1Z0-829 Working with Streams and Lambda Expressions Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✓

  books.stream() .collect(Collectors.groupingBy(Book::getGenre, Collectors.mapping(Book::getPrice, Collectors.toList()))) .entrySet().stream() .filter(e -> e.getValue().stream().mapToDouble(Double::doubleValue).average().orElse(0) > 20) .collect(Collectors.toMap(Map.Entry::getKey, e -> e.getValue().stream().mapToDouble(Double::doubleValue).sum()));

  Why this is correct

  Correct. It first groups books by genre, collecting prices into a list. Then it filters entries where the average price exceeds $20 using mapToDouble and average(), and finally sums the prices for those genres. This two-step process correctly computes the average per genre before filtering, then sums the total per genre.

• ✗

  books.stream() .filter(b -> b.getPrice() > 20) .collect(Collectors.groupingBy(Book::getGenre, Collectors.summingDouble(Book::getPrice)));

  Why it's wrong here

  Incorrect. It filters individual books with price >20 before grouping, so it only considers books with price above $20, not all books in the genre. The average should be computed on all books in the genre, not just those above $20. This would incorrectly exclude genres where many books are below $20 but the average still exceeds $20.

• ✗

  books.stream() .collect(Collectors.groupingBy(Book::getGenre, Collectors.averagingDouble(Book::getPrice))) .entrySet().stream() .filter(e -> e.getValue() > 20) .collect(Collectors.toMap(Map.Entry::getKey, e -> { return books.stream().filter(b -> b.getGenre().equals(e.getKey())).mapToDouble(Book::getPrice).sum(); }));

  Why it's wrong here

  Incorrect. Although it groups by genre and computes the average correctly, it then re-filters the original stream to compute the sum for each qualifying genre. While this may produce the correct result, it is inefficient and not an idiomatic use of streams. More importantly, the approach is error-prone if the source list is modified or if parallelism is used, and it does not follow the concise intent of the task.

• ✗

  books.stream() .collect(Collectors.groupingBy(Book::getGenre, Collectors.summingDouble(Book::getPrice))) .entrySet().stream() .filter(e -> e.getValue() > 20) .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

  Why it's wrong here

  Incorrect. It groups by genre and computes the sum directly, then filters based on the sum being greater than 20. However, the requirement is to filter based on the average price being greater than $20, not the total sum. This would incorrectly include genres with a small number of expensive books and exclude genres with many cheap books that still have a high average.

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