1 /*
2 * Copyright (c) 2012, 2014, 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;
26
27 import java.util.function.DoubleConsumer;
28 import java.util.stream.Collector;
29
30 /**
31 * A state object for collecting statistics such as count, min, max, sum, and
32 * average.
33 *
34 * <p>This class is designed to work with (though does not require)
35 * {@linkplain java.util.stream streams}. For example, you can compute
36 * summary statistics on a stream of doubles with:
37 * <pre> {@code
38 * DoubleSummaryStatistics stats = doubleStream.collect(DoubleSummaryStatistics::new,
39 * DoubleSummaryStatistics::accept,
40 * DoubleSummaryStatistics::combine);
41 * }</pre>
42 *
43 * <p>{@code DoubleSummaryStatistics} can be used as a
44 * {@linkplain java.util.stream.Stream#collect(Collector) reduction}
45 * target for a {@linkplain java.util.stream.Stream stream}. For example:
46 *
47 * <pre> {@code
48 * DoubleSummaryStatistics stats = people.stream()
49 * .collect(Collectors.summarizingDouble(Person::getWeight));
50 *}</pre>
51 *
52 * This computes, in a single pass, the count of people, as well as the minimum,
53 * maximum, sum, and average of their weights.
54 *
55 * @implNote This implementation is not thread safe. However, it is safe to use
56 * {@link java.util.stream.Collectors#summarizingDouble(java.util.function.ToDoubleFunction)
57 * Collectors.summarizingDouble()} on a parallel stream, because the parallel
58 * implementation of {@link java.util.stream.Stream#collect Stream.collect()}
59 * provides the necessary partitioning, isolation, and merging of results for
60 * safe and efficient parallel execution.
61 * @since 1.8
62 */
63 public class DoubleSummaryStatistics implements DoubleConsumer {
64 private long count;
65 private double sum;
66 private double sumCompensation; // Low order bits of sum
67 private double simpleSum; // Used to compute right sum for non-finite inputs
68 private double min = Double.POSITIVE_INFINITY;
69 private double max = Double.NEGATIVE_INFINITY;
70
71 /**
72 * Construct an empty instance with zero count, zero sum,
73 * {@code Double.POSITIVE_INFINITY} min, {@code Double.NEGATIVE_INFINITY}
74 * max and zero average.
75 */
76 public DoubleSummaryStatistics() { }
77
78 /**
79 * Records another value into the summary information.
80 *
81 * @param value the input value
82 */
83 @Override
84 public void accept(double value) {
85 ++count;
86 simpleSum += value;
87 sumWithCompensation(value);
88 min = Math.min(min, value);
89 max = Math.max(max, value);
90 }
91
92 /**
93 * Combines the state of another {@code DoubleSummaryStatistics} into this
94 * one.
95 *
96 * @param other another {@code DoubleSummaryStatistics}
97 * @throws NullPointerException if {@code other} is null
98 */
99 public void combine(DoubleSummaryStatistics other) {
100 count += other.count;
101 simpleSum += other.simpleSum;
102 sumWithCompensation(other.sum);
103 sumWithCompensation(other.sumCompensation);
104 min = Math.min(min, other.min);
105 max = Math.max(max, other.max);
106 }
107
108 /**
109 * Incorporate a new double value using Kahan summation /
110 * compensated summation.
111 */
112 private void sumWithCompensation(double value) {
113 double tmp = value - sumCompensation;
114 double velvel = sum + tmp; // Little wolf of rounding error
115 sumCompensation = (velvel - sum) - tmp;
116 sum = velvel;
117 }
118
119 /**
120 * Return the count of values recorded.
121 *
122 * @return the count of values
123 */
124 public final long getCount() {
125 return count;
126 }
127
128 /**
129 * Returns the sum of values recorded, or zero if no values have been
130 * recorded.
131 *
132 * <p> The value of a floating-point sum is a function both of the
133 * input values as well as the order of addition operations. The
134 * order of addition operations of this method is intentionally
135 * not defined to allow for implementation flexibility to improve
136 * the speed and accuracy of the computed result.
137 *
138 * In particular, this method may be implemented using compensated
139 * summation or other technique to reduce the error bound in the
140 * numerical sum compared to a simple summation of {@code double}
141 * values.
142 *
143 * <p>If any recorded value is a NaN or the intermediate sum is at
144 * any point a NaN, then the final sum will be NaN.
145 *
146 * If the recorded values contain infinities of opposite sign, the
147 * final sum will be NaN.
148 *
149 * It is possible for intermediate sums of finite values to
150 * overflow into opposite-signed infinities; if that occurs, the
151 * final sum will be NaN even if the recorded values are all
152 * finite.
153 *
154 * If the exact sum is infinite, a properly-signed infinity is
155 * returned.
156 *
157 * If all the recorded values are zero, the sign of zero is
158 * <em>not</em> guaranteed to be preserved in the final sum.
159 *
160 * @apiNote Values sorted by increasing absolute magnitude tend to yield
161 * more accurate results.
162 *
163 * @return the sum of values, or zero if none
164 */
165 public final double getSum() {
166 // Better error bounds to add both terms as the final sum
167 double tmp = sum + sumCompensation;
168 if (Double.isNaN(tmp) && Double.isInfinite(simpleSum))
169 // If the compensated sum is spuriously NaN from
170 // accumulating one or more same-signed infinite values,
171 // return the correctly-signed infinity stored in
172 // simpleSum.
173 return simpleSum;
174 else
175 return tmp;
176 }
177
178 /**
179 * Returns the minimum recorded value, {@code Double.NaN} if any recorded
180 * value was NaN or {@code Double.POSITIVE_INFINITY} if no values were
181 * recorded. Unlike the numerical comparison operators, this method
182 * considers negative zero to be strictly smaller than positive zero.
183 *
184 * @return the minimum recorded value, {@code Double.NaN} if any recorded
185 * value was NaN or {@code Double.POSITIVE_INFINITY} if no values were
186 * recorded
187 */
188 public final double getMin() {
189 return min;
190 }
191
192 /**
193 * Returns the maximum recorded value, {@code Double.NaN} if any recorded
194 * value was NaN or {@code Double.NEGATIVE_INFINITY} if no values were
195 * recorded. Unlike the numerical comparison operators, this method
196 * considers negative zero to be strictly smaller than positive zero.
197 *
198 * @return the maximum recorded value, {@code Double.NaN} if any recorded
199 * value was NaN or {@code Double.NEGATIVE_INFINITY} if no values were
200 * recorded
201 */
202 public final double getMax() {
203 return max;
204 }
205
206 /**
207 * Returns the arithmetic mean of values recorded, or zero if no
208 * values have been recorded.
209 *
210 * <p>The average returned can vary depending upon the order in
211 * which values are recorded.
212 *
213 * This method may be implemented using compensated summation or
214 * other technique to reduce the error bound in the {@link #getSum
215 * numerical sum} used to compute the average.
216 *
217 * <p>This method can return a NaN or infinite result in the same
218 * kind of numerical situations as {@linkplain #getSum() the sum}
219 * can be NaN or infinite, respectively.
220 *
221 * @apiNote Values sorted by increasing absolute magnitude tend to yield
222 * more accurate results.
223 *
224 * @return the arithmetic mean of values, or zero if none
225 */
226 public final double getAverage() {
227 return getCount() > 0 ? getSum() / getCount() : 0.0d;
228 }
229
230 /**
231 * {@inheritDoc}
232 *
233 * Returns a non-empty string representation of this object suitable for
234 * debugging. The exact presentation format is unspecified and may vary
235 * between implementations and versions.
236 */
237 @Override
238 public String toString() {
239 return String.format(
240 "%s{count=%d, sum=%f, min=%f, average=%f, max=%f}",
241 this.getClass().getSimpleName(),
242 getCount(),
243 getSum(),
244 getMin(),
245 getAverage(),
246 getMax());
247 }
248 }