The Java™ Tutorials
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Questions and Exercises
Trail: Collections
Lesson: Aggregate Operations

Questions and Exercises: Aggregate Operations


  1. A sequence of aggregate operations is known as a ___ .
  2. Each pipeline contains zero or more ___ operations.
  3. Each pipeline ends with a ___ operation.
  4. What kind of operation produces another stream as its output?
  5. Describe one way in which the forEach aggregate operation differs from the enhanced for statement or iterators.
  6. True or False: A stream is similar to a collection in that it is a data structure that stores elements.
  7. Identify the intermediate and terminal operations in this code:
    double average = roster
        .filter(p -> p.getGender() == Person.Sex.MALE)
  8. The code p -> p.getGender() == Person.Sex.MALE is an example of what?
  9. The code Person::getAge is an example of what?
  10. Terminal operations that combine the contents of a stream and return one value are known as what?
  11. Name one important difference between the Stream.reduce method and the Stream.collect method.
  12. 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?
  13. True or False: Aggregate operations make it possible to implement parallelism with non-thread-safe collections.
  14. Streams are always serial unless otherwise specified. How do you request that a stream be processed in parallel?


  1. 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) {
  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;
        if (hasFavorite)
    Collections.sort(favs, new Comparator<Album>() {
                               public int compare(Album a1, Album a2) {

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