Questions and Exercises: Aggregate Operations


  • A sequence of aggregate operations is known as a ___ .

  • Each pipeline contains zero or more ___ operations.

  • Each pipeline ends with a ___ operation.

  • What kind of operation produces another stream as its output?

  • Describe one way in which the forEach aggregate operation differs from the enhanced for statement or iterators.

  • True or False: A stream is similar to a collection in that it is a data structure that stores elements.

  • Identify the intermediate and terminal operations in this code:

double average = roster
    .filter(p -> p.getGender() == Person.Sex.MALE)
  • The code p -> p.getGender() == Person.Sex.MALE is an example of what?

  • The code Person::getAge is an example of what?

  • Terminal operations that combine the contents of a stream and return one value are known as what?

  • Name one important difference between the Stream.reduce method and the Stream.collect method.

  • 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?

  • True or False: Aggregate operations make it possible to implement parallelism with non-thread-safe collections.

  • Streams are always serial unless otherwise specified. How do you request that a stream be processed in parallel?


  • 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) {
  • 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;
    if (hasFavorite)
Collections.sort(favs, new Comparator<Album>() {
                           public int compare(Album a1, Album a2) {

Check your answers.