Answers to Questions and Exercises: Aggregate Operations

Questions

1. Q: A sequence of aggregate operations is known as a ___ .
   A: Pipeline
   
   
2. Q: Each pipeline contains zero or more ___ operations.
   A: Intermediate
   
   
3. Q: Each pipeline ends with a ___ operation.
   A: Terminal
   
   
4. Q: What kind of operation produces another stream as its output?
   A: Intermediate
   
   
5. Q: Describe one way in which the forEach aggregate operation differs from the enhanced for statement or iterators.
   A: The forEach aggregate operation lets the system decide "how" the iteration takes place. Using aggregate operations lets you focus on "what" instead of "how."
   
   
6. Q: True or False: A stream is similar to a collection in that it is a data structure that stores elements.
   A: False. Unlike a collection, a stream is not a data structure. It instead carries values from a source through a pipeline.
   
   
7. Q: Identify the intermediate and terminal operations in this code:

   double average = roster
       .stream()
       .filter(p -> p.getGender() == Person.Sex.MALE)
       .mapToInt(Person::getAge)
       .average()
       .getAsDouble();

   
   A: Intermediate: filter, mapToInt
   Terminal: average
   The terminal operation average returns an OptionalDouble. The getAsDouble method is then invoked on that returned object. It is always a good idea to consult the API Specification for information about whether an operation is intermediate or terminal.
   
   
8. Q: The code p -> p.getGender() == Person.Sex.MALE is an example of what?
   A: A lambda expression.
   
   
9. Q: The code Person::getAge is an example of what?
   A: A method reference.
   
   
10. Q: Terminal operations that combine the contents of a stream and return one value are known as what?
    A: Reduction operations.
    
    
11. Q: Name one important difference between the Stream.reduce method and the Stream.collect method.
    A: Stream.reduce always creates a new value when it processes an element. Stream.collect modifies (or mutates) the existing value.
    
    
12. Q: If you wanted to process a stream of names, extract the male names, and store them in a new List, would Stream.reduce or Stream.collect be the most appropriate operation to use?
    A: The collect operation is most appropriate for collecting into a List.
    
    Example:

    List<String> namesOfMaleMembersCollect = roster
        .stream()
        .filter(p -> p.getGender() == Person.Sex.MALE)
        .map(p -> p.getName())
        .collect(Collectors.toList());

    
    
    
13. Q: True or False: Aggregate operations make it possible to implement parallelism with non-thread-safe collections.
    A: True, provided that you do not modify (mutate) the underlying collection while you are operating on it.
    
    
14. Q: Streams are always serial unless otherwise specified. How do you request that a stream be processed in parallel?
    A: Obtain the parallel stream by invoking parallelStream() instead of stream().
    
    

Exercises

1. Exercise: Write the following enhanced for statement as a pipeline with lambda expressions. Hint: Use the filter intermediate operation and the forEach terminal operation.
   

   for (Person p : roster) {
       if (p.getGender() == Person.Sex.MALE) {
           System.out.println(p.getName());
       }
   }

   
   Answer:

   roster
       .stream()
       .filter(e -> e.getGender() == Person.Sex.MALE)
       .forEach(e -> System.out.println(e.getName());

2. Convert the following code into a new implementation that uses lambda expressions and aggregate operations instead of nested for loops. Hint: Make a pipeline that invokes the filter, sorted, and collect operations, in that order.

   List<Album> favs = new ArrayList<>();
   for (Album a : albums) {
       boolean hasFavorite = false;
       for (Track t : a.tracks) {
           if (t.rating >= 4) {
               hasFavorite = true;
               break;
           }
       }
       if (hasFavorite)
           favs.add(a);
   }
   Collections.sort(favs, new Comparator<Album>() {
                              public int compare(Album a1, Album a2) {
                                  return a1.name.compareTo(a2.name);
                              }});

   
   Answer:

   List<Album> sortedFavs =
     albums.stream()
           .filter(a -> a.tracks.anyMatch(t -> (t.rating >= 4)))
           .sorted(Comparator.comparing(a -> a.name))
           .collect(Collectors.toList());

   
   Here we have used the stream operations to simplify each of the three major steps -- identification of whether any track in an album has a rating of at least 4 (anyMatch), the sorting, and the collection of albums matching our criteria into a List. The Comparator.comparing() method takes a function that extracts a Comparable sort key, and returns a Comparator that compares on that key.