// Example 179 from page 141

//

import java.util.DoubleSummaryStatistics;

import java.util.stream.DoubleStream;

class Example179 {

public static void main(String[] args) {

simpleStatistics();

betterStatistics();

// uselesslyConsumedTwice();

}

private static void simpleStatistics() {

DoubleStream ds = DoubleStream.of(2, 4, 4, 4, 5, 5, 7, 9);

DoubleSummaryStatistics stats = ds.summaryStatistics();

// DoubleSummaryStatistics stats

// = ds.collect(DoubleSummaryStatistics::new,

// DoubleSummaryStatistics::accept,

// DoubleSummaryStatistics::combine);

System.out.printf("count=%d, min=%g, max=%g, sum=%g, mean=%g%n",

stats.getCount(), stats.getMin(), stats.getMax(),

stats.getSum(), stats.getAverage());

}

// How to extend DoubleSummaryStatistics to also compute standard

// deviation. Since a stream cannot be consumed twice, we do it in

// one pass by subclassing instead of, say, computing the mean in a

// first pass and then computing the standard deviation in the

// second pass, although that would be numerically better.

private static void betterStatistics() {

DoubleStream ds = DoubleStream.of(2, 4, 4, 4, 5, 5, 7, 9);

BetterDoubleStatistics stats

= ds.collect(BetterDoubleStatistics::new,

BetterDoubleStatistics::accept,

BetterDoubleStatistics::combine);

System.out.printf("count=%d, min=%g, max=%g, sum=%g, mean=%g, sdev=%g%n",

stats.getCount(), stats.getMin(), stats.getMax(),

stats.getSum(), stats.getAverage(), stats.getSdev());

}

private static void uselesslyConsumedTwice() {

DoubleStream ds = DoubleStream.of(2, 4, 4, 4, 5, 5, 7, 9);

DoubleSummaryStatistics stats = ds.summaryStatistics();

// This throws IllegalStateException: stream has already been operated upon or closed:

double sqsum = ds.map(x -> x*x).sum();

double sdev = Math.sqrt(sqsum/stats.getCount() - stats.getAverage()*stats.getAverage());

}

}

class BetterDoubleStatistics extends DoubleSummaryStatistics {

private double sqsum = 0.0;

@Override

public void accept(double d) {

super.accept(d);

sqsum += d * d;

}

public void combine(BetterDoubleStatistics other) {

super.combine(other);

sqsum += other.sqsum;

}

public double getSdev() {

double mean = getAverage();

return Math.sqrt(sqsum/getCount() - mean*mean);

}

}