s-chapter9.md

Chapter 9 Solutions to Practice Exercises

Exercise 1

With an anonymous class:

ArrayList<Movie> movies = new ArrayList<>(Movies.movies());
    movies.removeIf( new Predicate<Movie>()
    {
        @Override
        public boolean test(Movie pMovie)
        {
            return pMovie.year() < 1960;
        }
    });

As a lambda expression:

movies.removeIf( movie -> movie.year() < 1960 );

Exercise 2

If we add method isOld to class Movie:

public boolean isOld()
{
    return aYear < 1960;
}

then the expression can use a method reference:

movies.removeIf(Movie::isOld);

Exercise 3

With an anonymous class:

Movies.movies().forEach(new Consumer<Movie>() 
{
   @Override
   public void accept(Movie pMovie)
   {
      System.out.println(pMovie);
   }
});

With a lambda expression:

Movies.movies().forEach(movie -> System.out.println(movie));

With a reference to a static method, assumed to be in class Exercise3:

Movies.movies().forEach(Exercise3::print);
	
private static void print(Movie pMovie)
{
   System.out.println(pMovie);
}

And with a reference to System.out.println(Object):

Movies.movies().forEach(System.out::println);

Exercise 4

List<Movie> movies = Movies.movies();
movies.sort((movie1, movie2) -> movie1.time() - movie2.time());

If we move the lambda expression to a static factory:

private static Comparator<Show> createByTimeComparator()
{
   return (show1, show2) -> show1.time() - show2.time();
}

the client code becomes:

movies.sort(createByTimeComparator());

Exercise 5

We transform the factory into a simple comparison method:

private static int compareByTime(Show pShow1, Show pShow2)
{
   return pShow1.time() - pShow2.time();
}

We can then refer to it wherever a function type (Show,Show)->int is expected:

movies.sort(Exercise5::compareByTime);

Exercise 6

Assuming we have the static import:

import static java.util.Comparator.comparing;

the solution is:

List<Movie> movies = Movies.movies();
movies.sort(comparing(Movie::time).thenComparing(Movie::title));

Exercise 7

As usual with the Flyweight pattern the first step is to make the constructor(s) private. Then we need a static flyweight store:

private static final Map<String, Movie> aMovies = new HashMap<>();

and a static accessor method:

public static Movie get(String pTitle, int pYear, int pTime)
{
   return aMovies.computeIfAbsent(pTitle, title -> new Movie(pTitle, pYear, pTime));
}

In this simple exercise there's no way to get the year and time for a movie, so we have to provide all the information required to create an object (title, year, time) to the accessor method.

Exercise 8

I created a helper function to turn a number of minutes into the requires output:

private static String toHHMM(int pMinutes)
{
   assert pMinutes >=0;
   return String.format("%d:%02d", pMinutes/60, pMinutes%60);
}

The solution is then:

Movies.movies().stream()
   .map(Movie::time)
   .map(Exercise8::toHHMM)
   .forEach(System.out::println);

Exercise 9

To make the code more readable I import the following static members:

import static java.util.Comparator.comparing;
import static java.util.stream.Collectors.toList;

The solution:

List<Movie> mySelection = Movies.movies().stream()
   .filter(movie -> movie.year() < 2000)
   .sorted(comparing(Movie::time).reversed())
   .limit(3)
   .sorted(comparing(Movie::title).reversed())
   .collect(toList());

Exercise 10

With integers:

Random random = new Random();
System.out.println(
   IntStream.generate(()->random.nextInt(101))
      .limit(1000000)
      .summaryStatistics());

With doubles it's only a matter of changing the stream class and working around the 0-1.0 bounds for the random number generation:

DoubleStream.generate(()-> 100 * random.nextDouble())
   .limit(1000000)
   .summaryStatistics()

Exercise 11

The following code assumes Predicate.isEqual and Arrays.stream are statically imported:

long result = Movies.movies().stream()
   .map(Movie::title)
   .map(String::toLowerCase)
   .flatMap(string -> stream(string.split("[\\s,]+")))
   .filter(isEqual("the"))
   .count();

Exercise 12

Movie result = Movies.movies().stream()
   .reduce((movie1, movie2) ->
      movie1.time() > movie2.time() ? movie1 : movie2).get();

Exercise 13

This code assumes Collectors.toMap has been statically imported.

Map<String, Integer> result = 
   Movies.movies().stream()
      .collect(toMap(Movie::title, Movie::time));

Exercise 14

This exercise is easier to organize with a helper function that returns the decade a Movie was produced in, for example:

private static String decade(Movie pMovie)
{
   return String.format("%02ds", ((pMovie.year()-1900)%100)/10*10);
}

Assuming Collectors.groupingBy is statically imported, we have:

Map<String, List<Movie>> result = 
   Movies.movies().stream()
      .collect(groupingBy(Exercise14::decade));

Exercise 15

This solution relies on the fact that we can get a stream of all the entries in a Map:

Movies.movies().stream()
   .collect(groupingBy(Exercise15::decade))
   .entrySet().stream()
   .filter( entry -> entry.getKey().equals("50s") || entry.getKey().equals("60s"))
   .flatMap( entry -> entry.getValue().stream() );

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Copyright Martin P. Robillard 2019