DoubleSummaryStatistics.java revision 01c4dc04704e34c54162b52f390623038befa514 
1/*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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7 * published by the Free Software Foundation.  Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
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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).
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25package java.util;
26
27import java.util.function.DoubleConsumer;
28
29// TODO: Revert changes from {@linkplain} and {@link} to {@code} once
30// the streams changes have landed.
31/**
32 * A state object for collecting statistics such as count, min, max, sum, and
33 * average.
34 *
35 * <p>This class is designed to work with (though does not require)
36 * {@code java.util.stream streams}. For example, you can compute
37 * summary statistics on a stream of doubles with:
38 * <pre> {@code
39 * DoubleSummaryStatistics stats = doubleStream.collect(DoubleSummaryStatistics::new,
40 *                                                      DoubleSummaryStatistics::accept,
41 *                                                      DoubleSummaryStatistics::combine);
42 * }</pre>
43 *
44 * <p>{@code DoubleSummaryStatistics} can be used as a
45 * {@code java.util.stream.Stream#collect(Collector) reduction}
46 * target for a {@code java.util.stream.Stream stream}. For example:
47 *
48 * <pre> {@code
49 * DoubleSummaryStatistics stats = people.stream()
50 *     .collect(Collectors.summarizingDouble(Person::getWeight));
51 *}</pre>
52 *
53 * This computes, in a single pass, the count of people, as well as the minimum,
54 * maximum, sum, and average of their weights.
55 *
56 * @implNote This implementation is not thread safe. However, it is safe to use
57 * {@code java.util.stream.Collectors#summarizingDouble(java.util.function.ToDoubleFunction)
58 * Collectors.toDoubleStatistics()} on a parallel stream, because the parallel
59 * implementation of {@code java.util.stream.Stream#collect Stream.collect()}
60 * provides the necessary partitioning, isolation, and merging of results for
61 * safe and efficient parallel execution.
62 * @since 1.8
63 */
64public class DoubleSummaryStatistics implements DoubleConsumer {
65    private long count;
66    private double sum;
67    private double sumCompensation; // Low order bits of sum
68    private double simpleSum; // Used to compute right sum for non-finite inputs
69    private double min = Double.POSITIVE_INFINITY;
70    private double max = Double.NEGATIVE_INFINITY;
71
72    /**
73     * Construct an empty instance with zero count, zero sum,
74     * {@code Double.POSITIVE_INFINITY} min, {@code Double.NEGATIVE_INFINITY}
75     * max and zero average.
76     */
77    public DoubleSummaryStatistics() { }
78
79    /**
80     * Records another value into the summary information.
81     *
82     * @param value the input value
83     */
84    @Override
85    public void accept(double value) {
86        ++count;
87        simpleSum += value;
88        sumWithCompensation(value);
89        min = Math.min(min, value);
90        max = Math.max(max, value);
91    }
92
93    /**
94     * Combines the state of another {@code DoubleSummaryStatistics} into this
95     * one.
96     *
97     * @param other another {@code DoubleSummaryStatistics}
98     * @throws NullPointerException if {@code other} is null
99     */
100    public void combine(DoubleSummaryStatistics other) {
101        count += other.count;
102        simpleSum += other.simpleSum;
103        sumWithCompensation(other.sum);
104        sumWithCompensation(other.sumCompensation);
105        min = Math.min(min, other.min);
106        max = Math.max(max, other.max);
107    }
108
109    /**
110     * Incorporate a new double value using Kahan summation /
111     * compensated summation.
112     */
113    private void sumWithCompensation(double value) {
114        double tmp = value - sumCompensation;
115        double velvel = sum + tmp; // Little wolf of rounding error
116        sumCompensation = (velvel - sum) - tmp;
117        sum = velvel;
118    }
119
120    /**
121     * Return the count of values recorded.
122     *
123     * @return the count of values
124     */
125    public final long getCount() {
126        return count;
127    }
128
129    /**
130     * Returns the sum of values recorded, or zero if no values have been
131     * recorded.
132     *
133     * If any recorded value is a NaN or the sum is at any point a NaN
134     * then the sum will be NaN.
135     *
136     * <p> The value of a floating-point sum is a function both of the
137     * input values as well as the order of addition operations. The
138     * order of addition operations of this method is intentionally
139     * not defined to allow for implementation flexibility to improve
140     * the speed and accuracy of the computed result.
141     *
142     * In particular, this method may be implemented using compensated
143     * summation or other technique to reduce the error bound in the
144     * numerical sum compared to a simple summation of {@code double}
145     * values.
146     *
147     * @apiNote Values sorted by increasing absolute magnitude tend to yield
148     * more accurate results.
149     *
150     * @return the sum of values, or zero if none
151     */
152    public final double getSum() {
153        // Better error bounds to add both terms as the final sum
154        double tmp =  sum + sumCompensation;
155        if (Double.isNaN(tmp) && Double.isInfinite(simpleSum))
156            // If the compensated sum is spuriously NaN from
157            // accumulating one or more same-signed infinite values,
158            // return the correctly-signed infinity stored in
159            // simpleSum.
160            return simpleSum;
161        else
162            return tmp;
163    }
164
165    /**
166     * Returns the minimum recorded value, {@code Double.NaN} if any recorded
167     * value was NaN or {@code Double.POSITIVE_INFINITY} if no values were
168     * recorded. Unlike the numerical comparison operators, this method
169     * considers negative zero to be strictly smaller than positive zero.
170     *
171     * @return the minimum recorded value, {@code Double.NaN} if any recorded
172     * value was NaN or {@code Double.POSITIVE_INFINITY} if no values were
173     * recorded
174     */
175    public final double getMin() {
176        return min;
177    }
178
179    /**
180     * Returns the maximum recorded value, {@code Double.NaN} if any recorded
181     * value was NaN or {@code Double.NEGATIVE_INFINITY} if no values were
182     * recorded. Unlike the numerical comparison operators, this method
183     * considers negative zero to be strictly smaller than positive zero.
184     *
185     * @return the maximum recorded value, {@code Double.NaN} if any recorded
186     * value was NaN or {@code Double.NEGATIVE_INFINITY} if no values were
187     * recorded
188     */
189    public final double getMax() {
190        return max;
191    }
192
193    /**
194     * Returns the arithmetic mean of values recorded, or zero if no
195     * values have been recorded.
196     *
197     * If any recorded value is a NaN or the sum is at any point a NaN
198     * then the average will be code NaN.
199     *
200     * <p>The average returned can vary depending upon the order in
201     * which values are recorded.
202     *
203     * This method may be implemented using compensated summation or
204     * other technique to reduce the error bound in the {@link #getSum
205     * numerical sum} used to compute the average.
206     *
207     * @apiNote Values sorted by increasing absolute magnitude tend to yield
208     * more accurate results.
209     *
210     * @return the arithmetic mean of values, or zero if none
211     */
212    public final double getAverage() {
213        return getCount() > 0 ? getSum() / getCount() : 0.0d;
214    }
215
216    /**
217     * {@inheritDoc}
218     *
219     * Returns a non-empty string representation of this object suitable for
220     * debugging. The exact presentation format is unspecified and may vary
221     * between implementations and versions.
222     */
223    @Override
224    public String toString() {
225        return String.format(
226            "%s{count=%d, sum=%f, min=%f, average=%f, max=%f}",
227            this.getClass().getSimpleName(),
228            getCount(),
229            getSum(),
230            getMin(),
231            getAverage(),
232            getMax());
233    }
234}
235