Sum of Elements

Question

Sum of Elements

Accepted Answer

Summing elements in a stream of integers is done by converting to an IntStream with mapToInt, then calling its sum() terminal operation.

Key Points:
• mapToInt(Integer::intValue) converts a Stream into a primitive IntStream, avoiding boxing overhead during the sum.
• IntStream provides sum(), average(), min(), max(), and summaryStatistics() directly as terminal operations.
• For a Stream, calling reduce(0, Integer::sum) is an alternative that avoids the intermediate IntStream conversion.
• Using the primitive stream is generally more efficient for numeric aggregation than boxed Integer streams.
• sum() returns 0 for an empty stream, since 0 is the identity value for addition.

Example:
Given the list [1, 2, 3, 4, 5], converting to an IntStream and calling sum() produces 15.

Code Example:
```
int sum = numbers.stream()
        .mapToInt(Integer::intValue)
        .sum();
```

Interview Tip:
A concise interview answer is:

"I convert the boxed stream to a primitive IntStream with mapToInt(Integer::intValue), then call sum(), which avoids the boxing overhead of using reduce(0, Integer::sum) directly on a Stream and gives access to other numeric aggregates like average() and max()."