Given a list of employee names, return a list of their lengths

• map(String::length) transforms a Stream into a Stream, boxing each int.
• mapToInt(String::length) produces a primitive IntStream with no boxing, more efficient.
• IntStream terminals: sum(), average(), max(), summaryStatistics() work directly on primitives.
• Boxed conversion: IntStream.boxed() converts back to Stream for collecting into a List.
• UTF-16 caveat: length() counts code units, not visual characters.

What does mapping to length do?

The String.length() method returns the number of characters in the string as an int. Applying it through map, as map(String::length), transforms a stream of names into a stream of lengths, one length per name. It is a one-to-one transformation, so the result has the same number of elements as the input.

How do you get a list of name lengths?

package com.bank.hr;

import java.util.List;
import java.util.stream.Collectors;

public class NameLengths {
    public static void main(String[] args) {
        List<String> names = List.of("Rajesh", "Priya", "Amit", "Kavita");

        // Using map (boxes each int into Integer)
        List<Integer> lengths = names.stream()
            .map(String::length)
            .collect(Collectors.toList());

        System.out.println(lengths);
    }
}

[6, 5, 4, 6]

How is map different from mapToInt for this case?

| Method | Result type | Boxing? | Numeric terminals? |
|---|---|---|---|
| map(String::length) | Stream | Yes, each int boxed | No direct sum/avg |
| mapToInt(String::length) | IntStream | No boxing | sum(), average(), max() |

import java.util.IntSummaryStatistics;

// mapToInt: no boxing, direct numeric operations
IntSummaryStatistics stats = names.stream()
    .mapToInt(String::length)
    .summaryStatistics();

System.out.println("Max: " + stats.getMax());
System.out.println("Min: " + stats.getMin());
System.out.println("Avg: " + stats.getAverage());

Max: 6
Min: 4
Avg: 5.25

How do you collect IntStream into a List?

IntStream cannot be collected directly with Collectors.toList() because toList expects objects. Call boxed() first to convert each primitive int back to an Integer.

List<Integer> lengths = names.stream()
    .mapToInt(String::length)
    .boxed()
    .collect(Collectors.toList());

What are common mistakes?

Mistake: trying to collect an IntStream directly with Collectors.toList().

// WRONG: IntStream has no collect(Collector) overload
// names.stream().mapToInt(String::length)
//     .collect(Collectors.toList());  // compilation error!

// CORRECT: box first
List<Integer> lengths = names.stream()
    .mapToInt(String::length)
    .boxed()
    .collect(Collectors.toList());

What about the UTF-16 code unit caveat?

For text with characters outside the basic range, like emoji or certain scripts, length() counts UTF-16 code units, not visual characters. A single emoji can report length 2.

String emoji = "\uD83D\uDE00"; // smiley face
System.out.println(emoji.length());        // 2 (two code units)
System.out.println(emoji.codePointCount(0, emoji.length())); // 1 (one code point)

For employee names in English or Hindi this rarely matters, but a senior engineer knows the distinction.

What about using the lengths for further processing?

// Find the longest name
names.stream()
    .max(java.util.Comparator.comparingInt(String::length))
    .ifPresent(n -> System.out.println("Longest: " + n));

Longest: Rajesh

What should a senior engineer know?

• Prefer mapToInt over map for primitive results to avoid boxing overhead on large datasets.
• IntStream offers sum(), average(), max(), min(), and summaryStatistics() for free.
• length() is O(1) for String because the length is stored as a field, so mapping to lengths does not add significant overhead.
• The distinction between code units and code points matters when correctness over arbitrary Unicode is required.

What are common interview follow-up questions?

• "When would you use mapToInt instead of map?" When the result is a primitive int, to avoid boxing.
• "How do you get statistics from the lengths?" Use mapToInt(String::length).summaryStatistics().
• "What does length() actually count?" UTF-16 code units, not visual characters.