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@hoffrocket
Created November 20, 2013 05:48
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Java 8 Histogram
import java.util.Map;
import java.util.Random;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class Histogram {
public static <T> Map<T, Integer> histogram(Stream<T> stream) {
return stream.collect(
Collectors.groupingBy(Function.<T>identity(),
Collectors.mapping(i -> 1, Collectors.summingInt(s -> s.intValue()))));
}
public static void main(String [] a) {
Map<Integer, Integer> histo = histogram(new Random().ints(1, 11).limit(10000).boxed());
System.out.println(histo);
}
}
@hoffrocket
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Author

approximate equivalent in scala for comparison:

Stream.continually(util.Random.nextInt(10)).take(1000).groupBy(identity).mapValues(_.size)

@airborn
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airborn commented Aug 2, 2014

This could be reduced to (values changed to Long):

public static <T> Map<T, Long> histogram(Stream<T> stream) {
  return stream.collect(
    Collectors.groupingBy(
      Function.identity(),
      Collectors.counting()
    ));

@paskino
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paskino commented Nov 5, 2016

This will not be very useful with floating point numbers. And it does not allow to set the bin width, so it makes an histogram of bin of width 1. A little bit of more code you can get something more useful.
`
import java.util.;
import java.util.stream.
;
/**

  • Create a simple histogram from an array of numbers via stream
  • @author edo

*/
public class Histogram{
private int nBins, N=0;
private double min = Double.POSITIVE_INFINITY , max = Double.NEGATIVE_INFINITY;;
private double binWidth;
public Histogram(){

}
public Histogram(int bins){
    setNBins(bins);
}


public static void main(String[] args) {
	/**
	 * Create an arraylist of random numbers normally distributed 
	 */
	Random rn = new Random();
	int limit = 1000_000;
	DoubleStream s = DoubleStream.generate(() -> rn.nextGaussian());
	List<Double> data = s
			.limit(limit)
			.collect(()-> new LinkedList<>(), 
					LinkedList::add, 
					LinkedList::addAll);
	/**
	 * Create an Histogram with the stream API
	 */
	
	int nbins = 12; // number of bins in the histogram
	DoubleSummaryStatistics ds = data.stream().limit(limit)
			.collect(DoubleSummaryStatistics::new,
					DoubleSummaryStatistics::accept,
					DoubleSummaryStatistics::combine);
	Histogram h = new Histogram();
	h.setMax(ds.getMax());
	h.setMin(ds.getMin());
	h.setN((int)ds.getCount());
	h.setNBins(12);
	Map<Integer, Integer> histo8 = h.histogram(data.stream());
	System.out.println(histo8);
}




public void setNBins(int n){
	nBins = n;
	if (max != 0 && min != 0){
		binWidth = (max-min)/nBins;
	}
}
public void setN(int n){N=n;}
public void setMin(double val){
	min = val;
	if (max != 0 && nBins != 0){
		binWidth = (max-min)/nBins;
	}
}
public void setMax(double val){
	max = val;
	if (max != 0 && nBins != 0){
		binWidth = (max-min)/nBins;
	}
}


public Map<Integer, Integer> histogram(Stream<Double> stream) {
	  return stream.collect(
	    Collectors.groupingBy(
	      (x) -> findBin(x),
	      Collectors.mapping(i -> 1, 
	    		  Collectors.summingInt(s -> s.intValue()
	    		  ))
	    ));
}
public Integer findBin(Double datum){
	int ret = 0;
	if (datum == max) {
        ret = nBins-1; 
    } else if (datum == min) {
        ret = 0; 
    } else {
        ret = (int)Math.floor((datum-min)/binWidth);
    }
	return new Integer(ret);
}

}

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