1Z0-829 Working with Streams and Lambda Expressions Practice Question
A developer needs to process a stream of integers and collect the results into a Map<Integer, List<Integer>> where keys are the integers themselves and values are lists containing the number and its square. Which collector should be used?
⚠ Common exam trap
Oracle often tests the distinction between the three-argument and four-argument `toMap()` overloads, trapping candidates who omit the map supplier or who confuse `groupingBy()` with `toMap()` when the requirement is to store both the original element and a derived value in the map value.
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
✓
Collectors.toMap(Function.identity(), i -> Arrays.asList(i, i * i), (v1, v2) -> v1, HashMap::new)
It uses the four-argument overload of `Collectors.toMap()`: a key mapper (`Function.identity()`), a value mapper (a lambda that creates a `List<Integer>` containing the number and its square), a merge function (`(v1, v2) -> v1`) to handle duplicate keys (which won't occur here since keys are unique integers), and a `HashMap::new` supplier to ensure the map type is explicitly `HashMap`. This satisfies the requirement of producing a `Map<Integer, List<Integer>>` where each key maps to a list of the number and its square.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Collectors.toMap(Function.identity(), i -> Arrays.asList(i, i * i), (v1, v2) -> v1)
Why it's wrong here
Works only if keys are unique; merge function is redundant but still produces Map<Integer, List<Integer>>. However, it's not the best practice; also fails if duplicates exist.
- ✗
Collectors.groupingBy(Function.identity(), Collectors.mapping(i -> i * i, Collectors.toList()))
Why it's wrong here
Groups by the integer, but values are lists of squares only, not including the original number.
- ✓
Collectors.toMap(Function.identity(), i -> Arrays.asList(i, i * i), (v1, v2) -> v1, HashMap::new)
Why this is correct
Correctly creates map with list values, handles duplicates by keeping first.
- ✗
Collectors.toMap(Function.identity(), i -> i * i)
Why it's wrong here
Produces Map<Integer, Integer>, not Map<Integer, List<Integer>>.
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