */ public final double getMax() { return max; } /** * Returns the arithmetic mean of values recorded, or zero if no * values have been recorded. * * If any recorded value is a NaN or the sum is at any point a NaN * then the average will be code NaN. * *

The average returned can vary depending upon the order in * which values are recorded. * * This method may be implemented using compensated summation or * other technique to reduce the error bound in the {@link #getSum * numerical sum} used to compute the average. * * @apiNote Values sorted by increasing absolute magnitude tend to yield * more accurate results. * * @return the arithmetic mean of values, or zero if none */ public final double getAverage() { return getCount() > 0 ? getSum() / getCount() : 0.0d; } /** * {@inheritDoc} * * Returns a non-empty string representation of this object suitable for * debugging. The exact presentation format is unspecified and may vary * between implementations and versions. */ @Override public String toString() { return String.format( "%s{count=%d, sum=%f, min=%f, average=%f, max=%f}", this.getClass().getSimpleName(), getCount(), getSum(), getMin(), getAverage(), getMax()); } }