Working with Streams and Lambda Expressions →hardMultiple ChoiceObjective-mapped

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

Exhibit

List<String> names = Arrays.asList("Alice", "Bob", "Charlie", "David");
Map<Integer, List<String>> grouped = names.stream()
    .collect(Collectors.groupingBy(String::length));
System.out.println(grouped);

Refer to the exhibit. What is the output?

⚠ Common exam trap

The trap here is that candidates may miscompute string lengths (e.g., thinking "Charlie" has 5 characters) or confuse the grouping key with the index, leading to incorrect assignment of names to keys.

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

{3=[Bob], 5=[Alice, David], 7=[Charlie]}

The code groups names by the length of their string using `Collectors.groupingBy(String::length, Collectors.toList())`. The names "Alice" and "David" have length 5, "Bob" has length 3, and "Charlie" has length 7. The resulting map has keys 3, 5, and 7. Although `groupingBy` returns a `HashMap` without guaranteed order, the expected output in this context displays the keys in ascending numerical order: 3, 5, 7. Therefore, option C correctly shows the groups in that order with the corresponding names. Option B is incorrect because it lists the keys in non-ascending order (5, 3, 7), which does not match the typical output format.

Answer analysis

Option-by-option breakdown

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

✗
{3=[Bob], 5=[Alice, David, Charlie]}
Why it's wrong here
Incorrect; Charlie length is 7.
✗
{5=[Alice, David], 3=[Bob], 7=[Charlie]}
Why it's wrong here
Order not guaranteed but content is same as A; however, A is more standard.
✓
{3=[Bob], 5=[Alice, David], 7=[Charlie]}
Why this is correct
Correct.
✗
{3=[Bob], 5=[Alice], 7=[Charlie, David]}
Why it's wrong here
Incorrect; David length is 5, not 7.

Go deeper

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

Exhibit

Why each option matters

Option-by-option breakdown

Related to this question