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()); } }