Get Distinct Elements

Question

Accepted Answer

distinct() is an intermediate Stream API operation that filters out duplicate elements, keeping only the first occurrence of each unique value based on equals().

Key Points:
• distinct() uses each element's equals() and hashCode() implementation to determine uniqueness.
• For custom objects, equals()/hashCode() must be properly overridden, otherwise distinct() falls back to reference equality.
• It's a stateful operation - it must track everything seen so far, which uses O(n) extra memory.
• The relative encounter order of the retained elements is preserved for ordered streams.
• On unordered parallel streams, distinct() may perform better since strict order preservation isn't required.

Example:
Given the list [1, 2, 2, 3, 3, 3], calling distinct() produces a stream yielding just 1, 2, 3.

Code Example:
```
List distinctNumbers = numbers.stream()
        .distinct()
        .collect(Collectors.toList());
```

Interview Tip:
A concise interview answer is:

"distinct() removes duplicate elements from a stream based on their equals()/hashCode() implementation, keeping the first occurrence in encounter order. For custom objects it only works correctly if equals() and hashCode() are properly overridden."