Exercises.java

package solutions;

/*
 * LAMBDA PROGRAMMING LABORATORY
 *
 * For each exercise, develop a solution using Java SE 8 Lambda/Streams
 * and remove the @Ignore tag. Then run the tests.
 *
 * In NetBeans, Ctrl-F6 will run the project's tests, which default to
 * the unsolved exercises (as opposed to the solutions). Alt-F6 [PC] or
 * or Cmd-F6 [Mac] will run just the tests in the currently selected file.
 *
 * Several of the exercises read data from a text file. The field named
 * "reader" is a BufferedReader which will be opened for you on the text file.
 * In any exercise that refers to reading from the text file, you can simply
 * use the "reader" variable without worry about opening or closing it.
 * This is setup by JUnit using the @Before and @After methods at the bottom of
 * this file. The text file is "SonnetI.txt" (Shakespeare's first sonnet) which
 * is located at the root of this NetBeans project.
 */

import java.io.BufferedReader;
import java.io.IOException;
import java.math.BigInteger;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.IntSummaryStatistics;
import java.util.List;
import java.util.Map;
import java.util.Random;
import java.util.Set;
import java.util.TreeMap;
import java.util.function.IntConsumer;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;

import org.junit.After;
import org.junit.Before;
import org.junit.Ignore;
import org.junit.Test;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

//BEGINREMOVE

import java.util.ArrayDeque;
import java.util.concurrent.atomic.LongAdder;
import java.util.function.Function;
import java.util.stream.Collector;

import static java.util.Map.Entry;
import static java.util.AbstractMap.SimpleEntry;

import static java.util.Comparator.comparingInt;
import static java.util.Comparator.naturalOrder;
import static java.util.Comparator.reverseOrder;

import static java.util.stream.Collectors.counting;
import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.joining;
import static java.util.stream.Collectors.mapping;
import static java.util.stream.Collectors.partitioningBy;
import static java.util.stream.Collectors.toList;
import static java.util.stream.Collectors.toSet;

//ENDREMOVE

public class Exercises {


// ========================================================
// DEFAULT METHODS
// ========================================================


    /**
     * Create a string that consists of the first letters of each
     * word in the input list.
     */
    @Test
    public void ex01_accumulateFirstLetters() {
        List<String> input = Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot");

        //UNCOMMENT//String result = ""; // TODO
        //BEGINREMOVE
        StringBuilder sb = new StringBuilder();
        input.forEach(s -> sb.append(s.charAt(0)));
        String result = sb.toString();
        //ENDREMOVE

        assertEquals("abcdef", result);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Iterable.forEach().
    // </editor-fold>


    /**
     * Remove the words that have odd lengths from the list.
     */
    @Test
    public void ex02_removeOddLengthWords() {
        List<String> list = new ArrayList<>(Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot"));

        //UNCOMMENT//// TODO code to modify list
        //BEGINREMOVE
        list.removeIf(s -> (s.length() & 1) == 1);
        //ENDREMOVE

        assertEquals("[alfa, echo]", list.toString());
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Collection.removeIf().
    // </editor-fold>


    /**
     * Replace every word in the list with its upper case equivalent.
     */
    @Test
    public void ex03_upcaseAllWords() {
        List<String> list = new ArrayList<>(Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot"));

        //UNCOMMENT////TODO code to modify list
        //BEGINREMOVE
        list.replaceAll(s -> s.toUpperCase());
        //ENDREMOVE

        assertEquals("[ALFA, BRAVO, CHARLIE, DELTA, ECHO, FOXTROT]", list.toString());
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use List.replaceAll().
    // </editor-fold>


    /**
     * Convert every key-value pair of a map into a string and append them all
     * into a single string, in iteration order.
     */
    @Test
    public void ex04_stringifyMap() {
        Map<String, Integer> input = new TreeMap<>();
        input.put("c", 3);
        input.put("b", 2);
        input.put("a", 1);

        //UNCOMMENT//String result = ""; // TODO
        //BEGINREMOVE
        StringBuilder sb = new StringBuilder();
        input.forEach((k, v) -> sb.append(String.format("%s%s", k, v)));
        String result = sb.toString();
        //ENDREMOVE

        assertEquals("a1b2c3", result);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Map.forEach().
    // </editor-fold>


    /**
     * Given a list of words, create a map whose keys are the first letters of
     * each words, and whose values are the sum of the lengths of those words.
     */
    @Test
    public void ex05_mapOfStringLengths() {
        List<String> list = Arrays.asList(
            "aardvark", "bison", "capybara",
            "alligator", "bushbaby", "chimpanzee",
            "avocet", "bustard", "capuchin");
        Map<String, Integer> result = new TreeMap<>();

        //UNCOMMENT////TODO code to populate result
        //BEGINREMOVE
        list.forEach(s -> result.merge(s.substring(0, 1), s.length(), Integer::sum));
        // Instead of Integer::sum, something like (a, b) -> a + b may be used.
        //ENDREMOVE

        assertEquals("{a=23, b=20, c=26}", result.toString());
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Map.merge() within Iterable.forEach().
    // </editor-fold>


// ========================================================
// SIMPLE STREAM PIPELINES
// ========================================================


    /**
     * Given a list of words, create an output list that contains
     * only the odd-length words, converted to upper case.
     */
    @Test
    public void ex06_upcaseOddLengthWords() {
        List<String> input = new ArrayList<>(Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot"));

        //UNCOMMENT//List<String> result = null; // TODO
        //BEGINREMOVE
        List<String> result =
            input.stream()
                .filter(w -> (w.length() & 1) == 1)
                .map(w -> w.toUpperCase())
                .collect(toList());
            // Alternative:
            // Instead of Integer::sum, something like (a, b) -> a + b may be used.
        //ENDREMOVE

        assertEquals("[BRAVO, CHARLIE, DELTA, FOXTROT]", result.toString());
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Use filter() and map().
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Use collect() to create the result list.
    // </editor-fold>


    /**
     * Join the second letters of words 1 through 4 of the list (inclusive,
     * counting from zero), separated by commas, into a single string.
     */
    @Test
    public void ex07_joinStreamRange() {
        List<String> input = new ArrayList<>(Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot"));

        //UNCOMMENT//String result = ""; // TODO
        //BEGINREMOVE
        String result =
            input.stream()
                .skip(1)
                .limit(4)
                .map(word -> word.substring(1, 2))
                .collect(joining(","));
        //ENDREMOVE

        assertEquals("r,h,e,c", result);
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.skip() and Stream.limit().
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Use Collectors.joining().
    // </editor-fold>


    /**
     * Count the number of lines in the text file. (Remember to
     * use the BufferedReader named "reader" that has already been
     * opened for you.)
     *
     * @throws IOException
     */
    @Test
    public void ex08_countLinesInFile() throws IOException {
        //UNCOMMENT//long count = 0; // TODO
        //BEGINREMOVE
        long count =
            reader.lines()
                  .count();
        //ENDREMOVE

        assertEquals(14, count);
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Use BufferedReader.lines() to get a stream of lines.
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.count().
    // </editor-fold>


    /**
     * Find the length of the longest line in the text file.
     *
     * @throws IOException
     */
    @Test
    public void ex09_findLengthOfLongestLine() throws IOException {
        //UNCOMMENT//int longestLength = 0; // TODO
        //BEGINREMOVE
        int longestLength =
            reader.lines()
                  .mapToInt(String::length)
                  .max()
                  .getAsInt();
        //ENDREMOVE

        assertEquals(53, longestLength);
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.mapToInt() to convert to IntStream.
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Look at java.util.OptionalInt to get the result.
    // </editor-fold>


    /**
     * Find the longest line in the text file.
     *
     * @throws IOException
     */
    @Test
    public void ex10_findLongestLine() throws IOException {
        //UNCOMMENT//String longest = ""; // TODO
        //BEGINREMOVE
        String longest =
            reader.lines()
                  .max(comparingInt(String::length))
                  .get();
            // Alternative:
            // Instead of comparingInt(String::length), one could use something like
            //     (s1, s2) -> Integer.compare(s1.length(), s2.length())
        //ENDREMOVE

        assertEquals("Feed'st thy light's flame with self-substantial fuel,", longest);
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.max() with a Comparator.
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Use static methods on Comparator to help create a Comparator instance.
    // </editor-fold>


    /**
     * Select the set of words from the input list whose length is greater than
     * the word's position in the list (starting from zero) .
     */
    @Test
    public void ex11_selectByLengthAndPosition() {
        List<String> input = new ArrayList<>(Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot", "golf", "hotel"));

        //UNCOMMENT//List<String> result = null; // TODO
        //BEGINREMOVE
        List<String> result =
            IntStream.range(0, input.size())
                .filter(pos -> input.get(pos).length() > pos)
                .mapToObj(pos -> input.get(pos))
                .collect(toList());
        //ENDREMOVE

        assertEquals("[alfa, bravo, charlie, delta, foxtrot]", result.toString());
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Instead of a stream of words (Strings), run an IntStream of positions.
    // </editor-fold>


    /**
     * Given two lists of Integer, compute a third list where each element is the
     * difference between the corresponding elements of the two input lists
     * (first minus second).
     */
    @Test
    public void ex12_listDifference() {
        List<Integer> one = Arrays.asList(3, 1, 4, 1, 5, 9, 2, 6, 5, 3);
        List<Integer> two = Arrays.asList(2, 7, 1, 8, 2, 8, 1, 8, 2, 8);

        //UNCOMMENT//List<Integer> result = null; // TODO
        //BEGINREMOVE
        List<Integer> result =
            IntStream.range(0, one.size())
                .mapToObj(i -> one.get(i) - two.get(i))
                .collect(toList());
        //ENDREMOVE

        assertEquals("[1, -6, 3, -7, 3, 1, 1, -2, 3, -5]", result.toString());
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Run an IntStream of list positions (indexes).
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Deal with boxed Integers either by casting or by using mapToObj().
    // </editor-fold>


// ========================================================
// INTERMEDIATE STREAM PIPELINES
// ========================================================


    /**
     * Convert a list of strings into a list of characters.
     */
    @Test
    public void ex13_stringsToCharacters() {
        List<String> input = Arrays.asList("alfa", "bravo", "charlie");

        //UNCOMMENT//List<Character> result = null; // TODO
        //BEGINREMOVE
        List<Character> result =
            input.stream()
                .flatMap(word -> word.chars().mapToObj(i -> (char)i))
                .collect(toList());
        //ENDREMOVE

        assertEquals("[a, l, f, a, b, r, a, v, o, c, h, a, r, l, i, e]", result.toString());
        assertTrue(result.stream().allMatch(x -> x instanceof Character));
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.flatMap().
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Pay attention to the return type of String.chars() and boxing conversion.
    // </editor-fold>


    /**
     * Collect all the words from the text file into a list.
     * Use String.split(REGEXP) to split a string into words.
     * REGEXP is defined at the bottom of this file.
     *
     * @throws IOException
     */
    @Test
    public void ex14_listOfAllWords() throws IOException {
        //UNCOMMENT//List<String> output = null; // TODO
        //BEGINREMOVE
        List<String> output =
            reader.lines()
                  .flatMap(line -> Stream.of(line.split(REGEXP)))
                  .collect(toList());
        // Note: Arrays.stream() is acceptable instead of Stream.of().
        //ENDREMOVE

        assertEquals(
            Arrays.asList(
                "From", "fairest", "creatures", "we", "desire", "increase",
                "That", "thereby", "beauty's", "rose", "might", "never", "die",
                "But", "as", "the", "riper", "should", "by", "time", "decease",
                "His", "tender", "heir", "might", "bear", "his", "memory",
                "But", "thou", "contracted", "to", "thine", "own", "bright", "eyes",
                "Feed'st", "thy", "light's", "flame", "with", "self", "substantial", "fuel",
                "Making", "a", "famine", "where", "abundance", "lies",
                "Thy", "self", "thy", "foe", "to", "thy", "sweet", "self", "too", "cruel",
                "Thou", "that", "art", "now", "the", "world's", "fresh", "ornament",
                "And", "only", "herald", "to", "the", "gaudy", "spring",
                "Within", "thine", "own", "bud", "buriest", "thy", "content",
                "And", "tender", "churl", "mak'st", "waste", "in", "niggarding",
                "Pity", "the", "world", "or", "else", "this", "glutton", "be",
                "To", "eat", "the", "world's", "due", "by", "the", "grave", "and", "thee"),
            output);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.flatMap().
    // </editor-fold>


    /**
     * Read the words from the text file, and create a list containing the words
     * of length 8 or longer, converted to lower case, and sorted alphabetically.
     *
     * @throws IOException
     */
    @Test
    public void ex15_longLowerCaseSortedWords() throws IOException {
        //UNCOMMENT//List<String> output = null; // TODO
        //BEGINREMOVE
        List<String> output =
            reader.lines()
                  .flatMap(line -> Stream.of(line.split(REGEXP)))
                  .filter(word -> word.length() >= 8)
                  .map(String::toLowerCase)
                  .sorted()
                  .collect(toList());
        //ENDREMOVE

        assertEquals(
            Arrays.asList(
                "abundance", "beauty's", "contracted", "creatures",
                "increase", "niggarding", "ornament", "substantial"),
            output);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.sorted().
    // </editor-fold>


    /**
     * Read the words from the text file, and create a list containing the words
     * of length 8 or longer, converted to lower case, and sorted reverse alphabetically.
     * (Same as above except for reversed sort order.)
     *
     * @throws IOException
     */
    @Test
    public void ex16_longLowerCaseReverseSortedWords() throws IOException {
        //UNCOMMENT//List<String> result = null; // TODO
        //BEGINREMOVE
        List<String> result =
            reader.lines()
                  .flatMap(line -> Stream.of(line.split(REGEXP)))
                  .filter(word -> word.length() >= 8)
                  .map(String::toLowerCase)
                  .sorted(reverseOrder())
                  .collect(toList());
        //ENDREMOVE

        assertEquals(
            Arrays.asList(
                "substantial", "ornament", "niggarding", "increase",
                "creatures", "contracted", "beauty's", "abundance"),
            result);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Comparator.reverseOrder().
    // </editor-fold>


    /**
     * Read words from the text file, and sort unique, lower-cased words by length,
     * then alphabetically within length, and place the result into an output list.
     *
     * @throws IOException
     */
    @Test
    public void ex17_sortedLowerCaseDistinctByLengthThenAlphabetically() throws IOException {
        //UNCOMMENT//List<String> result = null; // TODO
        //BEGINREMOVE
        List<String> result =
            reader.lines()
                  .flatMap(line -> Stream.of(line.split(REGEXP)))
                  .map(String::toLowerCase)
                  .distinct()
                  .sorted(comparingInt(String::length)
                          .thenComparing(naturalOrder()))
                  .collect(toList());
        //ENDREMOVE

        assertEquals(
            Arrays.asList(
                "a", "as", "be", "by", "in", "or", "to", "we",
                "and", "art", "bud", "but", "die", "due", "eat", "foe",
                "his", "now", "own", "the", "thy", "too", "bear", "else",
                "eyes", "from", "fuel", "heir", "lies", "only",
                "pity", "rose", "self", "that", "thee", "this", "thou",
                "time", "with", "churl", "cruel", "flame", "fresh", "gaudy",
                "grave", "might", "never", "riper", "sweet", "thine",
                "waste", "where", "world", "bright", "desire", "famine",
                "herald", "mak'st", "making", "memory", "should", "spring",
                "tender", "within", "buriest", "content", "decease",
                "fairest", "feed'st", "glutton", "light's", "thereby", "world's", "beauty's",
                "increase", "ornament", "abundance", "creatures", "contracted", "niggarding",
                "substantial"),
            result);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.distinct().
    // </editor-fold>
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Comparator.theComparing().
    // </editor-fold>


    /**
     * Count the total number of words and the number of distinct, lower case
     * words in the text file, in one pass.
     */
    @Test
    public void ex18_countTotalAndDistinctWords() {
        //UNCOMMENT//long distinctCount = 0; // TODO
        //UNCOMMENT//long totalCount = 0; // TODO
        //BEGINREMOVE
        LongAdder adder = new LongAdder();
        long distinctCount =
            reader.lines()
                  .flatMap(line -> Stream.of(line.split(REGEXP)))
                  .map(String::toLowerCase)
                  .peek(s -> adder.increment())
                  .distinct()
                  .count();
        long totalCount = adder.longValue();
        //ENDREMOVE

        assertEquals("distinct count", 81, distinctCount);
        assertEquals("total count", 107, totalCount);
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.peek().
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Use LongAdder or AtomicLong/AtomicInteger to allow peek() to have side effects.
    // </editor-fold>


// ========================================================
// ADVANCED STREAMS: REDUCTION, COLLECTORS, AND GROUPING
// ========================================================


    /**
     * Compute the value of 21!, that is, 21 factorial. This value is larger than
     * Long.MAX_VALUE, so you must use BigInteger.
     */
    @Test
    public void ex19_bigFactorial() {
        //UNCOMMENT//BigInteger result = BigInteger.ONE; // TODO
        //BEGINREMOVE
        BigInteger result =
            IntStream.rangeClosed(1, 21)
                .mapToObj(n -> BigInteger.valueOf(n))
                .reduce(BigInteger.ONE, (m, n) -> m.multiply(n));
        //ENDREMOVE

        assertEquals(new BigInteger("51090942171709440000"), result);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use LongStream and reduction.
    // </editor-fold>


    /**
     * Get the last word in the text file.
     *
     * @throws IOException
     */
    @Test
    public void ex20_getLastWord() throws IOException {
        //UNCOMMENT//List<String> result = null; // TODO
        //BEGINREMOVE
        String result =
            reader.lines()
                  .flatMap(line -> Stream.of(line.split(REGEXP)))
                  .reduce((a, b) -> b)
                  .get();
        //ENDREMOVE

        assertEquals("thee", result);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.reduce().
    // </editor-fold>


    /**
     * Categorize the words from the text file into a map, where the map's key
     * is the length of each word, and the value corresponding to a key is a
     * list of words of that length. Don't bother with uniqueness or lower-
     * casing the words.
     *
     * @throws IOException
     */
    @Test
    public void ex21_mapLengthToWordList() throws IOException {
        //UNCOMMENT//Map<Integer, List<String>> result = null; // TODO
        //BEGINREMOVE
        Map<Integer, List<String>> result =
            reader.lines()
                  .flatMap(line -> Stream.of(line.split(REGEXP)))
                  .collect(groupingBy(String::length));
        //ENDREMOVE

        assertEquals(10, result.get(7).size());
        assertEquals(new HashSet<>(Arrays.asList("beauty's", "increase", "ornament")), new HashSet<>(result.get(8)));
        assertEquals(new HashSet<>(Arrays.asList("abundance", "creatures")), new HashSet<>(result.get(9)));
        assertEquals(new HashSet<>(Arrays.asList("contracted", "niggarding")), new HashSet<>(result.get(10)));
        assertEquals(Arrays.asList("substantial"), result.get(11));
        assertFalse(result.containsKey(12));
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Collectors.groupingBy().
    // </editor-fold>


    /**
     * Categorize the words from the text file into a map, where the map's key
     * is the length of each word, and the value corresponding to a key is a
     * count of words of that length. Don't bother with uniqueness or lower-
     * casing the words. This is the same as the previous exercise except
     * the map values are the count of words instead of a list of words.
     *
     * @throws IOException
     */
    @Test
    public void ex22_mapLengthToWordCount() throws IOException {
        //UNCOMMENT//Map<Integer, Long> result = null; // TODO
        //BEGINREMOVE
        Map<Integer, Long> result =
            reader.lines()
                  .flatMap(line -> Stream.of(line.split(REGEXP)))
                  .collect(groupingBy(String::length, counting()));
        //ENDREMOVE

        assertEquals( 1L, (long)result.get(1));
        assertEquals(11L, (long)result.get(2));
        assertEquals(28L, (long)result.get(3));
        assertEquals(21L, (long)result.get(4));
        assertEquals(16L, (long)result.get(5));
        assertEquals(12L, (long)result.get(6));
        assertEquals(10L, (long)result.get(7));
        assertEquals( 3L, (long)result.get(8));
        assertEquals( 2L, (long)result.get(9));
        assertEquals( 2L, (long)result.get(10));
        assertEquals( 1L, (long)result.get(11));

        IntSummaryStatistics stats = result.keySet().stream().mapToInt(i -> i).summaryStatistics();
        assertEquals("min key",  1, stats.getMin());
        assertEquals("max key", 11, stats.getMax());
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Use the "downstream" overload of Collectors.groupingBy().
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Use Collectors.counting().
    // </editor-fold>


    /**
     * Gather the words from the text file into a map, accumulating a count of
     * the number of occurrences of each word. Don't worry about upper case and
     * lower case. Extra challenge: implement two solutions, one that uses
     * groupingBy() and the other that uses toMap().
     *
     * @throws IOException
     */
    @Test
    public void ex23_wordFrequencies() throws IOException {
        //UNCOMMENT//Map<String, Long> result = null; // TODO
        //BEGINREMOVE
        Map<String, Long> result =
            reader.lines()
                  .flatMap(line -> Stream.of(line.split(REGEXP)))
                  .collect(groupingBy(Function.identity(), counting()));
                      // or use word -> word instead of Function.identity()

            // Alternative solution using toMap():

            // Map<String, Long> map =
            //     reader.lines()
            //           .flatMap(line -> Stream.of(line.split(REGEXP)))
            //           .collect(toMap(Function.identity(),
            //                          w -> 1L,
            //                          Long::sum));
        //ENDREMOVE

        assertEquals(2L, (long)result.get("tender"));
        assertEquals(6L, (long)result.get("the"));
        assertEquals(1L, (long)result.get("churl"));
        assertEquals(2L, (long)result.get("thine"));
        assertEquals(1L, (long)result.get("world"));
        assertEquals(4L, (long)result.get("thy"));
        assertEquals(3L, (long)result.get("self"));
        assertFalse(result.containsKey("lambda"));
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // For Collectors.groupingBy(), consider that each word needs to
    // be categorized by itself.
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // For Collectors.toMap(), the first occurrence of a word should be mapped to 1.
    // </editor-fold>


    /**
     * From the words in the text file, create nested maps, where the outer map is a
     * map from the first letter of the word to an inner map. (Use a string of length
     * one as the key.) The inner map, in turn, is a mapping from the length of the
     * word to a list of words with that length. Don't bother with any lowercasing
     * or uniquifying of the words.
     *
     * For example, given the words "foo bar baz bazz foo" the string
     * representation of the result would be:
     *     {b={3=[bar, baz], 4=[bazz]}, f={3=[foo, foo]}}
     *
     * @throws IOException
     */
    @Test
    public void ex24_nestedMaps() throws IOException {
        //UNCOMMENT//Map<String, Map<Integer, List<String>>> result = null; // TODO
        //BEGINREMOVE
        Map<String, Map<Integer, List<String>>> result =
            reader.lines()
                  .flatMap(line -> Stream.of(line.split(REGEXP)))
                  .collect(groupingBy(word -> word.substring(0,1),
                                      groupingBy(String::length)));
        //ENDREMOVE

        assertEquals("[abundance]", result.get("a").get(9).toString());
        assertEquals("[by, be, by]", result.get("b").get(2).toString());
        assertEquals("[flame, fresh]", result.get("f").get(5).toString());
        assertEquals("[gaudy, grave]", result.get("g").get(5).toString());
        assertEquals("[should, spring]", result.get("s").get(6).toString());
        assertEquals("[substantial]", result.get("s").get(11).toString());
        assertEquals("[the, thy, thy, thy, too, the, the, thy, the, the, the]",
            result.get("t").get(3).toString());
        assertEquals("[where, waste, world]", result.get("w").get(5).toString());
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // The nested map structure that's the desired is the result of applying
    // a "downstream" collector that's the same operation as the first-level collector.
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Both collection operations are Collectors.groupingBy().
    // </editor-fold>


    /**
     * Given a stream of strings, accumulate (collect) them into the result string
     * by inserting the input string at both the beginning and end. For example, given
     * input strings "x" and "y" the result should be "yxxy". Note: the input stream
     * is a parallel stream, so you MUST write a proper combiner function to get the
     * correct result.
     */
    @Test
    public void ex25_insertBeginningAndEnd() {
        Stream<String> input = Arrays.asList(
            "a", "b", "c", "d", "e", "f", "g", "h", "i", "j",
            "k", "l", "m", "n", "o", "p", "q", "r", "s", "t")
            .parallelStream();

        //UNCOMMENT//String result = input.collect(null, null, null); // TODO
        //BEGINREMOVE
        String result =
            input.collect(StringBuilder::new,
                          (sb, s) -> sb.insert(0, s).append(s),
                          (sb1, sb2) -> {
                              int half = sb2.length() / 2;
                              sb1.insert(0, sb2.substring(0, half));
                              sb1.append(sb2.substring(half));
                          })
                 .toString();
        //ENDREMOVE

        assertEquals("tsrqponmlkjihgfedcbaabcdefghijklmnopqrst", result);
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // The combiner function must take its second argument and merge
    // it into the first argument, mutating the first argument.
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // The second argument to the combiner function happens AFTER the first
    // argument in encounter order, so the second argument needs to be split
    // in half and prepended/appended to the first argument.
    // </editor-fold>


// ========================================================
// ADVANCED STREAMS: POTPOURRI
// ========================================================


    /**
     * Denormalize this map. The input is a map whose keys are the number of legs of an animal
     * and whose values are lists of names of animals. Run through the map and generate a
     * "denormalized" list of Animal objects using the provided Animal class, where
     * each Animal instance contains the name of the animal and the number of legs.
     * A simple Animal data class is provided. The ordering in the output list is not
     * considered significant.
     *
     * Input is Map<Integer, List<String>>:
     *   { 4=["ibex", "hedgehog", "wombat"],
     *     6=["ant", "beetle", "cricket"],
     *     ...
     *   }
     *
     * Output should be a List<Animal>:
     *   [ Animal("ibex", 4),
     *     Animal("hedgehog", 4),
     *     Animal("wombat", 4),
     *     Animal("ant", 6),
     *     Animal("beetle", 6),
     *     Animal("cricket", 6),
     *     ...
     *   ]
     */
    @Test
    public void ex26_denormalizeMap() {
        Map<Integer, List<String>> input = new HashMap<>();
        input.put(4, Arrays.asList("ibex", "hedgehog", "wombat"));
        input.put(6, Arrays.asList("ant", "beetle", "cricket"));
        input.put(8, Arrays.asList("octopus", "spider", "squid"));
        input.put(10, Arrays.asList("crab", "lobster", "scorpion"));
        input.put(750, Arrays.asList("millipede"));

        class Animal {
            final String name; final int legs;
            public Animal(String s, int i) { name = s; this.legs = i; }
            @Override public boolean equals(Object obj) {
                if (! (obj instanceof Animal)) return false;
                Animal other = (Animal)obj;
                return this.name.equals(other.name) && this.legs == other.legs;
            }
            @Override public int hashCode() { return name.hashCode() ^ legs; }
            @Override public String toString() { return String.format("(%s,%d)", name, legs); }
        }

        //UNCOMMENT//List<Animal> result = null; // TODO
        //BEGINREMOVE

        // Simple solution: use Map.forEach to iterate over each entry,
        // and use a nested List.forEach to iterate over each list entry,
        // and accumulate values into the result list.

        List<Animal> result = new ArrayList<>();
        input.forEach((legs, names) ->
                          names.forEach(name -> result.add(new Animal(name, legs))));

        // Alternative solution: stream over map entries, and use flatMap to generate
        // Animal instances for each animal name with the given number of legs. This
        // is more complicated, but it's a more general technique, and it can be run
        // in parallel.

//        List<Animal> result =
//            input.entrySet().stream()
//                 .flatMap(entry -> entry.getValue().stream()
//                                        .map(name -> new Animal(name, entry.getKey())))
//                 .collect(toList());

        //ENDREMOVE

        assertEquals(13, result.size());
        assertTrue(result.contains(new Animal("ibex", 4)));
        assertTrue(result.contains(new Animal("hedgehog", 4)));
        assertTrue(result.contains(new Animal("wombat", 4)));
        assertTrue(result.contains(new Animal("ant", 6)));
        assertTrue(result.contains(new Animal("beetle", 6)));
        assertTrue(result.contains(new Animal("cricket", 6)));
        assertTrue(result.contains(new Animal("octopus", 8)));
        assertTrue(result.contains(new Animal("spider", 8)));
        assertTrue(result.contains(new Animal("squid", 8)));
        assertTrue(result.contains(new Animal("crab", 10)));
        assertTrue(result.contains(new Animal("lobster", 10)));
        assertTrue(result.contains(new Animal("scorpion", 10)));
        assertTrue(result.contains(new Animal("millipede", 750)));
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // There are several ways to approach this. You could use a stream of map keys,
    // a stream of map entries, or nested forEach() methods.
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // If you use streams, consider using Stream.flatMap().
    // </editor-fold>


// ========================================================
// NEW FOR 2016
// ========================================================

    /**
     * Invert a "multi-map". (P. Sandoz)
     *
     * Given a Map<X, Set<Y>>, convert it to Map<Y, Set<X>>.