map, filter and collect

Definition

A stream is a pipeline for data. filter keeps items that match, map changes each item, and collect or toList gathers the result. Nothing runs until that final step.

Example code

Names of adults, in capitals

import java.util.List;

record Person(String name, int age) { }

class Demo {
    public static void main(String[] args) {
        List<Person> people = List.of(new Person("Asha", 30), new Person("Ravi", 17), new Person("Meera", 25));

        List<String> adults = people.stream()
                .filter(p -> p.age() >= 18)          // keep adults
                .map(p -> p.name().toUpperCase())    // turn each Person into a name
                .toList();                           // run the pipeline and collect

        System.out.println(adults);
    }
}

// Output:
// [ASHA, MEERA]