Chapter 84: map in Java

map(Function<? super T,? extends R>) runs a function per element and replaces the stream’s element type with R. It is the stream version of “for each item, compute a new value.”

filter (83) drops items; map keeps count but changes shape—including String → int via mapToInt.

1. Topic title

map: same conveyor length, each package repacked before the next stage

2. What it means

mapToInt(ToIntFunction) returns an IntStream—fewer Integer boxes on hot paths.

flatMap takes a function to a stream and concatenates those inner streams one level—classic example: line of text → words.

Avoid null: map should not return null elements—use flatMap(s -> Stream.empty()) or filter(Objects::nonNull) patterns instead of null bombs.

3. Why it is used

DTO shaping, parsing, normalizing strings, projecting fields—anywhere each input has exactly one output (or flatMap when zero or many).

4. Mental sketch

filter removes passengers before the photo. map gives everyone a hat—same number of people walk out, different prop.

5. Java code example

import java.util.List;
import java.util.stream.Collectors;
 
public class StreamMapDemo {
    public static void main(String[] args) {
        List<String> nums = List.of("1", "2", "3");
        int sum = nums.stream().mapToInt(Integer::parseInt).sum();
        System.out.println("sum=" + sum);
 
        List<List<Integer>> groups = List.of(List.of(1, 2), List.of(3), List.of());
        List<Integer> flat =
                groups.stream().flatMap(List::stream).collect(Collectors.toList());
        System.out.println(flat);
    }
}

flatMap(List::stream) turns nested lists into one stream of integers.

6. Explanation of code

mapToInt ends with sum()—a IntStream terminal. sum on IntStream is int; watch overflow on huge data in real finance code.

7. Common mistakes

map(mapper) returning streams accidentally—compiler usually catches wrong types; when in doubt, flatMap.

Chaining ten maps when one well-named method would read better—style matters.

8. Best practices

Prefer mapToInt/mapToLong/mapToDouble when the next steps are math-heavy.

Use flatMap for optional expansions: optional.stream() patterns in modern Java reduce ifPresent boilerplate.

9. Small practice task

Uppercase a list of words with map(String::toUpperCase), then filter those starting with "A", then collect(Collectors.toList()).

Beginner tip

Read map then filter top to bottom in time order: repack first, throw away next—order matters when map is expensive.