The STEM-AND-LEAF diagram from The Cartoon Guide to Statistics by Larry Gonick & Woollcott Smith

StemAndLeafDiagram.java

package p_fd0b04b007e395a7fcfc5066898d422d;

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.SortedMap;
import java.util.TreeMap;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;

import static java.util.Objects.requireNonNull;

public class StemAndLeafDiagram {

    private static final List<Integer> WEIGHTS_MALE_57 = Arrays.asList(
            140, 145, 160, 190, 155, 165, 150, 190, 195, 138, 160, 155, 153, 145, 170, 175, 175, 170, 180,
            135, 170, 157, 130, 185, 190, 155, 170, 155, 215, 150, 145, 155, 155, 150, 155, 150, 180, 160,
            135, 160, 130, 155, 150, 148, 155, 150, 140, 180, 190, 145, 150, 164, 140, 142, 136, 123, 155
    );

    private static final List<Integer> WEIGHTS_FEMALE_35 = Arrays.asList(
            140, 120, 130, 138, 121, 125, 116, 145, 150, 112, 125, 130, 120, 130, 131, 120, 118, 125, 135,
            125, 118, 122, 115, 102, 115, 150, 110, 116, 108, 95, 125, 133, 110, 150, 108
    );

    private static SortedMap<Integer, List<Integer>> collectStemAndLeaf(final Stream<Integer> weights) {
        return weights.collect(Collectors.groupingBy(
                w -> w / 10,
                TreeMap::new,
                Collectors.mapping(
                        v -> v % 10,
                        Collectors.collectingAndThen(
                                Collectors.toList(),
                                l -> {
                                    l.sort(Comparator.naturalOrder());
                                    return l;
                                }
                        )
                )
        ));
    }

    private static void printStemAndLeafDiagram(final List<Integer> weights) {
        if (requireNonNull(weights, "weights is null").isEmpty()) {
            throw new IllegalArgumentException("weights is empty");
        }
        final Map<Integer, List<Integer>> map = collectStemAndLeaf(weights.stream());
        {
            final int min = weights.stream().mapToInt(Integer::intValue).map(w -> w / 10).min().getAsInt();
            final int max = weights.stream().mapToInt(Integer::intValue).map(w -> w / 10).max().getAsInt();
            IntStream.rangeClosed(min, max).forEach(v -> {
                if (!map.containsKey(v)) {
                    map.put(v, null);
                }
            });
        }
        map.forEach((k, v) -> {
            System.out.printf("%1$2d_: ", k);
            if (v != null) {
                v.forEach(System.out::print);
            }
            System.out.println();
        });
    }

    public static void main(final String... args) {
        System.out.println("------------------------------------------------------------------------------------ MALE");
        printStemAndLeafDiagram(WEIGHTS_MALE_57);
        System.out.println("---------------------------------------------------------------------------------- FEMALE");
        printStemAndLeafDiagram(WEIGHTS_FEMALE_35);
    }
}

------------------------------------------------------------------------------------ MALE
12_: 3
13_: 005568
14_: 000255558
15_: 0000000355555555557
16_: 000045
17_: 000055
18_: 0005
19_: 00005
20_: 
21_: 5
---------------------------------------------------------------------------------- FEMALE
 9_: 5
10_: 288
11_: 002556688
12_: 0001255555
13_: 0001358
14_: 05
15_: 000