map

Learn to transform elements with map.

Transforming with map

map(Function) transforms each element into a new value and returns a new Stream. Function<T, R> is a functional interface with a single apply(T) method; T is the input type, R the output type.

Example: names.stream().map(s -> s.length()) turns each String into its length; the result is a Stream<Integer>. map is a transformation; it does not change the element count. The stream continues even if the type changes.

mapToInt and primitive specialization

For primitive results use mapToInt, mapToLong, mapToDouble. They return IntStream, LongStream, DoubleStream; these allow primitive operations like sum() and average().

For example names.stream().mapToInt(s -> s.length()).sum() gives the total length of names. This is more concise than map(s -> s.length()).reduce(0, (a, b) -> a + b). The average() method returns Optional, safely read with .orElse(0.0).

Exercise Sum name lengths

Declare List<String> names = new ArrayList<>();; add "Ayşe" and "Alex". Compute the total of name lengths with int sum = names.stream().mapToInt(s -> s.length()).sum();. Print it with System.out.println(sum);.

Show example solution

import java.util.List;
import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();
        names.add("Ayşe");
        names.add("Alex");
        int sum = names.stream().mapToInt(s -> s.length()).sum();
        System.out.println(sum);
    }
}

Test yourself

Match the map concepts

map(...)

Transforms each element
Function<T, R>

Transform function
apply(T)

R-typed result
s -> s.length()

Lambda Function

Complete the map call

names.stream().____(s -> s.length())

Show answer

names.stream().map(s -> s.length())

What does names.stream().map(s -> s.length()) return?
- Stream<String>
- Stream<Integer>
- long
- List<Integer>
Show answer

Stream<Integer> — map turns each String into an Integer; the result is a Stream<Integer>.

Next lesson: forEach and count

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