1 /*
2 * Copyright (c) 2012, 2013, 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.toDoubleStatistics()} 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 min = Double.POSITIVE_INFINITY;
68 private double max = Double.NEGATIVE_INFINITY;
69
70 /**
71 * Construct an empty instance with zero count, zero sum,
72 * {@code Double.POSITIVE_INFINITY} min, {@code Double.NEGATIVE_INFINITY}
73 * max and zero average.
74 */
75 public DoubleSummaryStatistics() { }
76
77 /**
78 * Records another value into the summary information.
79 *
80 * @param value the input value
81 */
82 @Override
83 public void accept(double value) {
84 ++count;
85 sumWithCompensation(value);
86 min = Math.min(min, value);
87 max = Math.max(max, value);
88 }
89
90 /**
91 * Combines the state of another {@code DoubleSummaryStatistics} into this
92 * one.
93 *
94 * @param other another {@code DoubleSummaryStatistics}
95 * @throws NullPointerException if {@code other} is null
96 */
97 public void combine(DoubleSummaryStatistics other) {
98 count += other.count;
99 sumWithCompensation(other.sum);
100 sumWithCompensation(other.sumCompensation);
101 min = Math.min(min, other.min);
102 max = Math.max(max, other.max);
103 }
104
105 /**
106 * Incorporate a new double value using Kahan summation /
107 * compensated summation.
108 */
109 private void sumWithCompensation(double value) {
110 double tmp = value - sumCompensation;
111 double velvel = sum + tmp; // Little wolf of rounding error
112 sumCompensation = (velvel - sum) - tmp;
113 sum = velvel;
114 }
115
116 /**
117 * Return the count of values recorded.
118 *
119 * @return the count of values
120 */
121 public final long getCount() {
122 return count;
123 }
124
125 /**
126 * Returns the sum of values recorded, or zero if no values have been
127 * recorded.
128 *
129 * If any recorded value is a NaN or the sum is at any point a NaN
130 * then the sum will be NaN.
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 * @apiNote Values sorted by increasing absolute magnitude tend to yield
144 * more accurate results.
145 *
146 * @return the sum of values, or zero if none
147 */
148 public final double getSum() {
149 // Better error bounds to add both terms as the final sum
150 return sum + sumCompensation;
151 }
152
153 /**
154 * Returns the minimum recorded value, {@code Double.NaN} if any recorded
155 * value was NaN or {@code Double.POSITIVE_INFINITY} if no values were
156 * recorded. Unlike the numerical comparison operators, this method
157 * considers negative zero to be strictly smaller than positive zero.
158 *
159 * @return the minimum recorded value, {@code Double.NaN} if any recorded
160 * value was NaN or {@code Double.POSITIVE_INFINITY} if no values were
161 * recorded
162 */
163 public final double getMin() {
164 return min;
165 }
166
167 /**
168 * Returns the maximum recorded value, {@code Double.NaN} if any recorded
169 * value was NaN or {@code Double.NEGATIVE_INFINITY} if no values were
170 * recorded. Unlike the numerical comparison operators, this method
171 * considers negative zero to be strictly smaller than positive zero.
172 *
173 * @return the maximum recorded value, {@code Double.NaN} if any recorded
174 * value was NaN or {@code Double.NEGATIVE_INFINITY} if no values were
175 * recorded
176 */
177 public final double getMax() {
178 return max;
179 }
180
181 /**
182 * Returns the arithmetic mean of values recorded, or zero if no
183 * values have been recorded.
184 *
185 * If any recorded value is a NaN or the sum is at any point a NaN
186 * then the average will be code NaN.
187 *
188 * <p>The average returned can vary depending upon the order in
189 * which values are recorded.
190 *
191 * This method may be implemented using compensated summation or
192 * other technique to reduce the error bound in the {@link #getSum
193 * numerical sum} used to compute the average.
194 *
195 * @apiNote Values sorted by increasing absolute magnitude tend to yield
196 * more accurate results.
197 *
198 * @return the arithmetic mean of values, or zero if none
199 */
200 public final double getAverage() {
201 return getCount() > 0 ? getSum() / getCount() : 0.0d;
202 }
203
204 /**
205 * {@inheritDoc}
206 *
207 * Returns a non-empty string representation of this object suitable for
208 * debugging. The exact presentation format is unspecified and may vary
209 * between implementations and versions.
210 */
211 @Override
212 public String toString() {
213 return String.format(
214 "%s{count=%d, sum=%f, min=%f, average=%f, max=%f}",
215 this.getClass().getSimpleName(),
216 getCount(),
217 getSum(),
218 getMin(),
219 getAverage(),
220 getMax());
221 }
222 }