Java tutorial
/* * Copyright (c) 2012, 2017, 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; import java.util.stream.Collector; import java.util.stream.DoubleStream; /** * A state object for collecting statistics such as count, min, max, sum, and * average. * * <p>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: * <pre> {@code * DoubleSummaryStatistics stats = doubleStream.collect(DoubleSummaryStatistics::new, * DoubleSummaryStatistics::accept, * DoubleSummaryStatistics::combine); * }</pre> * * <p>{@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: * * <pre> {@code * DoubleSummaryStatistics stats = people.stream() * .collect(Collectors.summarizingDouble(Person::getWeight)); *}</pre> * * 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.summarizingDouble()} 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; /** * Constructs an empty instance with zero count, zero sum, * {@code Double.POSITIVE_INFINITY} min, {@code Double.NEGATIVE_INFINITY} * max and zero average. */ public DoubleSummaryStatistics() { } /** * Constructs a non-empty instance with the specified {@code count}, * {@code min}, {@code max}, and {@code sum}. * * <p>If {@code count} is zero then the remaining arguments are ignored and * an empty instance is constructed. * * <p>If the arguments are inconsistent then an {@code IllegalArgumentException} * is thrown. The necessary consistent argument conditions are: * <ul> * <li>{@code count >= 0}</li> * <li>{@code (min <= max && !isNaN(sum)) || (isNaN(min) && isNaN(max) && isNaN(sum))}</li> * </ul> * @apiNote * The enforcement of argument correctness means that the retrieved set of * recorded values obtained from a {@code DoubleSummaryStatistics} source * instance may not be a legal set of arguments for this constructor due to * arithmetic overflow of the source's recorded count of values. * The consistent argument conditions are not sufficient to prevent the * creation of an internally inconsistent instance. An example of such a * state would be an instance with: {@code count} = 2, {@code min} = 1, * {@code max} = 2, and {@code sum} = 0. * * @param count the count of values * @param min the minimum value * @param max the maximum value * @param sum the sum of all values * @throws IllegalArgumentException if the arguments are inconsistent * @since 10 */ public DoubleSummaryStatistics(long count, double min, double max, double sum) throws IllegalArgumentException { if (count < 0L) { throw new IllegalArgumentException("Negative count value"); } else if (count > 0L) { if (min > max) throw new IllegalArgumentException("Minimum greater than maximum"); // All NaN or non NaN var ncount = DoubleStream.of(min, max, sum).filter(Double::isNaN).count(); if (ncount > 0 && ncount < 3) throw new IllegalArgumentException("Some, not all, of the minimum, maximum, or sum is NaN"); this.count = count; this.sum = sum; this.simpleSum = sum; this.sumCompensation = 0.0d; this.min = min; this.max = max; } // Use default field values if count == 0 } /** * 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. * * <p> 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. * * Because of the unspecified order of operations and the * possibility of using differing summation schemes, the output of * this method may vary on the same input values. * * <p>Various conditions can result in a non-finite sum being * computed. This can occur even if the all the recorded values * being summed are finite. If any recorded value is non-finite, * the sum will be non-finite: * * <ul> * * <li>If any recorded value is a NaN, then the final sum will be * NaN. * * <li>If the recorded values contain one or more infinities, the * sum will be infinite or NaN. * * <ul> * * <li>If the recorded values contain infinities of opposite sign, * the sum will be NaN. * * <li>If the recorded values contain infinities of one sign and * an intermediate sum overflows to an infinity of the opposite * sign, the sum may be NaN. * * </ul> * * </ul> * * It is possible for intermediate sums of finite values to * overflow into opposite-signed infinities; if that occurs, the * final sum will be NaN even if the recorded values are all * finite. * * If all the recorded values are zero, the sign of zero is * <em>not</em> guaranteed to be preserved in the final sum. * * @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. * * <p> The computed average can vary numerically and have the * special case behavior as computing the sum; see {@link #getSum} * for details. * * @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; } /** * 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()); } }